Patent Publication Number: US-2022233324-A1

Title: Hydrogel implants for mid-foot

Description:
FIELD OF DISCLOSURE 
     The present disclosure relates generally to orthopedic implants, and more specifically, to hydrogel implants for repairing articulation surfaces in joints in the mid-foot region. 
     BACKGROUND 
     Implants can be used to replace deteriorated or otherwise damaged cartilage within a joint. Such devices can be used to treat osteoarthritis, rheumatoid arthritis, other inflammatory diseases, generalized joint pain, and joint damages. 
     SUMMARY 
     Disclosed herein is an implant for replacing a portion of an articulation surface of a joint, the implant comprising: a main portion configured for inserting into a joint, wherein the main portion comprises: a porous material portion having a first bone-engaging surface; and 
     a hydrogel portion that is bonded to the porous material portion and forming an articulation surface opposite from the first bone-engaging surface; and 
     a bone plate portion configured for securing the implant to a bone that forms the joint;
         wherein, the main portion having a leading end and a trailing end, wherein the leading end is configured for being inserted into the joint;   wherein the bone plate portion is integrally formed with the porous material portion and extends from the trailing end, forming a second bone-engaging surface that is also formed of the porous material and extends from the first bone-engaging surface in a direction opposite from the articulation surface at an angle with respect to the first bone-engaging surface;   wherein the bone plate portion comprises a solid metal portion that forms all exterior surfaces of the bone plate portion except for the second bone-engaging surface; and   wherein the bone plate portion has at least one screw hole for receiving a bone screw.       

     An implant for replacing a portion of an articulation surface of a joint according to another embodiment is disclosed. The implant comprises: a main portion configured for inserting into a joint, wherein the main portion comprises: 
     a hydrogel portion forming a bone-contacting surface and an articulation surface opposite from the bone-contacting surface; 
     wherein, the main portion having a leading end and a trailing end, wherein the leading end is configured for being inserted into the joint; and 
     a bone plate portion configured for securing the implant to a bone that forms the joint; 
     wherein the bone plate portion comprises:
         a first part having a perforated structure that is embedded in the hydrogel portion; and   a second part that is not embedded in the hydrogel portion and extending from the trailing end in a direction opposite from the articulation surface at an angle ≤160° but ≥80° with respect to the bone-contacting surface;   wherein the second part has at least one screw hole for receiving a bone screw.       

     An implant for replacing a portion of an articulation surface of a joint according to another embodiment is also disclosed. The implant comprises: a main portion configured for inserting into a joint, wherein the main portion comprises: 
     a hydrogel portion forming a bone-contacting surface and an articulation surface opposite from the bone-contacting surface; 
     wherein the bone-contacting surface comprises a protruding part; 
     wherein, the main portion having a leading end and a trailing end, wherein the leading end is configured for being inserted into the joint; and 
     a bone plate portion configured for securing the implant to a bone that forms the joint; wherein the bone plate portion comprises:
         a first part having a perforated structure that is embedded in the protruding part of the hydrogel portion; and   a second part that is not embedded in the protruding part of the hydrogel portion and extending from the trailing end in a direction opposite from the articulation surface at an angle ≤160° but ≥80° with respect to the bone-contacting surface;   wherein the second part has at least one screw hole for receiving a bone screw.       

     An implant for replacing a portion of an articulation surface of a joint according to yet another embodiment is disclosed. The implant comprises: a main portion configured for inserting into a joint and comprising a leading end, a trailing end, an articulation surface and a bone-contacting surface extending between the leading end and the trailing end, wherein the leading end is configured for being inserted into the joint, wherein the main portion further comprises: 
     a porous material portion; and 
     a hydrogel portion forming the articulation surface and the bone-contacting surface opposite from the articulation surface; 
     wherein the porous material portion is bonded to the hydrogel portion, extending from the trailing end and partially towards the leading end and forms a portion of the bone-contacting surface; 
     wherein the porous material portion comprises a tapered hole at the trailing end; and 
     a bone plate configured for securing the implant to a bone that forms the joint; 
     wherein the bone plate is formed of a solid metal; 
     wherein the bone plate comprises a tapered stem that is configured to be inserted into the tapered hole in the porous material portion, whereby the tapered stem and the tapered hole cooperate to urge the bone-contacting surface of the implant toward the bone when the implant is inserted into the joint; and 
     wherein the bone plate has at least one screw hole for receiving a bone screw. 
