Abstract:
A meniscus implant having a compressible bearing element with an articulation surface. The implant also includes a bone-securing element extending downwardly from the bearing element and configured to be engaged within a channel created within a tibial plateau.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention pertains to prosthetic devices. More particularly, the invention pertains to knee joint prosthesis, which may be surgically implanted between the femoral condyle and tibial plateau of the knee joint. 
         [0002]    A meniscal cartilage provides the mobile weight bearing surfaces of the knee joint. Damage to these surfaces is generally due to genetic predisposition, trauma, and/or aging. The result is usually the development of chondromalacia, thinning and softening of the articular cartilage, and degenerative tearing of the meniscal cartilage. Various methods of treatment are available to treat these disease processes. Each option usually has specific indications and is accompanied by a list of benefits and efficiencies that may be compared to other options. 
         [0003]    The healthy knee joint has a balance of joint cartilage across the four surfaces of this bi-compartmental joint (medical femoral condyle, medial tibial plateau, lateral femoral condyle and lateral tibial plateau). In patients with osteoarthritis, knee degenerative process typically leads to an asymmetric wear pattern that leaves one compartment with symmetrically less articular cartilage covering the distal portions (or weight bearing area) of the tibia and the femur than the other compartment. Most commonly, the medial compartment of the knee joint is affected more often than the lateral compartment. 
         [0004]    As the disease progresses, large amounts of articular cartilage are worn away. Due to the asymmetrical nature of the erosion, the alignment of the mechanical axis of rotation of the femur relative to the tibia becomes tilted down towards the compartment which is suffering the majority of the erosion. This results in VARUS (bow-leg) deformity in the case of a medial compartment disease predominates, or a VALGUS (knock-kneed) deformity in a case of lateral compartment disease predominance. Factors such as excessive body weight, previously traumatic injury, knee instability, the absence of meniscus and genetic predisposition, all affect the rate of the disease. 
         [0005]    It is important to understand that the disease manifests itself as periodic continuous pain that can be quite uncomfortable for the patient. The cause of this pain is subject to many opinions, but it is apparent that, as the joint compartment collapses, the collateral ligament on the side of the predominant diseased area becomes increasingly slack (like one side of a pair of loose suspenders), and the tibial and femoral axis move, for example, from a VALGUS to VARUS condition. This increases the stress of the opposing collateral ligament (and cruciate ligaments as well) and shifts the load bearing function of this bi-compartmental joint increasingly towards the disease side. This increasing joint laxity is suspected as causing some of the pain one feels. In addition, as the bearing loads are shifted, the body responds to the increased loading of the diseased compartment with increased production of bony-surfaced areas in an attempt to reduce the ever-increasing area unit loading. All of the shifting of the knee component geometry causes a misalignment of the mechanical axis of the joint. The misalignment causes an increase in the rate of degenerative change to the diseased joint surfaces causing an ever-increasing amount of cartilage debris to build up in the joint, further causing joint inflammation and subsequent pain. 
         [0006]    Currently there is a void in options to treat the relatively young patient with moderate to severe chondromalacia involving mainly one compartment of the knee. Current treatments include cortisone injections, hyaluronic acid (HA) injections and arthroscopic debridement. Repeated cortisone injections actually weaken articular cartilage after a long period of time. HA has shown promising results but is only a short-term solution for pain. Arthroscopic debridement alone frequently does not provide long-lasting relief of symptoms. Unfortunately, the lack of long-term success of these treatments leads to more invasive treatment methods. Osteochondral allografts and micro fracture techniques are indicated for small cartilage defects that are typically the result of trauma. These procedures are not suitable for addressing large areas of degeneration. In addition, osteochondral allografts can only be used to address defects on the femoral condyle. Tibial degeneration can not be addressed with this technique. 
         [0007]    The only true solution is to rebuild the defective joint by (filling) the joint space with more articular bearing material through complete resurfacing of the existing femoral condyle and tibial plateau. By replacing the original cartilage to its pre-disease depth, the joint mechanical axis alignment is restored to its original condition. Unfortunately these natural articular materials and surgical technology required to accomplish replacement tasks do not yet exist. 
