Patent Publication Number: US-2022211511-A1

Title: Talonavicular joint prosthesis and method of implanting the prosthesis

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a non-provisional application based upon U.S. provisional patent application Ser. No. 63/133,978, entitled “TALONAVICULAR JOINT PROSTHESIS AND METHOD OF IMPLANTING THE PROSTHESIS”, filed Jan. 5, 2021, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to orthopaedic prostheses and, more particularly, prostheses for repair and treatment of a talonavicular joint and an associated method of preparing the implantation site and implanting the prosthesis. 
     2. Description of the Related Art 
     Disease and damage to bones of the foot, such as the talus and/or the navicular, can severely limit a person&#39;s ability to walk. The traditional approach to repairing loss of function is fusing the joint together to stabilize the bones, but this approach limits the range of motion of the foot. More recent treatment approaches have utilized prostheses that are implanted in the foot, but such implants have not, thus far, adequately restored function, such as natural biomechanics of the foot. 
     What is needed in the art is a talonavicular joint prosthesis that can address some of the issues with known prostheses. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method of replacing the natural talonavicular joint that exists between a patient&#39;s talus and navicular bones includes the steps of providing talar and navicular components, surgically preparing the joint surfaces of the patient&#39;s talus and navicular bones so as to create a new joint space to accommodate these components, and surgically implanting these components. The implants may be created as a general design and/or patient-specific design utilizing 3D- and/or 4D-imaging and/or a variety of manufacturing techniques including additive manufacturing. 4D-based designs would include the ability to restore specific motion(s) for the patient. 
     Surgical preparation of the bones is facilitated by establishing a central axis or axes to prepare the joint surfaces from. The central axis (or axes) is defined by either patient specific guides that match the dorsal surface of the talus based on CT, Mill, ultrasound or other imaging modalities, or specific anatomic landmarks of the bones intraoperatively, and an algorithm that is based off of either the navicular or the talar joint surface geometry. The central axis is then mechanically established in the talar bone to guide the preparation of the bone surface, through cutting devices curved in biplanar fashion to minimize the amount of bone removal and match the general curvature of the navicular or talar joint surface. 
     Alternatively, robotic instrumentation may be utilized to prepare the bone surface using the previously described algorithm, axes, and/or features provided according to the present invention. Exemplary robotic instrumentation includes, but is not limited to, robotic surgical systems such as the da Vinci surgical system. 
     Alternatively, the cutting guides/central axis can be used to create planar cuts that minimize bone removal based on the arthritic condition of the bone. For instance, the talar surface is typically flattened in arthritic joints so preparation of the bone surface may best be restored by a planar cut defined by the central axis. 
     The cutting guide/central axis can also establish a way to allow for alignment correction of, or relative to, adjacent bones and provide a way to hold the bones in space during preparation of the navicular or talar joint surface. In addition, they can provide a way to fix the alignment correction through various embodiments of the navicular component, including screws, grafts, wedges or other configurations that replace lost or damaged bone. Modular components can allow for various articular geometries and bone surface geometries to be combined to allow for deformity correction and restoration of motion. 
     In some embodiments, fixation features, such as pegs, are provided that are designed and located to facilitate dorsal insertion, avoid areas of poor vascularity and have a circular or non-circular geometry to press fit the porous surface into bone prepared via a drill (e.g. round hole) or other cutting tool to create other geometries in the bone. 
     The design may also avoid interruption of associated joints that are not arthritic or may integrate within the implant (or one or more additional implants) to treat arthritic joints adjacent to the talonavicular joint, such as the subtalar, calcaneal, cuboid, etc. 
     Features of the design may be applied to, for example, an all-metal talus prosthesis to articulate with one or more of the joints in either hemi-arthroplasty or joint replacement. 
     In some exemplary embodiments provided according to the present disclosure, a talonavicular joint prosthesis includes a navicular component configured to be implanted in a navicular bone and including a base and a surface having an articulation recess formed therein; and a talar component configured to be implanted in a talus bone and including a bearing section configured to rest within the articulation recess. The talar component and the navicular component are shaped such that, when brought together, there is more conformity between the navicular component and the talar component in a dorsal-plantar plane than in a medial-lateral plane. 