     The novel implants disclosed herein provides hydrogel implants having hybrid structures that allow repair of articular cartilage surfaces in various joint spaces that were not easily repaired and provide robust and durable repaired surfaces utilizing the benefits of utilizing hydrogel material for articulation surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various embodiments of the inventive hydrogel implant of the present disclosure will be described in more detail in conjunction with the following drawing figures. The structures in the drawing figures are illustrated schematically and are not intended to show actual dimensions. 
         FIG. 1  is a perspective view illustration of a hydrogel implant according to a first embodiment of the present disclosure. 
         FIG. 2  is an exploded view illustration of the hydrogel implant of  FIG. 1 . 
         FIG. 3A  is a side view illustration of the hydrogel implant of  FIG. 1 . 
         FIG. 3B  is a side view illustration of the hydrogel implant of  FIG. 1  in implanted position in a joint. 
         FIG. 4  is a cross-section view illustration of the hydrogel implant of  FIG. 1  taken through the section line H-H shown in  FIG. 3 . 
         FIG. 5  is a top view illustration of the hydrogel implant of  FIG. 1  showing only the porous metal foam portion, i.e., the implant without the hydrogel portion and the solid metal portion. 
         FIG. 6  is a cross-section view illustration of the structure shown in  FIG. 5  taken through the section line T-T. 
         FIG. 7  is a side view illustration of the hydrogel implant of  FIG. 1  showing only the porous metal foam and the solid metal portions, i.e., the implant without the hydrogel portion. 
         FIG. 8  is a cross-section view illustration of the structure shown in  FIG. 7  taken through the section line E-E. 
         FIG. 9  is a top view illustration of the structure shown in  FIG. 7 . 
         FIG. 10  is a top view illustration of the hydrogel implant of  FIG. 1 . 
         FIG. 11  is a cross-section view illustration of the hydrogel implant taken through the section line M-M shown in  FIG. 10 . 
         FIG. 12  is a detailed view of the region N identified in the sectional view of  FIG. 11 . 
         FIG. 13  is a perspective view of a hydrogel implant according to a second embodiment. 
         FIG. 14  is an exploded view illustration of the hydrogel implant of  FIG. 13 . 
         FIG. 15  is a top view of the bone plate portion of the hydrogel implant of  FIG. 13 . 
         FIG. 16  is a side view of the bone plate portion shown in  FIG. 15 . 
         FIG. 17  is a cross-section view of the bone plate portion taken through the section line F-F shown in  FIG. 15 . 
         FIG. 18  is a detailed view of the region G identified in the sectional view of  FIG. 17 . 
         FIG. 19  is a detailed view of the region N identified in the sectional view of  FIG. 17 . 
         FIG. 20  is a perspective view of a hydrogel implant according to a third embodiment. 
         FIG. 21  is an exploded view illustration of the hydrogel implant of  FIG. 20 . 
         FIG. 22  is a top view of the bone plate portion of the hydrogel implant of  FIG. 20 . 
         FIG. 23  is a cross-section view of the bone plate portion taken through the section line C-C shown in  FIG. 22 . 
         FIG. 24  is a detailed view of the region E identified in the sectional view of  FIG. 23 . 
         FIG. 25  is an illustration of an example of a mold that can be used to form the hydrogel implants of the present disclosure by injection molding. 
         FIG. 26  is a cross-section view of the injection molding set up shown in  FIG. 25 . 
         FIG. 27  is a perspective view illustration of a hydrogel implant according to a fourth embodiment. 
         FIGS. 28-29  are illustrations showing the hydrogel implant of  FIG. 27  implanted into some of the mid-foot joints. 
         FIG. 30  is a perspective view illustration of the main portion of the hydrogel implant of  FIG. 27 . 
         FIG. 31  is an illustration showing a view of the bone-contacting surface of the main portion shown in  FIG. 30 . 
         FIG. 32  is a top view illustration of the main portion shown in  FIG. 31 . 
         FIG. 33  is a side view illustration of the main portion shown in  FIG. 31 . 
         FIG. 34  is a cross-section view of the main portion shown in  FIG. 31  taken through the section line A-A shown in  FIG. 32 . 
         FIG. 35  is a perspective view of the hydrogel implant of  FIG. 27  secured to a bone in its fully implanted configuration. 
         FIG. 36  is a cross-section view of the hydrogel implant shown in  FIG. 35  in which the bone plate portion has not yet been seated in its fully implanted configuration. 
         FIG. 37  is a cross-section view of the hydrogel implant shown in  FIG. 35  in which the bone plate portion is seated in its fully implanted configuration. 