         [0008]    Therefore, what is needed is a uni-compartmental interpositional spacer, which by effectively replacing worn articular material, restores normal joint alignment and provides an anatomical correct bearing surface for the femoral condyle to articulate against. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed toward the method of performing surgery and various implants that may be used during knee reconstruction or surgery. In one aspect of the present invention, the method of performing surgery may include forming a groove in a tibial plateau. Next, an implant having a bone-securing element and an articulation element is provided and placed within the groove. Specifically, the bone securing element of the implant is positioned within the groove such that the articulation element is disposed adjacent a tibial plateau. In one aspect, the groove extends from the anterior of the tibia to the posterior. While forming a groove, at least some damaged cartilage may be removed from the tibial plateau. 
         [0010]    The groove may include a first side extending from the medial to lateral side and a second width extending in the same direction. And the first width may be larger than the second width. The bone-securing element of the implant may include a corresponding geometric shape that is similarly shaped to the geometric shape of the groove. 
         [0011]    In yet another aspect of the present invention, the implant may include an intermediate portion that is attached to both the bone-securing element and the articulation element such that the intermediate portion connects the articulation element to the bone-securing element. 
         [0012]    Another aspect of the present invention, a meniscus implant may include a compressible bearing element having an articulation surface and a bone-securing element extending downwardly from the bearing element and configured to be engaged within a channel created within a tibial plateau. 
         [0013]    At least a portion of the compressible bearing element may be embedded within a portion of the bone-securing element. And the bone-securing element may have a porosity that promotes bone ingrowth. The bone-securing element may include a first side wall and a second side wall. Each of the side walls may include a transitional portion that transitions the first side and second side walls from a separation that is equal to a first distance to a separation that is equal to a second distance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a side view of one embodiment of the present invention; 
           [0015]      FIG. 2  is a top perspective view of the embodiment of  FIG. 1 ; 
           [0016]      FIG. 3  is a bottom perspective view of the embodiment of  FIG. 1 ; 
           [0017]      FIG. 4  illustrates a step according to one method of the present invention; 
           [0018]      FIG. 5  illustrates an additional step according to the method also illustrated in  FIG. 4 ; 
           [0019]      FIG. 6  illustrates the method as discussed with regards to  FIGS. 4 and 5  however done on an opposite side of the tibial plateau; and 
           [0020]      FIGS. 7 and 8  illustrate a perspective and side view, respectively of yet another alternate embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The prosthesis meniscal devices of the subject invention are uni-compartmental devices suitable for minimally invasive, surgical implantation. By the term “meniscal” it is meant that the devices are positioned within a compartment in which a portion of the natural meniscus is ordinarily located. The natural meniscus may be maintained in position or may be wholly or partially removed, depending upon its condition. Under ordinary circumstances, pieces of a natural meniscus, which have been torn away, are removed, and damaged areas may be trimmed as necessary. In somewhat rare instances, the entire portion of the meniscus residing in meniscal cavity may be removed. Thus, the term “meniscal device” is descriptive of the location of the device rather than implying that it is a replacement for or has the shape of the natural meniscus. In most cases, the meniscal device of the present invention does not have the same shape as the natural meniscus and will not entirely replace the meniscus. 
         [0022]    By the term “uni-compartmental” it is meant that each device is suitable for implantation into but one department defined by the space between the femoral condyle and it&#39;s associated tibial plateau. In other words, the device is not a “bi-compartmental” device which, in one rigid device, could be inserted into both of the two femoral condyle/tibial plateau compartments, unless it is specifically called out that the device is bi-compartmental. In many, if not most cases, a device will be inserted into one compartment only, generally the medial compartment, as meniscus and associated articular surfaces in these components (left knee medial and right knee medial compartments) are most subject to wear and damage. However, it is possible to insert two separate devices into the medial and lateral compartments of the same knee, or to use two such devices that are mechanically but not rigidly bi-compartmental. 
         [0023]    With reference to  FIGS. 1-3 , a first embodiment of an implantable prosthesis is illustrated in the form of a meniscal implant  12 . The meniscal implant  12  includes a femoral facing surface  14  and an oppositely-facing tibial surface  16 . These two surfaces generally are convex or concave. For instance, as shown in  FIGS. 1-3 , the femoral facing surface  14  is concaved so as to provide an articulation surface for a femoral condyle as will be described below. In contrast, the tibial facing surface  16  has a generally convex shape so as to conform to the distal end of a tibial plateau. The femoral facing surface  14  and tibial facing surface  16  are in communication via an edge  18  extending between the two surfaces  20  of the meniscal implant. The femoral facing surface  14 , tibial facing surface  16  and edge  18  comprise the soft-flexible articulation portion  20  of the meniscal implant. The “articulation portion” refers to the part of the implant that either replaces or supplements the normal cartilage found on a tibial plateau. 