     In some exemplary embodiments, a method of implanting a talonavicular joint prosthesis in a navicular bone and a talus bone is provided. The method includes: establishing a central axis based on a radius of curvature of the navicular bone in a dorsal view and a width for articulation of the navicular bone, establishing the central axis includes defining at least two arcs from the radius of curvature within the width for articulation and defining a point where the arcs intersect as being on the central axis; aligning at least a portion of a guide with the central axis; preparing the navicular bone and the talus bone using at least one cutting device and the guide to guide movement of the at least one cutting device; and placing a navicular component in the prepared navicular bone and a talar component in the prepared talus bone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a top view of an exemplary embodiment of a talar component of a talonavicular joint prosthesis provided according to the present invention; 
         FIG. 2  is a side view of the talar component of  FIG. 1 ; 
         FIG. 3A  is a top view of an exemplary embodiment of a navicular component of a talonavicular joint prosthesis provided according to the present invention; 
         FIG. 3B  is a bottom view of the navicular component of  FIG. 3A ; 
         FIG. 4  is a side view of the navicular component of  FIGS. 3A and 3B ; 
         FIG. 5A  is a side view of an exemplary embodiment of a talonavicular joint prosthesis including the talar component of  FIGS. 1-2  and the navicular component of  FIGS. 3A-4 ; 
         FIG. 5B  is another side view of the talonavicular joint prosthesis of  FIG. 5A ; 
         FIG. 5C  is a cross-sectional view of the talonavicular joint prosthesis of  FIG. 5A  taken along line  5 C- 5 C; 
         FIG. 5D  is a cross-sectional view of the talonavicular joint prosthesis of  FIG. 5B  taken along line  5 D- 5 D; 
         FIG. 5E  is a cross-sectional view of another exemplary embodiment of a talonavicular joint prosthesis with a talar component and a navicular component that have differing conformities in a medial-lateral plane; 
         FIG. 6A  illustrates an exemplary embodiment of a modular navicular component provided according to the present invention with an articulating section partially coupled to a base; 
         FIG. 6B  illustrates the articulating section inserted in the base of  FIG. 6A ; 
         FIG. 6C  illustrates an instrument being used to lock together the articulating section and the base of the navicular component of  FIGS. 6A-6B ; 
         FIG. 6D  illustrates the navicular component of  FIGS. 6A-6C  with slots that can be used to remove the articulating section from the base; 
         FIG. 6E  illustrates another exemplary embodiment of a modular navicular component provided according to the present invention with an articulating section coupled to a base; 
         FIG. 6F  illustrates the articulating section of the modular navicular component of  FIG. 6E  by itself; 
         FIG. 7  is a cross-sectional view of an exemplary embodiment of a modular talar component provided according to the present invention; 
         FIG. 8A  illustrates an exemplary embodiment of a guide provided according to the present invention that is coupled to a talus bone; 
         FIG. 8B  is another view of the guide illustrated in  FIG. 8A ; 
         FIG. 9A  is an illustration of an image of a talus bone; 
         FIG. 9B  is an illustration of another image of the talus bone of  FIG. 9A ; 
         FIG. 10  is an illustration of the talus bone illustrated in  FIGS. 9A and 9B  with a perimeter and area of the talar head defined; 
         FIG. 11  illustrates how a central axis may be established according to the present invention; 
         FIG. 12  is a side view of the guide of  FIGS. 8A and 8B  aligned with a central axis defined according to the present invention; 
         FIG. 13  is another view of the guide of  FIG. 12 ; 
         FIG. 14  is another view of the guide of  FIGS. 12-13 ; 
         FIG. 15  is a side view of an exemplary embodiment of a cutting guide assembly provided according to the present invention that includes the guide of  FIG. 12  aligned with the central axis defined according to the present invention and a cutting guide coupled to the guide; 
         FIG. 16  is another view of the cutting guide assembly of  FIG. 15 ; 
         FIG. 17  is another view of the cutting guide assembly of  FIGS. 15-16 ; 
         FIG. 18  is a top view of an exemplary of a drill guide assembly provided according to the present invention that includes the guide of  FIG. 12  aligned with the central axis defined according to the present invention and a drill guide coupled to the guide; 
         FIG. 19  is a side view of the drill guide assembly of  FIG. 18 ; 
         FIG. 20  is another view of the guide of  FIGS. 18-21 ; 
         FIG. 21A  illustrates drill holes formed in a bone using the drill guide assembly of  FIGS. 18-20 ; 
         FIG. 21B  illustrates a cutting device connecting the drill holes formed in the bone of  FIG. 21A  and creating a mating surface for bone; 
         FIG. 22A  illustrates an exemplary embodiment of a cutting device assembly and a cutting device that may be used to connect the drill holes illustrated in  FIGS. 20A-20B ; 
         FIG. 22B  illustrates another view of the cutting device assembly and cutting device illustrated in  FIG. 22A ; 
         FIG. 22C  illustrates another exemplary embodiment of a cutting device assembly that may be used to connect the drill holes illustrated in  FIGS. 20A-20B , the cutting device assembly having fixation arms that may be pinned to one or more adjacent bones; 
         FIG. 23A  illustrates a side view of a step of preparing a talonavicular joint to accept a joint prosthesis using a cutting device in the embodiment of a cylindrical mill; 
         FIG. 23B  illustrates a side view of another step of preparing the talonavicular joint of  FIG. 23A  to accept a joint prosthesis using a curved mill; 
         FIG. 24  illustrates a cross-sectional view of another exemplary embodiment of a talonavicular joint prosthesis provided according to the present invention that includes a wedge implanted in a foot; 