         FIG. 38  is a perspective view illustration of a hydrogel implant according to another embodiment of the present disclosure. 
         FIG. 39  is an exploded view illustration of the hydrogel implant of  FIG. 38 . 
         FIG. 40  is a side view illustration of the hydrogel implant of  FIG. 38 . 
         FIG. 41  is a cross-section view illustration of the hydrogel implant of  FIG. 38  taken through the section line V-V shown in  FIG. 40 . 
         FIG. 42  is a top view illustration of the hydrogel implant of  FIG. 38 . 
         FIG. 43  is a cross-section view illustration of the hydrogel implant taken through the section line AF-AF shown in  FIG. 42 . 
         FIG. 44  is a detailed view of the region AG identified in the sectional view of  FIG. 43 . 
     
    
    
     DETAILED DESCRIPTION 
     This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. The drawing figures are not necessarily to scale and certain features may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness. In the description, relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. When only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship. In the claims, means-plus-function clauses, if used, are intended to cover the structures described, suggested, or rendered obvious by the written description or drawings for performing the recited function, including not only structural equivalents but also equivalent structures. 
     According to an embodiment illustrated in  FIGS. 1-12 , an implant  100  for replacing a portion of an articulation surface of a joint is disclosed. As shown in  FIG. 1 , the implant comprises a main portion  110  configured for inserting into a joint and a bone plate portion  120  extending from the main portion  110  at an angle and configured for securing the implant  100  to a bone that forms the joint. As shown in the exploded view of  FIG. 2 , the main portion  110  comprises a porous material portion  115  having a first bone-engaging surface  130 , and a hydrogel portion  112  that is bonded to the porous material portion  115 . 
     Referring to  FIGS. 2 and 3A , the hydrogel portion  112  forms an articulation surface  114  located opposite from the first bone-engaging surface  130 . In other words the articulation surface  114  and the first bone-engaging surface  130  face away from each other. The bone plate portion  120  comprises a solid metal portion  122  that forms all exterior surfaces of the bone plate portion  120  except for the second bone-engaging surface  140 . The second bone-engaging surface  140  of the bone plate portion  120  is formed of the same porous material as the porous material portion  115  and is preferably integrally formed with the porous material portion  115  as a unitary structure for ease of manufacturing and producing a more compact structure. 
     The bone plate portion  120  comprises at least one screw hole  150  for receiving a bone screw that is used to secure the implant  100  to a bone. There can be more than one screw holes provided in the bone plate portion  120  for implanting into a joint repair site that may require more than one bone screw to secure the implant. 
     The main portion  110  of the implant  100  has a leading end  111  and a trailing end  113 , where the leading end is configured for being inserted into the joint. Here, the terms “leading” and “trailing” references generally the implant&#39;s orientation in its implanted position in a joint space and also the orientation as the implant is being inserted into the joint space. 
     The bone plate portion  120  is integrally formed with the porous material portion  115  and extends from the trailing end, forming a second bone-engaging surface  140 . Because the extension piece  117  is formed of the same porous material as the porous material portion  115 , the second bone-engaging surface  140  also promotes the cancellous bone&#39;s growth into the second bone-engaging surface  140  and enhance the implant&#39;s stability in the repair site. 
     As shown by the dashed lines in the side view of the implant  100  in  FIG. 3A , the porous material portion  115  has an extension piece  117  that extends from the first bone-engaging surface  130  in a direction opposite from the articulation surface  114  at an angle θ with respect to the first bone-engaging surface  130 . The angle θ between the first and second bone-engaging surfaces  130 ,  140  is selected to enable secure attachment of the implant to the bone. In some embodiments, that angle can be substantially 90°. This means that the angle can be 90°±2°. In some embodiments, the angle is an obtuse angle. In some embodiments, the obtuse angle is ≥110° and ≤160°. In some embodiments, the obtuse angle is ≥130° and ≤140°. 
     The extension piece  117  is provided to form the second bone-engaging surface  140 . The porous material portion  115  and the extension piece  117  together provide a skeletal base structure on which the hydrogel portion  112  is applied and bonded thereto. This skeletal structure is shown in  FIGS. 5 and 6 . The solid metal portion  122  fills the space  118  between the extension piece  117  and the porous material portion  115 . In some embodiments, the solid metal portion  122  can be integrally formed with the porous material portion  115  and the extension piece  117 .  FIGS. 7-9  show the porous material structures  115 ,  117  and the solid metal portion  122  together. 