         [0024]    The articulation portion  20  is generally flexible and is preferably made from a polymer such as polyurethane, Delrin, Ultem, PVA, PEEK, and/or polyethylene. The articulation portion  20  thus uses a flexible, low profile layer of smooth material to create a smooth articulating surface as defined by the femoral facing surface  14 . The femoral facing surface  14  is designed to slide against mating cartilage, bone, and/or additional implants. Preferably the new artificial joining surface created by the femoral facing surface  14  supports any type of motion that naturally occurs within the older joint surface prior to the damage that was incurred. 
         [0025]    The meniscus implant  12  further includes a securing element such as a keel  30 . Keel  30  is preferably comprised of a three dimensional titanium mesh having a predetermined porosity. The keel includes a bone anchoring portion  32  and an intermediate portion  34 . The keel  30  may be constructed using various methods known to those in the art. And in certain preferred environments, the keel  30  may be formed using methods as described and commonly assigned U.S. patent application Ser. Nos. 11/448,954 entitled “Flexible Joint Implant”; 10/704,270 entitled “Laser-Produced Porous Surface”; 11/027,421 entitled, “Gradiant Porous Implant”; 11/295,008 entitled “Laser-Produced Porous Surface”, the disclosures of which are hereby incorporated by reference herein. 
         [0026]    As discussed in U.S. patent application Ser. No. 10/704,270, the keel  30  may be constructed using a selective laser melting or sintering process, which hereby grows the structure in a layer by layer process. In an alternate process, the keel  30  may be built using a method described in U.S. patent application Ser. No. 10/704,270, wherein the intermediate portion  34  acts as a base or substrate on which the bone-anchoring portion  32  is built, also in a layer by layer fashion. Additional techniques for constructing metalized structures i.e., keel  30 , may also be constructed employing methods as disclosed in commonly assigned in U.S. patent application Ser. No. 10/071,667 entitled “Porous Metallic Scaffold for Tissue”, the disclosure of which is hereby incorporated by reference herein, as well as additional methods as known to those in the art such as that disclosed in Patent Cooperation Treaty Application 2005/023118 entitled “Porous Metal Articles Formed Using an Extractable Particulate” filed on Jul. 22, 2004 the disclosure of which is hereby incorporated by reference herein. 
         [0027]    The keel  30  generally has a height from a first end  36  of the keel to a second end  38  of the keel of approximately 4 mm to 20 mm The bone-engaging portion  32  of the keel  30  preferably has a height of between 2 mm and 18 mm. 
         [0028]    As shown in  FIG. 1 , the keel  30  includes two opposing side walls  40  and  42 . The opposing side walls  40  and  42  are generally symmetrical although this is not required. Each side wall  40 ,  42  preferably defines a narrow portion  44  and a wide portion  46 . The narrow portion  44  may be substantially placed within the intermediate portion  34  of the keel  30  while the wide portion  46  may be substantially placed within the bone-anchoring portion  32  of the keel. As shown in  FIG. 1 , the side walls  40  and  42  preferably may include a curved transition wall  44  that allows a smooth transition between the narrow portion  44  of the keel to the wide portion  46 . Although the transition wall is illustrated as being curved, the wall may be straight, slanted or have different configurations, as well as including steps and the like. Similarly, the keel  30  may simply include two straight side walls thereby giving the keel a constant width as opposed to the varying width shown in the figures. 
         [0029]    With reference to  FIG. 1 , a longitudinal plane  50  slices the meniscus implant  12  in half. The longitudinal plane  50  extends from the anterior  52  to the posterior  54  of the implant. Thus, the longitudinal plane  50  dissects the meniscus implant  12  into an inside segment  56  and an outside segment  58 . The reference to an “inside segment” or “outside segment” has no geometry meaning as to the superior articular surface onto which the meniscus implant  12  may be positioned, be it on the medial or lateral side of the tibial plateau. But rather the inside segment refers to the fact that the meniscus implant  12  will be positioned with the inside segment  56  being closer positioned to a line passing through a mechanical axis of the tibia in a lengthwise direction while the outside segment  58  is farther away from the mechanical axis. 