         FIG. 25A  illustrates another exemplary embodiment of a modular navicular component provided according to the present invention with a base that is configured to be locked to an articulating component by a locking clip; 
         FIG. 25B  illustrates an exemplary embodiment of a locking clip that may be used to lock the base and articulating component of  FIG. 25A  together; 
         FIG. 25C  illustrates the locking clip of  FIG. 25B  locking together the base and articulating component of  FIG. 25A  together; 
         FIG. 26A  illustrates a medial-lateral radius of the talar component of  FIGS. 1-2  by itself when aligned along the central axis defined according to the present invention; and 
         FIG. 26B  illustrates a dorsal-plantar radius of the talar component of  FIGS. 1-2 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIGS. 1-5D and 26A-26B , there is shown an exemplary embodiment of a talonavicular joint prosthesis  500  (illustrated in  FIGS. 5A-5D ) which generally includes a talar component  100 , illustrated by itself in  FIGS. 1-2 and 26A-26B , and a corresponding navicular component  300 , illustrated by itself in  FIGS. 3A-4 , that are configured to be implanted at a prepared talonavicular site, i.e., in a prepared talus bone and a prepared navicular bone, respectively. The components  100 ,  300  both comprise one or more biocompatible materials that are suitable for implantation in an anatomical space. Exemplary materials include but are not limited to: biocompatible metals such as titanium, cobalt-chrome, tantalum, and stainless steel; and polymers such as ultra-high molecular weight polyethylene (UHMWPE), polyaryl ether ketones (PAEK) such as polyether ether ketone (PEEK), polylactic acid (PLA), and polyglycolic acid (PGA). In some embodiments, the components  100 ,  300  are formed with different materials. For example, the talar component  100  may comprise a cobalt chrome molybdenum alloy that is plasma sprayed with titanium and coated with hydroxyapatite while the navicular component  300  comprises a titanium base  310  and a UHMWPE articulating surface  321 . It should be appreciated that a wide variety of materials may be used to form the components  100 ,  300  of the talonavicular joint prosthesis  500 , with the previously described materials being exemplary only. In some embodiments, portions of the components  100 ,  300  may be non-porous, i.e., have a porosity of less than 5%, to prevent significant tissue ingrowth therein and other portions may be highly porous, i.e., have a porosity greater than 40%, to encourage tissue ingrowth therein. 
     Referring specifically to  FIGS. 1-2 and 26A-26B  illustrating the talar component  100 , it can be seen that the talar component  100  has a bearing section  110  and a fixation section  120  that is coupled to the bearing section  110 . The bearing section  110  may have the shape of an oval dome or a spherical dome. Referring to  FIGS. 26A and 26B , it is illustrated that the bearing section  110  of the talar component  100  may be formed in the shape of an oval dome to have a medial-lateral radius MLR that is greater than a dorsal-plantar radius DPR relative to an axis, which may be a central axis CA as defined according to the present invention and is described further herein. If the bearing section  110  were to have equal radii, in contrast, the bearing section  110  would be in the shape of a spherical dome. It should be appreciated that these shapes are exemplary only, and may be adjusted as desired or necessary to account for patient-specific variables or otherwise. The fixation section  120  may include one or more fins  121  that will press-fit into a groove or other recess formed in the prepared talar site. The fin(s)  121  may have a rectangular cross-section with one or more beveled edges so the fin(s)  121  have at least one portion that decreases in width with an increase in distance from the bearing section  110 . It should be appreciated that the fin(s)  121  may be integrally formed with the talar component  100  or, alternatively, may be reversibly coupled components that can be removed from the talar component  100  without damaging the talar component  100 , allowing for the talar component  100  to have modular fixation elements. The fin(s)  121  may comprise, or be coated with, a porous ingrowth material that encourages tissue ingrowth into the fins  121  to provide stable fixation of the talar component  100  in the talus bone. 
     Referring specifically to  FIGS. 3A-4  illustrating the navicular component  300 , it can be seen that the navicular component  300  has a base  310  with an articulating section  320  coupled to the base  310 . The base  310  may comprise titanium, as previously described, and include one or more fixation fins  311 , illustrated as two fixation fins  311 , that are similar to the fins  121  of the talar component  100  and may be reversibly coupled to the navicular component  300 . The base  310  may have a curved surface  312 , from which the fins  311  extend, and a flat surface  313  opposite the curved surface  312  that is coupled to the articulating section  320  having an articulation recess  322  formed therein. Following implantation, the bearing section  110  of the talar component  100  may rest within and articulate against the articulation recess  322 , allowing articulation of the joint.  FIGS. 5A-5D  illustrate the talar component  100  and the navicular component  300  together forming the talonavicular joint prosthesis  500 . The base  310  of the navicular component  300  may be 4 mm thick (top-to-bottom) while the articulation section  320  may have varying thicknesses of UHMWPE. The thickness of the UHMWPE of the articulation section  320  may be defined with respect to an interface surface  313 A where the articulating section  320  has a greatest surface area interface with the base  310 , with the thickness being between 3 mm at a minimum (between the interface surface  313 A and a bottom of the articulation recess  322 ) and 11 mm at a maximum (between the interface surface  313 A and a portion of the articulating section  320  that does not have the articulation recess  322  formed therein). 