     In  FIGS. 5 and 6 , because only porous material structures  115  and  117  are shown, without the solid metal portion  122 , the hole  150 A in the extension piece  117  is larger than the bone screw hole  150  which is the final dimension screw hole that is formed by the solid metal portion  122  that overlays on the extension piece  117 . 
     In a preferred embodiment, the porous material structures  115 ,  117  and the solid metal portion  122  are formed as a unitary structure. For example, the porous material structures and the solid metal portion  122  can be 3-D printed and sintered to form a unitary structure. 
     In some embodiments, the bone plate portion  120  and the porous material portions  115  and are formed of surgical grade metal. In a preferred embodiment, the surgical grade metal used is titanium. In more preferred embodiment, the solid metal portion  122  is formed of titanium metal and the porous material portion  115  and the extension piece  117  are made of porous titanium metal foam. 
     The hydrogel portion  112  is bonded to the porous material portion by having some hydrogel material infiltrate into pores of the porous material portion. In preferred embodiments where the porous material is porous titanium metal foam, the hydrogel material infiltrate into pores of the porous titanium metal foam. 
     The porous material may comprise an oxide material. The porous material can comprise at least one of surgical grade materials such as aluminum, alumina, zirconia, titanium, titania, stainless steel, PEEK, and steatite that are approved for implantation in humans. The porous material can have a porosity between 45 ppi and 80 ppi. Pores of the porous material can have a dimension between 100 μm and 500 μm. The porous material can be ceramic, metal, or plastic. In some embodiments, the porous material comprises porous ceramic material (e.g., oxide-ceramic), metal (e.g., titanium (e.g., titanium mesh, printed titanium), stainless steel (e.g., stainless steel wool), plastic (e.g., polyaryl ether ketone (PAEK) (e.g., polyether ether ketone (PEEK)), other biocompatible materials, combinations thereof, etc.) In some preferred embodiments, the porous material is porous metal foam material that has open-celled three-dimensional scaffold structure for bone and tissue growth. 
     In more preferred embodiments, the porous metal foam material is porous titanium foam. An example of such porous titanium foam material is Wright Medical Technology&#39;s BIOFOAM® Cancellous Titanium™ technology. The titanium matrix of BIOFOAM® Cancellous Titanium™ technology has fully interconnected porosity of up to 70% providing an ideal environment for optimum bone ingrowth and incorporation. The titanium matrix of BIOFOAM® Cancellous Titanium™ technology has: compressive strength that is between that of cortical and cancellous bone, thus minimizing deformation under dynamic loading conditions; compressive modulus that is close to that of cancellous bone, allowing the natural transfer of dynamic loads away from the implant to the surrounding bone; and high surface coefficient of friction that provides initial stability in the interface between the implant and the bone, minimizing micro motion and creating a stable environment for rapid ingrowth and fixation. Examples of alternative materials for the porous metal foam is titanium dioxide foam and porous tantalum foam. 
     Referring to  FIG. 3B , when the implant  100  is implanted in a patient to repair or replace a portion of an articulation surface (e.g., articular cartilage) in a joint, the damaged articulation surface and the adjacent bone region would be prepared to receive the implant  100 . The prepared site would have resected bone surfaces B 1  and B 2  corresponding to the first bone-engaging surface  130  and the second bone-engaging surface  140  of the implant  100 . All or much of the resected bone surfaces B 1  and B 2  would generally be comprised of cancellous bone and because the first and second bone-engaging surfaces  130 ,  140  are formed of porous metal foam material that has a mesh-like structure with many pores mimicking the cancellous bone structure, the cancellous bone grows into the porous metal foam structure and further enhances the securement of the implant  100  in the repair site. 
     The hydrogel portion  112  can be formed by applying the hydrogel material in a liquid form on the porous material structure  115  in a mold and then allowing the hydrogel material to cross-link by conducting the appropriate processes that are appropriate for the particular type of hydrogel material that is selected for a given application for the implant. 
     In some embodiments of the implant  100 , the bond between the hydrogel portion and the porous material portion is enhanced by having some hydrogel material infiltrating into the pores in a portion of the porous material along the surface that comes in contact with the hydrogel material. Thus, in a region in the porous material structure  115  along the hydrogel portion  112 , both the hydrogel material and the porous material co-exist while in the remainder of the porous material structure  115  toward the bone-engaging surface  130 , only the porous material exists without any hydrogel material. That allows the bone-engaging surface  130  to present pores that enable cancellous bone ingrowth. 
