Abstract:
The present invention relates to a stabilized ankle prosthesis configured for use in patients with compromised soft tissue in the ankle. The prosthesis of the present invention is a two-component design comprising a stabilizing lip configured to constrain movement in the general direction of compromised soft tissue.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    An ankle joint may become severely damaged and painful due to arthritis from prior ankle surgery, bone fracture, infection, osteoarthritis, posttraumatic osteoarthritis or rheumatoid arthritis, for example. Options for treating the injured ankle have included anti-inflammatory and pain medications, braces, physical therapy, amputation, joint arthrodesis, and total ankle replacement. 
         [0002]    In the past the main-stay of ankle arthrosis has been joint arthrodesis, due to the poor prosthetic survival rate of total ankle replacements. This is primarily due to the clinical results of early ankle designs. Therefore, arthrodesis has been the only choice for many surgeons and patients. Arthrodesis improves stability and reduces pain, but also severely inhibits normal function of the ankle joint. Although some patients have very good results from ankle fusion, surrounding joints above and below the fusion may become arthritic and painful because the lack of ambulation places additional stress on these joints. 
         [0003]    There have been numerous ankle joint replacement prostheses developed over the last 30 years. The Agility ankle is an example of an early implant design. It is comprised of two components—one part is cemented to the tibia and the other part is cemented to the talus. An issue with the design of the early ankle prostheses is that although they allow for some dorsiflexion/plantarflexion motion, the articulation surfaces restrict varus/valgus and rotation motions. Another problem surrounding early implant designs is their reliance on the surrounding soft tissues of the ankle to stabilize the implant. As a result, they are not well suited for implantation in individuals with compromised soft tissues. 
         [0004]    Another example is the Salto Talaris ankle device, which is a fixed-bearing ankle prosthesis. This two-component ankle system utilizes a conical talar component with two different radii of curvature and a curved groove in the sagittal plane. The medial radius is smaller than the lateral to allow equal tensioning of the collateral ligaments. The tibial component is designed for a fixed insertion of a polyethylene bearing piece that is replaceable. Some issues with the fixed-bearing prostheses include high wear rate of the articulation surfaces, ambulatory constraint, and loosening of the implant. 
         [0005]    Another ankle replacement device is the Scandanavian Total Ankle Replacement (STAR). In this device the tibial component is designed for less bone resection and has two parallel bars for insertion into the subchondral bone. The talar component is meant to mimic the talar dome and has a central ridge for stabilization of a polyethylene piece. The STAR prosthesis inhibits inversion/eversion coupling with plantarflexion/dorsiflexion motion. This leads to straining and potential damage to the deltoid ligaments on the medial side of the ankle. Another issue with this device is edge loading, which puts a great amount of stress on the ridge of the implant and results in the implant retracting from the talus. 
         [0006]    More modern designs have attempted to increase the range of motion while maintaining the integrity of the surrounding soft tissues of the ankle. For example, U.S. Pat. No. 7,625,409 discloses a prosthesis designed to allow full range of motion while minimizing edge loading and subsidence. However, this prosthesis fails to address the need for an implant for use in patients with compromised soft tissues in the ankle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows an anterior view of an ankle prosthesis according to an embodiment of the present invention; 
           [0008]      FIG. 2  shows an anterior view of an ankle joint and surrounding soft tissues; 
           [0009]      FIG. 3  shows an anterior view of exemplary means of attachment of an ankle prosthesis according to an embodiment of the present invention; and 
           [0010]      FIG. 4  shows a posterior view of an ankle prosthesis according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Although detailed embodiments of the present invention are disclosed herein, it is to be understood that the invention is not restricted to the details of the embodiments. Many changes in design, composition, configuration and dimensions are possible without departing from the spirit and scope of the instant invention. Further, the figures are not necessarily to scale. Some features may be exaggerated to show details of particular components. Specific structural and functional details disclosed herein are not to be interpreted as limiting, but as an aid for teaching one skilled in the art how to variously employ the present invention. Accordingly, it should be readily understood that the embodiments described and illustrated herein are illustrative only, and are not to be considered as limitations upon the scope of the present invention. 
         [0012]    An ankle joint is a very complex joint having three motions that occur simultaneously: dorsiflexsion/plantarflexion, varus/valgus, and internal/external rotation. In a healthy ankle, the ankle joint relies on soft tissues, including ligaments, to provide stability. These tissues include, for example, an anterior inferior tibiofibular ligament, a calcaneal-fibular ligament, a posterior talo-fibular ligament, a syndesmotic ligament, an anterior capsule of the ankle joint (which helps keep the ankle from anterior movement), and a deltoid ligament. These soft tissues contribute to the overall function of the ankle by ensuring joint stability. 