     The talar component  100  and the navicular component  300  are shaped such that, when brought together, there is more conformity between the navicular component  300  and the talar component  100  in a dorsal-plantar plane than in a medial-lateral plane, as can be appreciated from  FIGS. 5A-5D . As used herein, “conformity” generally refers to the degree in match between the shapes and dimensions of the talar component  100  and the navicular component  300 . Conformity may be measured by measuring a volume in a particular region between the talar component  100  and the navicular component  300 ; the more volume that is present in the region, indicating a mismatch in the geometry, the less the conformity there is, and vice-versa. For example, the shape of the articulation recess  322  of the navicular component  300  and the bearing section  310  of the talar component  300  may be such that there is differing conformity along different axes. As can be appreciated from comparing  FIG. 5D , which illustrates the components  100 ,  300  in the medial-lateral plane when brought together, and  FIG. 5C , which illustrates the components  100 ,  300  in the dorsal-plantar plane when brought together, there is more conformity between the components  100 ,  300  in the dorsal-plantar plane (indicated by a smaller volume between the components  100 ,  300 ) than in the medial-lateral plane. To provide differing conformities, the articulation recess  322  and a bearing surface  111  of the bearing section  110  of the talar component  100  may be shaped so an articulation radius of curvature in the dorsal-plantar plane ARCDP of the articulation recess  322  and a bearing radius of curvature in the dorsal-plantar plane BRCDP of the bearing surface  111  are similar in a direction of a short axis of the natural talonavicular joint in order to support flexion and extension of the joint. In contrast, the articulation recess  322  and the bearing surface  111  of the bearing section  110  may be shaped so there is a mismatch between an articulation radius of curvature in the medial-lateral plane ARCML and a bearing radius of curvature in the medial-lateral plane BRCML in a direction of the long axis of the natural talonavicular joint. In this respect, the articulation radius of curvature in the medial-lateral plane ARCML of the articulation recess  322  may be greater than the bearing radius of curvature in the medial-lateral plane BRCML of the bearing surface  111  of the bearing section  110  of the talar component  100  to allow for translational movement between the components  100 ,  300  in the long axis. Additionally, along the long axis, the curvature of the short axis may be less conforming medially than laterally to allow for potential translation in all directions medially. In some embodiments, a ratio of the articulation radius of curvature ARCML to the bearing radius of curvature BRCML is between 1.25:1 and 2:1 in the medial-lateral plane ( FIG. 5D ) and/or a ratio of the articulation radius of curvature ARCDP to the bearing radius of curvature BRCDP is between 1:1 and 1.5:1 in the dorsal-plantar plane ( FIG. 5C ). 
     As can be appreciated from  FIGS. 5C and 5D , the articulation recess  322  and the bearing surface  111  are symmetric in both the medial-lateral plane and the dorsal-plantar plane so the conformity between the components  100 ,  300  in both planes on one side of the formed prosthesis, e.g., a medial side, is the same as the conformity between the components  100 ,  300  in both planes on an opposite side, e.g., a lateral side. In some embodiments, the bearing surface  111  and the articulation recess  322  are shaped such that conformity between the components  100 ,  300  in the medial-lateral plane and the dorsal-plantar plane differs on a medial side of the prosthesis  500  compared to a lateral side of the formed prosthesis  500 , i.e., the bearing surface  111  and the articulation recess  322  are asymmetric. For example, and referring now to  FIG. 5E , a difference between a first articulation radius of curvature in the medial-lateral plane ARCML 1  and the bearing radius of curvature in the medial-lateral plane BRCML may be greater on a medial side  326  of the formed prosthesis than the difference between a second articulation radius of curvature in the medial-lateral plan ARCML 2  and the bearing radius of curvature in the medial-lateral plane BRCML on an opposite lateral side  327  of the formed prosthesis, i.e., the conformity between the components  100 ,  300  in the medial-lateral plane is less on the medial side  326  than the lateral side  327 . Similarly, a difference between a first articulation radius of curvature in the dorsal-plantar plane and the bearing radius of curvature in the dorsal-plantar plane may be greater on the medial side  326  of the formed prosthesis than the difference between a second articulation radius of curvature in the dorsal-plantar plane and the bearing radius of curvature in the dorsal-plantar plane on the opposite lateral side  327  of the formed prosthesis, i.e., the conformity between the components  100 ,  300  in the dorsal-plantar plane is less on the medial side  326  than the lateral side  327 . In such embodiments, the conformity between the components  100 ,  300  in the dorsal-plantar plane may still be greater than in the medial-lateral plane on the respective sides of the formed prosthesis. In some embodiments, the conformity between the components  100 ,  300  on the lateral side decreases at a non-constant rate in the medial-to-lateral direction. It should be appreciated that even though the articulation recess  322  is illustrated and described as having differing radii of curvature on the medial side  326  and the lateral side  327 , in some embodiments the bearing surface  111  has differing radii of curvature on the medial side  326  and the lateral side  327  to provide different conformities, which may be alternatively to or in addition to the multiple radii of curvature of the articulation recess  322 . 