     The hydrogel material referred to herein refers to a three-dimensional solid resulting from cross-linked hydrophilic polymer chains formed of polyvinyl alcohol (PVA). The hydrogel material can comprise one or more other materials in addition to PVA, such as, for example, other hydrogels, other polymeric materials, additives, and/or the like. In some embodiments, the PVA content of the hydrogel in the implants disclosed herein can be about 40% by weight. The PVA content of the hydrogel can range from about 10% by weight to about 80% by weight, as appropriate for particular application. 
     The hydrogel can comprise water, saline, other liquids, combinations thereof, and/or the like. In some embodiments, saline may be preferred over water, because, under certain circumstances, saline can help maintain osmotic balance with surrounding anatomical tissues following implantation. The exact composition of the hydrogel component in an implant can be selected for optimal performance in a particular application to achieve the desired or required strength, load bearing capacity, compressibility, flexibility, longevity, durability, resilience, coefficient of friction, and/or other properties and characteristics. 
     In some embodiments, such hydrogel portions of the implants can be formulated for drug delivery and/or is seeded with growth factors and/or cells. In such embodiments, the hydrogel component can comprise one or more of the following: chondrocytes, growth factors, bone morphogenetic proteins, collagen, hyaluronic acid, nucleic acids, and stem cells. Such factors and/or any other materials included in the implants can help facilitate and/or promote long-term fixation of the implants at the joint site. 
       FIG. 10  shows a top view illustration of the hydrogel implant  100 .  FIG. 11  is a cross-section view illustration of the hydrogel implant  100  taken through the section line M-M shown in  FIG. 10 .  FIG. 12  is a detailed view of the region N identified in the sectional view of  FIG. 11 . 
     Referring to  FIGS. 38-41 , an implant  100 A for replacing a portion of an articulation surface of a joint according to another embodiment is disclosed. As shown in  FIG. 38 , the implant comprises a main portion  110 A configured for inserting into a joint and a bone plate portion  120 A extending from the main portion  110 A at an angle and configured for securing the implant  100 A to a bone that forms the joint. As shown in the exploded view of  FIG. 39 , the implant  100 A comprises four different components that are bonded together in the following order: a first porous material portion  117 A, a solid metal portion  116 A, a second porous material portion  115 A, and a hydrogel portion  112 A. 
     Referring to  FIGS. 39 and 40 , which is a side view of the implant  100 A, the first porous material portion  117 A has a first bone-engaging surface  130 A, and a second bone-engaging surface  140 A. The top portion  121 A of the first porous material portion  117 A form the second bone-engaging surface  140 A and the remaining portion of the first porous material portion  117 A form the first bone-engaging surface  130 A. On the side opposite of the two bone-engaging surfaces  130 A,  140 A, the solid metal portion  116 A is bonded to the first porous material portion  117 A. 
     The solid metal portion  116 A comprises a top portion  122 A, which together with the top portion  121 A of the first porous material portion  117 A form the bone plate portion  120 A of the implant  100 A. Similar to the implant  100  described above, the bone plate portion  120 A of the implant  100 A also comprises at least one screw hole for receiving a bone screw that is used to secure the implant  100 A to a bone. In the illustrated example shown, two screw holes  150 A and  151 A are provided in the bone plate portion  120 A for implanting into a joint repair site that may require more than one bone screw to secure the implant. The top portion  121 A of the first porous material portion  117 A comprises holes  150 A′ and  151 A′ that correspond to the two screw holes  150 A and  151 A. 
     The top portion  122 A of the solid metal portion  116 A forms the exterior surface of the bone plate portion  120 A while the second bone-engaging surface  140 A is formed by the first porous material portion  117 A. 
     The second porous material portion  115 A is positioned between and bonded to both the solid metal portion  116 A and the hydrogel portion  112 A. The hydrogel portion  112 A forms an articulation surface  114 A located opposite from the first bone-engaging surface  130 A. In other words, the articulation surface  114 A and the first bone-engaging surface  130 A face away from each other. 
     The main portion  110 A of the implant  100 A has a leading end  111 A and a trailing end  113 A, where the leading end  111 A is configured for being inserted into the joint. Here, the terms “leading” and “trailing” references generally the implant&#39;s orientation in its implanted position in a joint space and also the orientation as the implant is being inserted into the joint space. 