         [0013]    In a healthy ankle these ligaments naturally stretch according to specific ankle motions in order to keep the joint secure. It is desirable that an ankle replacement prosthesis prevents stretching these ligaments beyond their natural range of motion. Further, because the ankle joint absorbs a stress greater than four times the body&#39;s weight with every step, an ankle replacement prosthesis ideally will be able to withstand the pressures associated with weight-bearing and motion. 
         [0014]    In a patient with healthy soft tissue in the ankle, this tissue will provide stability to a prosthesis. However, when the soft tissue is compromised, a prosthesis can fail for various reasons, including instability. Therefore, there exists a need for an implant that provides ambulation of the ankle joint similar to that of a natural ankle and that remains stable when implanted in a patient with compromised soft tissues. 
         [0015]    An ankle prosthesis  1  of the present invention addresses these and additional problems. Ankle prosthesis  1  of the present invention provides adequate range of motion for the primary degrees of freedom of the talar joint, including movement in a frontal plane and a sagittal plane. Additionally, ankle prosthesis  1  of the present invention provides stability for implantation into an ankle joint with compromised soft tissues by constraining movement in the general direction of compromised soft tissues. 
         [0016]    As may be seen with reference to  FIG. 2 , the natural anatomy of a talus bone  3  has a bicondylar contour  5 . The ankle prosthesis of the present invention comprises a talar component and a tibial component. In one embodiment, the talar component mimics the natural anatomy of the talus bone and the tibial component comprises a complimentary contour. In another embodiment depicted in  FIG. 1 , the natural anatomy is mimicked in a reverse orientation, where a bicondylar contour is on a tibial component  7  and a talar component  9  comprises a complimentary contour. 
         [0017]    In one embodiment of the present invention, ankle prosthesis  1  comprises a two-component design which is sufficiently sized to prevent subsidence. With reference to  FIG. 1 , ankle prosthesis  1  comprises tibial component  7  and talar component  9 . Tibial component  7  is configured for attachment to a tibia  11 . As may be seen in  FIG. 3 , tibial component  7  comprises an attachment surface  13  positioned on a proximal portion of tibial component  7  and an articulation surface  15  positioned on a distal portion of tibial component  7 , wherein at least a portion of attachment surface  13  is configured for attachment to tibia  11 . The attachment surface and the articulation surface of tibial component  7  may comprise a unitary piece, or in an alternative embodiment the surfaces may be two distinct units attached to one another by any suitable means of attachment. 
         [0018]    Tibial component  7  may be attached to tibia  11  by any suitable means of attachment, e.g., one or more screws or one or more rods. An exemplary means of attachment  17  may be seen with reference to  FIG. 3 . In one embodiment, tibial component  7  functions as the “male” component of ankle prosthesis  1 . For example, referring to  FIGS. 1 and 3 , articulation surface  15  of tibial component  7  comprises at least one convex contour  19  extending anteriorly-posteriorly on a medial portion of tibial component  7 , at least one convex contour  21  extending anteriorly-posteriorly on a lateral portion of tibial component  7 , and at least one concave contour  23  extending anteriorly-posteriorly in a sagittal plane of tibial component  7 . In this embodiment, articulation surface  15  comprises a bicondylar contour which mimics the natural anatomy of the proximal portion of talus bone  3 . Radii of curvature of the at least one convex contour  19  on the medial portion and the at least one convex contour  21  on the lateral portion of articulation surface  15  of tibial component  13  may be the same, or in an alternative embodiment a radius of curvature of the medial portion may be greater, or in another embodiment a radius of curvature of the lateral portion may be greater. 
         [0019]    In an alternative embodiment, tibial component  7  functions as the “female” component of ankle prosthesis  1 . In this embodiment articulation surface  15  of tibial component  7  comprises at least one concave contour extending anteriorly-posteriorly on a medial portion of tibial component  7 , at least one concave contour extending anteriorly-posteriorly on a lateral portion of tibial component  7 , and at least one convex contour extending anteriorly-posteriorly in a sagittal plane of tibial component  7 . In this embodiment, articulation surface  15  comprises a contour which is complimentary to a bicondylar contour. Radii of curvature of the at least one concave contour on the medial portion and the at least one concave contour on the lateral portion of articulation surface  15  of tibial component  13  may be the same, or in an alternative embodiment a radius of curvature of the medial portion may be greater, or in another embodiment a radius of curvature of the lateral portion may be greater. 
         [0020]    Complimentary talar component  9  is configured for attachment to talus  3 . Talar component  9  comprises an attachment surface  25  on a distal portion of the component and an articulation surface  27  on a proximal side of the component. At least a portion of attachment surface  25  is configured for attachment to talus  3 . The attachment surface and the articulation surface of talar component  9  may comprise a unitary piece, or in an alternative embodiment the surfaces may be two distinct units attached to one another by any suitable means of attachment. 