     In accordance with the present invention, the talar and navicular components  100 ,  300  each have an inner (fixation) surface  114 ,  324  and an outer (articular) surface  111 ,  323  and a defined average thickness that quantifies the average distance between these component surfaces. Additionally, each component&#39;s inner surface  114 ,  324  is configured to generally follow the anatomic contour of the original joint surface to which each component  100 ,  300  is to be attached and to also minimize its average thickness consistent with providing sufficient strength and rigidity in the prosthesis&#39; components, so as to yield minimum bone resection in the creation of the new prosthesis-accommodating joint space. 
     The outer surfaces  111 ,  323  are designed to articulate with each other to preserve motion of this joint and provide the ability to restore motion equivalent to the natural anatomy. The relationship of the mating outer surfaces  111 ,  323  is configured so that the radius of curvature is relatively conforming in the sagittal plane, to allow for dorsi-plantar flexion movement, and is less conforming in the coronal plane to allow for translation movement and/or adjust for misalignment of the implant axes relative to each other. Combined, these will allow for restoration of inversion and eversion of the foot. 
     Referring now to  FIGS. 6A-7 , exemplary embodiments of a talar component  700  and navicular components  600 ,  600 A are illustrated that are modular. As can be seen, an articulating section  620  of the navicular component  600  may be formed with a section lip  621  and locking grooves  622  that are configured to accept and lock with one or more corresponding base lips  611  of a base  610 . In some embodiments, the section lip  621  is flexible, e.g., formed of UHMWPE, while the base lip(s)  611  is rigid, e.g., formed of metal. The lips  611 ,  621  may be pressed together so the flexible section lip  621  flexes and snaps onto the base lip  611 . As illustrated in  FIGS. 6A and 6B , the flexible section lip  621  may be initially hooked onto the base lip  611  on a dorsal side before pushing down the articulating section  620  to snap the rest of the section lip  621  with the base lip  611  and lock the articulating section  620  to the base  610 . It should be appreciated that the section lip  621  may alternatively be initially hooked onto the base lip  611  on the plantar side. In some embodiments, the flexible section lip(s)  621  may be solely located on the dorsal side of the articulating section  620 . In some embodiments, the section lip(s)  621  are all flexible. In an alternative embodiment, and referring now to  FIGS. 6E-6F , the base  610  is provided similarly to what is illustrated in  FIGS. 6A-6B  but an articulating section  620 A is provided that has a flexible section lip  621 A and a rigid section lip  621 B. The flexible section lip  621 A may be on a dorsal side of the articulating section  620  while the rigid section lip  621 B may be on the plantar side of the articulating section  620 A. The section lip  621 A may be made flexible by making an undercut  622 A adjacent to the section lip  621 , which is not present adjacent to the rigid section lip  621 B. The rigid section lip  621 B has a configuration that allows the rigid section lip  621 B to be inserted when the navicular component  600 A is already implanted. The rigid section lip  621 B engages the base lip  611  as the articulating section  620  is rotated onto the base  610 . To facilitate insertion and rotation, the rigid section lip  621 B may have angled surfaces  623  that correspond to surfaces of the base lip  611 . An instrument  630 , illustrated in  FIG. 6C , may be used to further press the base lips  611  and the section lips  621 ,  621 A,  621 B together, locking the base  610  and the articulating section  620  together. As illustrated in  FIG. 6D , the navicular component  600  may also include one or more slots  623  on the dorsal side that are shaped to accept an instrument, which may be the instrument  630  or a different instrument, allowing the instrument to grab hold of the talar component  700  to remove the talar component  700  from the navicular component  600 . 
     The talar component  700  may be formed similarly by separating a bearing section  710  into two separable pieces  711 ,  712  that are held together by a press-fit or other interference fit. In some embodiments, there is no engagement between the material, such as UHMWPE, of the articulating section  620  and the base  610  on one or more sides of the articulating section  620 . By forming the components  600 ,  600 A,  700  as modular components, proper tension or restoration of the joint line can be achieved and/or one or both pieces can be replaced if there is damage to the components. 