     Both the first and second porous material portions  117 A and  115 A are preferably made of the same porous material. The first porous material portion  117 A which forms the first and second bone-engaging surfaces,  130 A and  140 A, respectively, promotes the cancellous bone&#39;s growth into the bone-engaging surfaces  130 A,  140 A and enhance the implant&#39;s stability in the repair site. 
     As shown in  FIG. 40 , the first bone-engaging surface  130 A is substantially parallel to the articulation surface  114 A of the hydrogel portion  112 A. The second bone-engaging surface  140 A of the bone plane portion  120 A and the first bone-engaging surface  130 A form an angle θ with respect to the first bone-engaging surface  130 . The angle θ between the first and second bone-engaging surfaces  130 A,  140 A is selected to enable secure attachment of the implant to the bone. In some embodiments, that angle can be substantially 90°. This means that the angle can be 90°±2°. In some embodiments, the angle is an obtuse angle. In some embodiments, the obtuse angle is ≥110° and ≤160°. In some embodiments, the obtuse angle is ≥130° and ≤140°. 
     The first porous material portion  117 A, the solid metal portion  116 A, and the second porous material portion  115 A together provide a skeletal base structure on which the hydrogel portion  112 A is applied and bonded thereto. In some embodiments, the solid metal portion  116 A can be integrally formed with the first and second porous material portions  117 A and  115 A as a unitary structure. For example, the porous material structures and the solid metal portion  116 A can be 3-D printed and sintered to form a unitary structure. 
     As in the implant embodiment  100 , the solid metal portion  116 A and the porous material portions  117 A,  115 A can be formed of surgical grade metal such as titanium and/or titanium alloys. 
     The hydrogel portion  112 A is bonded to the second porous material portion  115 A by having some hydrogel material infiltrate into pores of the porous material portion. In preferred embodiments where the porous material is porous titanium metal foam, the hydrogel material infiltrate into pores of the porous titanium metal foam. The porous material may comprise of the materials described above in connection with the implant  100 . 
     When implanted in a patient, the implant  100 A&#39;s arrangement will be similar to the example for implant  100  shown in  FIG. 3B . 
     The hydrogel portion  112 A can be formed by applying the hydrogel material in a liquid form on the porous material structure  115 A in a mold and then allowing the hydrogel material to cross-link by conducting the appropriate processes that are appropriate for the particular type of hydrogel material that is selected for a given application for the implant. 
     In some embodiments of the implant, the bond between the hydrogel portion and the porous material portion is enhanced by having some hydrogel material infiltrating into the pores in a portion of the porous material along the surface that comes in contact with the hydrogel material. Thus, in a region in the porous material structure  115 A along the hydrogel portion  112 A, both the hydrogel material and the porous material co-exist while in the remainder of the porous material structure  115 A toward the bone-engaging surface  130 A, only the porous material exists without any hydrogel material. That allows the bone-engaging surface  130 A to present pores that enable cancellous bone ingrowth. 
       FIG. 42  is a top view illustration of the hydrogel implant  100 A.  FIG. 43  is a cross-section view illustration of the hydrogel implant  100 A taken through the section line AF-AF shown in  FIG. 42 .  FIG. 44  is a detailed view of the region AG identified in the sectional view of  FIG. 43 . 
     Referring to  FIGS. 13-19 , an implant  200  for replacing a portion of an articulation surface of a joint according to another embodiment is disclosed. The implant  200  comprises a main portion  210  configured for inserting into a joint. The main portion  210  can comprise a hydrogel portion  212  forming a bone-contacting surface  230  and an articulation surface  214  opposite from the bone-contacting surface  230 . The main portion  210  has a leading end  211  and a trailing end  213 , wherein the leading end is configured for being inserted into the joint. A bone plate portion  220  configured for securing the implant  200  to a bone that forms the joint. The bone plate portion  220  comprises a first part  223  having a perforated structure that is embedded in the hydrogel portion  212 ; and a second part  225  that is not embedded in the hydrogel portion and extending from the trailing end  213  in a direction opposite from the articulation surface  214  at an angle ≤160° but ≥80° with respect to the bone-contacting surface  230 . The second part  225  has at least one screw hole  250  for receiving a bone screw (not shown). The second part  225  can have generally circular configuration around the screw hole  250  as shown in  FIG. 15 , but the shape of the second part  225  can be designed to have any appropriate shape to fit into the structure (e.g. contour) of the bones around the particular joint space into which the implant  200  will be implanted. 