         [0021]    Talar component  9  may be attached to talus  3  by any suitable means of attachment, e.g., one or more screws or one or more rods. An exemplary means of attachment  29  may be seen with reference to  FIG. 3 . Articulation surface  27  of talar component  9  is configured to compliment articulation surface  15  of tibial component  7 , and accordingly may comprise the “female” or “male” component of ankle prosthesis  1  depending on the configuration of tibial component  7 . Articulation surface  27  of talar component  9  comprises a contour for receiving articulation surface  15  of tibial component  7 . In one embodiment of the present invention in which talar component  9  comprises the “female” component of ankle prosthesis  1 , articulation surface  27  of talar component  9  has at least one concave contour  31  extending anteriorly-posteriorly on a medial portion of talar component  9 , at least one concave contour  33  extending anteriorly-posteriorly on a lateral portion of talar component  9 , and at least one convex contour  35  extending anteriorly-posteriorly on a sagittal plane of talar component  9 . Convex contour  35  is configured to compliment the at least one concave contour  23  on the sagittal plane of articulation surface  15  of tibial component  7 . At least one concave contour  31  on medial portion of articulation surface  27  of talar component  9  is configured to compliment the at least one convex contour  19  on the medial portion of articulation surface  15  of tibial component  7  and the at least one concave contour  33  on the lateral portion of articulation surface  27  of talar component  9  is configured to compliment the at least one convex contour  21  on the lateral portion of articulation surface  15  of tibial component  7 . Radii of curvature of the at least one concave contour  31  on the medial portion of talar component  9  and the at least one concave contour  33  on the lateral portion of talar component  9  may be the same, or in an alternative embodiment a radius of curvature on the medial portion may be greater, or in another embodiment a radius of curvature of the lateral portion may be greater. 
         [0022]    In an alternative embodiment, talar component  9  comprises the “male” component of ankle prosthesis  1 . In this embodiment, articulation surface  27  of talar component  9  has at least one convex contour extending anteriorly-posteriorly on a medial portion of talar component  9 , at least one convex contour extending anteriorly-posteriorly on a lateral portion of talar component  9 , and at least one concave contour extending anteriorly-posteriorly on a sagittal plane of talar component  9 . Concave contour is configured to compliment the at least one convex contour on the sagittal plane of articulation surface  15  of tibial component  7 . At least one convex contour on medial portion of articulation surface  27  of talar component  9  is configured to compliment the at least one concave contour on the medial portion of articulation surface  15  of tibial component  7 , and the at least one convex contour on the lateral portion of articulation surface  27  of talar component  9  is configured to compliment the at least one concave contour on the lateral portion of articulation surface  15  of tibial component  7 . Radii of curvature of the at least one convex contour on the medial portion of talar component  9  and the at least one convex contour  33  on the lateral portion of talar component  9  may be the same, or in an alternative embodiment a radius of curvature on the medial portion may be greater, or in another embodiment a radius of curvature of the lateral portion may be greater. 
         [0023]    Articulation surface  15  of tibial component  7  and articulation surface  27  of talar component  9  form an articulation interface  37 . Congruence of the articulation surfaces is maintained in all positions of an ankle joint movement, including dorsiflexion/plantarflexion, inversion/eversion, and internal/external rotation. 
         [0024]    In order to maintain congruence of the articulation surfaces of prosthesis  1  when implanted in an ankle with compromised soft tissue, one embodiment of the present invention comprises a lip on the female component. The lip is configured to maintain congruence of the articulation surfaces in patients having compromised soft tissues by at least partially limiting mobility in a direction toward the compromised soft tissue. Congruence of the articulation surfaces may be maintained when the articulation surfaces correspond to one another and are in agreement for each direction of motion. 
         [0025]    In one embodiment of the present invention, a lip  39  is configured to stabilize prosthesis  1  when implanted in patients with compromised soft tissues on a medial side of the ankle. For example, lip  39  may be configured for implantation in a patient with a compromised deltoid ligament  41  (depicted in  FIG. 2  with talar component  9  being the “female” component). In this embodiment, lip  39  comprises a raised surface on a medial edge  43  of articulation surface  27  of talar component  9  such that the raised surface extends to a position which is in superior relation to a medial edge  45  of articulation surface  15  of tibial component  7 . In this embodiment, the raised surface constrains movement in a medial direction, or eversion of the implant. In an alternative embodiment in which tibial component  7  is the “female” component, the lip comprises a raised surface on a medial edge of articulation surface of tibial component  7  such that the raised surface extends to a position which is in inferior relation to a medial edge of the articulation surface of talar component  9  (not depicted). 