     Referring now to  FIGS. 8A and 8B , an exemplary embodiment of a patient specific guide  1200 , which is illustrated in further detail in  FIGS. 12-14 , provided according to the present invention is illustrated. The guide  1200  is illustrated coupled to a talus bone and is configured such that a central axis CA within the talus is defined. The guide  1200  matches a dorsal surface DS of the talus and rigidly fixates hindfoot and midfoot bones in the proper orientation during bone preparation. The guide  1200  may be modular so the guide  1200  can be easily modified to prepare bones with differing degrees of deformity, as will be described further herein. In some embodiments, the talus is oriented at an angle α, such as 50°, that is off-axis to the coronal plane for resecting once the guide  1200  is fixated in the proper location and orientation. 
     The shape of the guide  1200  may be established based on imaging of the talus, as illustrated in  FIGS. 9A and 9B , so the central axis CA is generally oriented perpendicular to an axis that extends through a length L of the talar head and/or parallel to an axis that extends through a width W of the talar head. An algorithm based off of either the navicular or the talar joint surface geometry may additionally, or alternatively, be used to determine the location of the central axis CA. As illustrated in  FIG. 10 , the guide  1200  can be used to define a perimeter P of the talar head and an area AR of the talar head. The area of the talar head is defined so an area adjacent to the perimeter P of the talus is excluded from bone preparation, in order to protect the surface of the talus below the exterior surface, and from articulation with the navicular component  300  following implantation. Once the guide  1200  is properly positioned, the central axis CA can be established in the talar bone using a mechanical element to guide the preparation of the bone surface. Utilizing the central axis CA to prepare the fixation surface in all planes helps to preserve bone. Cutting devices that can be used to prepare the bone surface may include, for examples, a mill  2300 B that is curved in biplanar fashion (illustrated in  FIG. 23B ) to minimize the amount of bone removed and match the general curvature of the navicular or talar joint surface. The guide  1200  can be used in conjunction with a cutting mill to sweep the cutting mill along the curve in the long axis, with the mill  2300 B being oval shaped and matching the curvature in the short axis. 
     Referring now to  FIG. 11 , a step that may be performed to prepare a navicular bone according to the present invention is illustrated, which may be performed via pre-operative planning in a view that is similar to what is illustrated in  FIG. 9A . As can be seen, the location of a central axis CA can be established using a width for articulation N w  of the navicular as well as a radius of curvature R N  of the navicular bone in the dorsal view. To establish the central axis CA, the radius of curvature R N  of the navicular bone and the width for articulation N w  are measured. The radius of curvature R N  is used to define two or more arcs, such as two arcs A 1 , A 2 , within the width for articulation N w . The arcs A 1 , A 2  may be, for example, respectively centered on a medial edge of articulation ME and a lateral edge of articulation LE, with the intersection defining a point P 1  on the central axis CA. It should be appreciated that more than two arcs may be utilized, so long as the utilized arcs are on the radius of curvature R N  and between the medial and lateral edges ME, LE of the width for articulation N w . Once the central axis CA is established, the guide  800  may then be positioned so a central post of the guide  800  is aligned with the central axis CA. Milling and drilling of the bone may then be performed, using the guide  800  to guide formation of the holes and cuts. For preparation of navicular bone surfaces, 2-4 mm may be added to the radii of curvature R N ; for preparation of the talar surface, 4-8 mm may be subtracted from the radii of curvature R N . 
     Referring now to  FIGS. 12-14 , the guide  1200  that may be provided and used according to the present invention is illustrated in further detail. As illustrated, the guide  1200  has been placed so an opening  1201  formed in the guide  1200  is aligned with a central axis CA of the navicular curvature, which may be determined as previously described, to assist a surgeon with preparing the site for the prosthesis. The guide  1200  may also have a plurality of additional openings  1202  that can act as guide openings for a drill or other cutting instrument, which may be used to cut the openings for fixation fins of the talar component. The guide  1200  may have one or more modular posts  1203  that are configured to attach to a cutting device guide and/or a cutting device, as will be described further herein, which may then be utilized to cut and/or mill the surface of the bone. Because the guide  1200  is positioned so the central axis CA of the navicular curvature is defined, the surface of the bone can be prepared in a manner that follows the natural curvature of the top surface of the bone and reduces the amount of healthy bone tissue that is resected in order to implant the prosthesis. 
     Referring now to  FIGS. 15-17 , an exemplary embodiment of a cutting guide assembly  1500  provided according to the present invention is illustrated that includes a cutting guide  1510  coupled to the modular posts  1203  of the guide  1200 , which is aligned with the central axis CA as previously described in the context of  FIGS. 12-14 . The cutting guide  1510  has a cutting slot  1511  formed therein. The cutting guide  1510  may guide one or more cutting devices to cut into bone tissue. The guide  1200  is aligned with the central axis CA of the navicular curvature, as previously described, to properly orient the guide  1200  and the attached cutting guide  1510  (and any guided cutting devices) for preparation of the bones. The cutting guide  1510  is placed so the bone can be prepared without removing the plantar head, with cuts then being made using a cutting device inserted in the cutting slot  1511 . In other words, the navicular bone and the talus bone can be prepared using at least one cutting device, and in some embodiments multiple cutting devices, and the cutting guide assembly  1500  to guide movement of the cutting device(s). Once the bones are prepared, the navicular component  300  can be placed in the prepared navicular bone and the talar component  100  can be placed in the prepared talus bone. 