     In some embodiments of the implant  200 , the second part  225  extends from the trailing end  213  at an angle that is ≤110° and ≥80°. In some embodiments of the implant  200 , the second part  225  extends from the trailing end  213  at an angle that is substantially 90° (i.e., 90±2°). In some embodiments of the implant  200 , the first part  223  of the bone plate portion  220  is embedded in the hydrogel portion  212  and located closer to the bone-contacting surface  230  than the articulation surface  214 . In some embodiments of the implant  200 , the bone-contacting surface  230  is a flat surface. When the bone-contacting surface  230  is a flat surface, the first part  223  of the bone plate portion  220  has substantially flat configuration as shown in  FIGS. 13 and 14  to correspond to the flat contour of the bone-contacting surface  230 . 
     The implant  200  can be formed by molding the hydrogel material around the first part  223  of the bone plate portion  220  using injection molding or open cavity molding processes known to those in the art. As shown in  FIGS. 14 and 19 , the first part  223  of the bone plate portion  220 , the part that gets embedded in the hydrogel portion  212 , can be perforated with holes  227  to better enable the hydrogel material to intimately surround and envelope the first part  223  during the molding process so that the resulting implant  200  has the optimal structural integrity. 
     The bone plate portion  220  is made of a surgical grade metal, such as stainless steel, cobalt based superalloys, titanium, titanium alloys, etc. In some embodiments, the surgical grade metal is titanium. 
     Referring to  FIGS. 20-24 , an implant  300  according to another embodiment is disclosed. The implant  300  is similar to the implant  200  just described with one of the differences being the provision of a protruding part  316  on the bone-contacting surface  330 . 
     The implant  300  for replacing a portion of an articulation surface of a joint comprises a main portion  310  configured for inserting into a joint. The main portion  310  comprises a hydrogel portion  312  forming a bone-contacting surface  330  and an articulation surface  314  opposite from the bone-contacting surface  330 . The bone-contacting surface  330  comprises the protruding part  316  that provides additional structural stability at the interface between the bone and the bone-contacting surface  330  when the implant  300  is implanted in position in a joint space. Preferably, the bone surface that is receiving the implant  300  would be prepared to have a contour that is complementary to the contour of the bone-contacting surface  330  that includes the protruding part  316 . 
     Similar to the implant  200 , the main portion  310  of the implant  300  comprises a leading end  311  and a trailing end  313 , where the leading end  311  is configured for being inserted into the joint. The implant  300  further comprises a bone plate portion  320  configured for securing the implant  300  to a bone that forms the joint. The bone plate portion  320  comprises a first part  323  having a perforated structure that is embedded in the protruding part  316  of the hydrogel portion  312 , and a second part  325  that is not embedded in the protruding part of the hydrogel portion. The second part  325  extends from the trailing end  313  in a direction opposite from the articulation surface  314  at an angle ≤160° but ≥80° with respect to the base flat portion of the bone-contacting surface  330  (i.e., the part of the bone-contacting surface  330  excluding the protruding part  316 . The second part has at least one screw hole  350  for receiving a bone screw (not shown). Similar to the implant  200 , the second part  325  can have generally circular configuration around the screw hole  350  as shown in  FIG. 21 , but the shape of the second part  325  can be designed to have any appropriate shape to fit into the structure (e.g. contour) of the bones around the particular joint space into which the implant  300  will be implanted. 
     In some embodiments of the implant  300 , the second part  325  extends from the trailing end  313  at an angle that is ≤110° and ≥80°. In some embodiments of the implant  300 , the second part  325  extends from the trailing end  313  at an angle that is substantially 90° (i.e., 90±2°). In some embodiments, the first part  323  of the bone plate portion is embedded in the hydrogel portion and located closer to the bone-contacting surface  330  than the articulation surface  314 . Preferably, the first part  323  of the bone plate portion  320  has a contour that substantially matches the contour of the protruding part  316  of the hydrogel portion  312 . 
     In some embodiments, the protruding part  316  of the bone-contacting surface  330  has a half-cylinder contour and the first part  323  of the bone plate portion has a complementary curved contour. In some embodiments of the implant  300 , the bone plate portion  320  is made of a surgical grade metal, such as stainless steel, cobalt based superalloys, titanium, titanium alloys, etc. In some embodiments, the surgical grade metal is titanium. 