         [0026]    In another embodiment of the present invention, the lip is configured to stabilize prosthesis  1  when implanted in patients with compromised soft tissues on a lateral side of the ankle, such as a compromised anterior inferior tibiofibular ligament  47  (depicted in  FIG. 2 ), for example. In this embodiment, a lip  49  comprises a raised surface on a lateral edge  51  of talar component  9  such that the raised surface extends to a position which is in superior relation to a lateral edge  53  of articulation surface  15  of tibial component  7 . In this embodiment, the raised surface constrains movement in a lateral direction, or inversion of the implant. In an alternative embodiment in which tibial component  7  is the “female” component, the lip comprises a raised surface on a lateral edge of articulation surface of tibial component  7  such that the raised surface extends to a position which is in inferior relation to a lateral edge of the articulation surface of talar component  9  (not depicted). 
         [0027]    In another embodiment, prosthesis  1  is provided for implantation in patients with compromised soft tissues on both the medial and lateral sides of the ankle. In this embodiment, the lip comprises a first raised surface  39  on medial edge  43  of talar component  9  such that the raised surface extends to a position which is in superior relation to medial edge  45  of articulation surface  15  of tibial component  7  and a second raised surface  49  on lateral edge  51  of talar component  9  such that the raised surface extends to a position which is in superior relation to lateral edge  53  of articulation surface  15  of tibial component  7 . In this embodiment, varus/valgus freedom of movement is constrained, but there is freedom of movement for dorsiflexion/plantarflexion. In an alternative embodiment, tibial component  7  is the “female” component and comprises the lip. 
         [0028]    With reference to  FIG. 4 , in another embodiment prosthesis  1  is provided for implantation in patients with compromised soft tissue on a posterior side of the ankle. In this embodiment, the lip comprises a raised surface (not shown) on a posterior edge  55  of talar component  9  such that the raised surface extends to a position which is in superior relation to a posterior edge  57  of articulation surface  15  of tibial component  7 . In this embodiment, posterior displacement of tibial component  7  is prevented. In an alternative embodiment, tibial component  7  is the “female” component and comprises the lip to prevent posterior displacement of talar component  9 . 
         [0029]    The lip may also be configured for implantation in patients with compromised soft tissue on an anterior side of the ankle. In this embodiment, the lip comprises a raised surface (not shown) on an anterior edge of talar component  9  such that the raised surface extends to a position which is in superior relation to an anterior edge of articulation surface  15  of tibial component  7 . In this embodiment, anterior displacement of tibial component  7  is prevented. In an alternative embodiment, tibial component  7  is the “female” component and comprises the lip to prevent anterior displacement of talar component  9 . 
         [0030]    In another embodiment, prosthesis  1  is provided for implantation in patients with compromised soft tissue such that stabilization of the implant and constraint of movement is necessary in more than one direction of mobility. The lip comprises a raised surface (not shown) extending from medial edge  43  of articulation surface  27  of talar component  9  posteriorly curving to lateral edge  51  of articulation surface  27  of talar component  9  and continuing around returning to medial edge  43 . The lip extends to a position which is in superior relation to articulation surface  15  of tibial component  7 . In this configuration, the lip comprises a general cup-like contour providing stabile implantation in patients with more severe soft tissue impairment. In an alternative embodiment, tibial component  7  is the “female” component and comprises the cup-like lip. 
         [0031]    Another consideration for providing a successful total ankle replacement prosthesis is the material or materials of construction. In order to reduce wearing of the components, and therefore failure of the prosthesis, it is desirable to use a material, or a combination of materials, which create minimal friction between the two components. Suitable materials include those which minimize friction and resultant wear of the articulation surfaces. Some exemplary materials include a metal, a polymer, or a ceramic material. However, other suitable materials are contemplated within the spirit and scope of the present invention. In one embodiment of the present invention, articulation surface  15  of tibial component  7  is comprised of a first material and articulation surface  27  of talar component  9  is comprised of a second material. The first material and the second material may be substantially the same. In an alternative embodiment, the first and second material are substantially different. Further, tibial component and talar component are each comprised of an attachment surface and an articulation surface. The articulation surface and attachment surface of tibial component  7  may comprise materials which are substantially the same, or in an alternative embodiment the surfaces may comprise materials which are substantially different. Likewise, the articulation surface and attachment surface of talar component  9  may comprise materials which are substantially the same, or in an alternative embodiment the surfaces may comprise materials which are substantially different. 
         [0032]    Additionally, it may be desirable to use a material which promotes osseointegration, so that a direct interface between attachment surface  25  of talar component  9  and talus  3 , and in between attachment surface  13  of tibial component  7  and tibia  11 , are formed. To this end, one embodiment of the present invention comprises a material of construction which promotes osseointegration. For example, the material may comprise pores into which osteoblasts and supporting tissues can migrate.