     Referring now to  FIGS. 18-21B , an exemplary embodiment and use of a drill guide assembly  1800  for preparing a talonavicular joint according to the present invention is illustrated. The drill guide assembly  1800  includes a drill guide  1810  coupled to the modular posts  1203  of the guide  1200 , which is aligned with the central axis CA as previously described in the context of  FIGS. 12-14 . The drill guide  1810  has a plurality of drill openings  1811  formed therein. The drill guide assembly  1800  of  FIGS. 18-21B  has the opening  1201  of the guide  1200  aligned with a central axis CA of the navicular curvature, which may be established as previously described, to properly orient the drill guide  1810  for preparation of the bone. As can be seen, the drill guide  1810  has two series of drill openings  1811 , with the drill openings  1811  of each series being aligned on a respective arc. A drill may be inserted into each of the openings  1811  to form one or two arcs of drill holes in the bone, as illustrated in  FIGS. 21A and 21B , providing a template for cutting the bone. Once the drill holes are formed, a cutting device, such as a mill, may be moved along the series of drill holes to “connect the dots” and create the proper cut in the bone, as illustrated in  FIG. 21B . 
     Referring now to  FIGS. 22A and 22B , an exemplary embodiment of a cutting device assembly  2200  provided according to the present invention is illustrated that may be used in conjunction with the guide  1200  to connect drill holes formed in bone, e.g., connect the drill holes illustrated in  FIGS. 21A and 21B  using a cutting device, and/or otherwise prepare the joint for implantation of an implant. The cutting device assembly  2200  may be oriented so a central post  2201  of the cutting device assembly  2200  is aligned with the central axis CA, which may be established as previously described. The cutting device assembly  2200  may, for example, be placed so the central post  2201  is inserted in the opening  1201  of the guide  1200 , which is aligned with the central axis CA. A first sleeve  2202  may be coupled to the central post  2201  and a rotatable arm  2203  may be coupled thereto in order to couple the first sleeve  2202  to a second sleeve  2204  that is configured to hold a cutting device, such as an orthopaedic mill  2210 . The rotatable arm  2203  may rotate about the central post  2201 , which may be aligned with the central axis CA, so the second sleeve  2204  and held cutting device  2210  also rotate about the central axis CA, allowing a user to rotate the orthopaedic mill  2210  along the formed drill holes of  FIGS. 21A and 21B . The rotatable arm  2203  may be adjustable so a rotation radius R defined between the first sleeve  2202  and the second sleeve  2204  is adjustable, allowing the radius of the mill&#39;s  2210  movement to be adjusted. The rotation radius R may be adjusted, for example, by placing the rotatable arm  2203  in an arm slot formed in the first sleeve  2202  and displacing the rotatable arm  2203  within the arm slot. In some embodiments, the rotatable arm  2203  is graduated with distance lines to indicate the rotation radius R of the second sleeve  2204  and held cutting device  2210  relative to the first sleeve  2202 . 
     Referring now to  FIG. 22C , another exemplary embodiment of a cutting device assembly  2200 C is illustrated that is similar to the cutting device assembly  2200  but includes one or more fixation arms, illustrated as a pair of fixation arms  2220 . The fixation arms  2220  may be coupled to the central post  2201  and be configured to be fixated to a bone adjacent to the bone(s) being prepared, such as a cuneiform. Each fixation arm  2220  may have a fixation feature  2221  at its end  2222  that is configured to be fixated to bone. For example, the fixation feature  2221  may be an opening that is shaped and sized to accept a pin or other fixation element so the fixation feature  2221  is fixated to bone tissue. It should be appreciated that the fixation feature  2221  may also be configured to fixate directly into bone, e.g., by being a spike or similar construction. Fixating the fixation arm(s)  2220  to adjacent bone(s) can improve anchoring of the cutting device assembly  2200  during bone preparation to increase the reliability of cuts and reduce the risk of the cutting device  2210  moving in an undesired way during preparation. 