     Referring to  FIGS. 25 and 26 , an example of a molding process for forming the implants  200  and  300  is disclosed. A mold  500  having a plurality of mold cavities  510  is provided. Each of the mold cavity  510  is configured with the outline shape of either the implant  200  or the implant  300 . A bone plate portion  220  or  320  is first placed in each of the mold cavity  510 . Then, a nozzle  550  for dispensing the hydrogel material is positioned into the mold cavity  510  as shown in  FIG. 26 . Next, the hydrogel material, represented by the arrow  600 , is dispensed into each of the mold cavity  510 . Next, with each of the mold cavities  510  holding a bone plate portion  220  and filled with the hydrogel material, an appropriate post processing is carried out for cross-linking the hydrogel material to form the finished implant product  200 . This process is equally applicable to the implant  300 . The specifics of this post processing would be determined by the particular hydrogel material being used but would be well known to those in the art for the particular formulation of hydrogel. 
     Referring to  FIGS. 27-37 , another embodiment of an implant  400  for replacing a portion of an articulation surface of a joint is disclosed. The implant  400  comprises a main portion  410  configured for inserting into a joint and a bone plate  420  configured for securing the implant  400  to a bone that forms a joint. 
     Referring to  FIG. 30 , the main portion  410  comprises a leading end  411 , a trailing end  413 , an articulation surface  414  and a bone-contacting surface  430  extending between the leading end and the trailing end. The leading end  411  is configured for being inserted into the joint. The main portion  410  further comprises a porous material portion  415  and a hydrogel portion  412  forming the articulation surface  414  and the bone-contacting surface  430  opposite from the articulation surface  414 . The porous material portion  415  is bonded to the hydrogel portion  412  and the porous material portion  415  extends partially from the trailing end  413  towards the leading end  411  and forms a portion of the bone-contacting surface  430 . 
     Referring to  FIG. 30  and the cross-sectional view in  FIG. 34 , the porous material portion  415  comprises a tapered hole  460  at the trailing end  413 . 
     Referring to the cross-sectional views of  FIGS. 36 and 37 , in some embodiments, the bone plate  420  is formed of a solid metal and comprises a tapered stem  427  that is configured to be inserted into the tapered hole  460  in the porous material portion  415 . The tapered stem  427  and the tapered hole  460  cooperate to urge the bone-contacting surface  430  of the implant  400  toward the bone when the implant  400  is inserted into the joint. Referring to  FIGS. 27, 35-36 , the bone plate  420  comprises at least one screw hole  450  for receiving a bone screw S. 
     The porous material for the porous material portion  415  can be the same material as the porous material portion  115  of the implant  100  discussed above. 
     In some embodiments, the hydrogel portion  412  is bonded to the porous material portion  415  by having some hydrogel material infiltrating into pores in a portion of the porous material portion. The main portion  410  comprising the hydrogel portion  412  and the porous material portion  415  can be formed by an appropriate process such as an injection molding or open cavity molding process as described above in connection with the implant embodiment  100 . 
       FIGS. 35-37  are illustrated with the bone of a joint in which the implant  400  is secured. A portion of the bone immediately surrounding the implant  400  is illustrated conceptually as a box-like volume Bone for illustration purposes.  FIGS. 36 and 37  show how the tapered stem  427  of the bone plate  420  and the tapered hole  460  of the porous material portion  415  engage each other and cooperate to urge the bone-contacting surface  430  of the implant  400  toward the bone when the implant  400  is inserted into the joint.  FIG. 36  shows the implant  400  positioned in place in the Bone. The bone-contacting surfaces  430  are contacting the prepared bone surface B 3 . The tapered stem  427  of the bone plate  420  is partially inserted into the mating tapered hole  460  in the porous material portion  415  and a bone screw S is placed through the screw hole  450  in the bone plate  420  and starting to engage the pre-drilled hole in the Bone.  FIG. 37  shows the implant  400  where the bone screw S is fully screwed into the Bone and has secured the bone plate  420  to the Bone. With the bone plate  420  in its fully-seated position, the tapered stem  427  is fully inserted into the tapered hole  460 . The tapered surface of the tapered stem  427  pushes against the sidewall of the tapered hole  460  as the tapered stem  427  reaches its fully-seated position and securely holds the main portion  410  of the implant  400  in place. 
     In  FIGS. 28 and 29 , two examples of the implant  400  are shown implanted in joint spaces between a metatarsal bone and a cuneiform bone. In  FIG. 29 , a third implant  400  is shown implanted in a subtalar joint space between the talus and the calcaneus. 
     Although the devices, kits, systems, and methods have been described in terms of exemplary embodiments, they are not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the devices, kits, systems, and methods, which may be made by those skilled in the art without departing from the scope and range of equivalents of the devices, kits, systems, and methods.