     In some exemplary embodiments provided according to the present invention, and referring further to  FIGS. 23A-23B , a method of preparing a bone site and implanting the previously described talonavicular joint prosthesis  500  is provided. The method may include making a dorsal incision and retracting soft tissues around a natural talonavicular site TN. A central axis CA may be established. The central axis CA may be established from a radius of curvature R N  using a guide, which may be a patient specific guide formed according to the present invention, to guide bone removal and preparation. A guide, such as the previously described guide  1200 , may be aligned with the central axis CA such that the guide  1200  has at least a portion, such as an opening  1201 , aligned with the central axis CA. The guide  1200  may couple to various other guides and/or instruments to prepare the bones, such as a surgical drill and various attachments/bits for the surgical drill. A cutting guide  1510  may be coupled to the guide  1200  to create an opening along the joint central axis CA in a dorsal view. The guide  1200  may be attached to the drill guide  1810  then the cutting device assembly  2200  to prepare the navicular bone by milling the navicular surface in the dorsal view to create medial-to-lateral curvature that minimizes the amount of navicular bone surface that is removed, as previously described in the context of  FIGS. 18-22C . The talar surface may then be prepared by milling the talar bone using the cutting device assembly  2200 ,  2200 C coupled to the guide  1200  in combination with the cutting device  2210 , a cylindrical orthopaedic mill  2300 A ( FIG. 23A ), and/or a curved (bi-planar) orthopaedic mill  2300 B ( FIG. 23B ). Fixation element holes for the navicular component  300  and the talar component  100  are formed in the respective bones. Trial components for the navicular component  300  and the talar component  100  may be inserted in the formed fixation element holes to confirm proper placement and orientation. Articular surface trial components may also be inserted to establish the thickness of the polyethylene. Range of motion and stability may also be assessed using the trial components. The navicular component  300  may then be inserted so its fixation element(s), such as the fins  311 , are properly located in their respective opening(s) in the prepared navicular bone and the talar component  100  may also be inserted so its fin(s)  121  are properly located in the respective opening(s) in the prepared talus bone. The UHMWPE insert of the navicular component  300 , if the navicular component  300  is modular, may be inserted into the navicular component  300  and locked into place. The wound may then be closed. 
     In some instances, there is dorsal-plantar curvature in the navicular surface that can result in an undesirable amount of navicular bone being removed. In such an instance, an alternative approach may be used to prepare the joint for the prosthesis  500 . The central axis CA may be established by defining at least two arcs A 1 , A 2  from the radius of curvature R N  within the width for articulation N w  and defining a point P 1  where the arcs A 1 , A 2  intersect as being on the central axis CA, as previously described and illustrated in  FIG. 11 . At least a portion of the guide, such as an opening, is aligned with the established central axis CA. A drill is inserted at the central axis CA through the guide and the talus bone may be resected through the guide to create space. The drill is used as a center of rotation for drilling and milling of the bone about the central axis CA, which may be assisted using the various guides and guide assemblies  1510 ,  1810 ,  2200 ,  2200 A described previously. A curved shaped mill  2300 B, which is illustrated in  FIG. 23B  and may have a biplanar curve, is used to shape the navicular bone on a three-dimensional curved shape, which preserves the dorsal aspect of the navicular bone for support of the prosthesis  500 . The milling design may be selected to match the navicular curvature that is measured. 
     In some embodiments, and referring now to  FIG. 24 , an additional wedge  2400  may be provided to address bone deficits, such as when there is a fracture or other type of significant damage to the talus bone and/or navicular bone. As illustrated, the wedge  2400  may be placed over an area of the bone deficit, shown in the navicular bone, and bear on the bone and an adjacent bone, such as a cuneiform, for stability. The adjacent bone may be fused to another adjacent bone, also illustrated as another cuneiform, to further stabilize the bones. 
     Referring now to  FIGS. 25A-25C , another exemplary embodiment of a modular navicular component  2500  provided according to the present invention is illustrated. The navicular component  2500  includes a base  2510  removably coupled to an articulating section  2520  including an articulating surface  2521  with an articulation recess  2522 . The articulating section  2520  is removably coupled to the base  2510  by a locking clip  2530 , illustrated by itself in  FIG. 25B , that fits in corresponding slots  2511 ,  2523  formed in the base  2510  and the articulating section  2520 . The locking clip  2530  can be unclipped from the base  2510  and/or the articulating section  2520  to unlock the base  2510  and the articulating section  2520  from one another, allowing the base  2510  and the articulating section  2520  to be uncoupled. 
     From the foregoing, it should be appreciated that the talonavicular joint prosthesis  500  provided according to the present invention, and the method of implanting the prosthesis  500 , provides a way to repair damaged talonavicular joints while minimizing the amount of removed healthy tissue and restoring the natural range of motion. The talar component  100  and the navicular component  300  of the prosthesis  500  are shaped so there is a high degree of conformity between the components  100 ,  300  in the dorsal-plantar plane to get a hinge effect that mimics the natural joint movement with less conformity in the medial-lateral plane so there is greater articulation in that plane. For preparing the bone, the previously described algorithm and method may be used to establish a central axis CA using the navicular radius of curvature R N  and the intersection of arcs A 1 , A 2  defined by the radius of curvature R N . Patient specific guides and a central axis can be used for surface preparation and to establish the central axis of rotation for bone preparation. A guide and the central axis can be used to make deformity corrections for proper alignment of components and bone preparation of the talus and navicular surfaces while minimizing navicular bone resection in the medial-lateral and dorsal-plantar directions. 
     While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.