         [0030]    In  FIGS. 1-3 , the keel  30  is positioned offset from a longitudinal plane  50  and adjacent to the edge  18  of the articulation portion  20  of the implant that is within the inside segment  56  of the implant. The keel  30 , however, may be positioned at any position or in any orientation relative to the articulation portion  20  including centrally. Therefore, as shown in the figures, the keel extends from the anterior  52  to the posterior  54  of the meniscus implant  12  within the inside segment  56  of the implant. 
         [0031]    In a method of assembly, the keel  30 , including the bone-anchoring portion  32  and intermediate portion  34  may be constructed using the various processes described herein. Once the keel  30  has been constructed, the keel may be placed within a mold cavity as discussed in U.S. patent application Ser. No. 11/448,954 and the other various references of which are incorporated by reference herein or known to those in the art. The mold cavity may include a forming area that enables a polymer material to be disposed therein. The polymer material is introduced into the forming area of the mold cavity and is allowed to cure into a desired shape so as to form the articular portion of the meniscal implant  12 . During this process, the polymer material is also allowed to creep into the intermediate portion  34  of the keel  30 . The intermediate portion  34  of the keel  30  preferably has a porosity that enables the polymer to adhere to the various metal lattice constructed within the intermediate portion. Once cured, the combination the polymer locked within the metal lattice of the intermediate portion  34  secures the articulation portion  20  to the bone-engaging portion  32  of the keel  30 . 
         [0032]    As described in the various references disclosed herein, the keel  30  may include areas with various gradient porosities as well as barrier layers and other features described in the references incorporated, which aid the keel in promoting bone ingrowth and the like including a desirable porosity and pore size. 
         [0033]    In a method of implantation, a small trough  60  of bone may be removed from a central section of the tibia adjacent to the tibial spine as shown in  FIG. 4 . The trough  60  preferably has a geometry that corresponds to the keel  30  or at least to the bone-anchoring portion  32  of the keel. The small trough  60  extends from the anterior  64  to the posterior  66  of the tibia. Thus, the trough  60  includes a lower portion  66  having a greater width than an upper portion  68  of the trough. This difference in width helps lock the meniscus implant within the tibial plateau as will be described shortly. 
         [0034]    Once the small trough  60  has been created within the tibial plateau by methods known to those in the art, the meniscus implant  12  may be introduced to the tibial plateau. As shown in  FIG. 5 , the keel  30  may be received within the trough  60  by placing an edge of the keel adjacent a first edge of the small trough  60 . The entire meniscus implant  12  may then be slid in a direction as denoted by arrow A such that the meniscus implant  12  enters the trough  60  in a anterior to posterior direction. Of course, the meniscus implant may also be slid in reverse if the surgeon is using a posterior to anterior approach. The meniscus implant is slid forward until the articulation portion  20  of the meniscus implant  12  is positioned in a desired location sitting atop the tibial plateau. The inferior portion of the implant also known as the keel  30  may be inserted through this trough, while the superior portion of the meniscal implant  12  also known as the articulation portion  20  sits a top the tibial plateau  62  of the tibia  64 . Since the keel includes a wide portion  46  beneath the narrow portion  44 , the keel is unable to be removed from its location by applying a force out of the tibia and upwards. The only way to remove the meniscus implant  12  is to slide the keel anteriorly or posteriorly until the entire keel is removed from the trough  60 . The keel  30  is now firmly placed within the trough  60  and is surrounded by adjacent bone. And with the bone-securing portion  32  of the keel promoting bone ingrowth, the keel will become firmly locked within the bone over time. Of course, if the keel has certain geometric shapes such as straight walls, it may be lowered into the trough. 
         [0035]    Since the keel includes a narrow portion and a wide portion, the keel is designed to fit into the small trough or key-way in a key like fashion so as to lock the meniscal implant  12  into the bone and prevent loosening. Therefore, the key way allows the meniscal device to be implanted into the proximal end of the tibia using a anterior to posterior approach. Once the keel  30  is positioned within a small trough or key-way, the tibial facing surface  16  of the implant is disposed adjacent and against the tibia. Thus the meniscus device sits atop the tibia condyle thereby providing a low profile articulation surface. Besides the various key and key-way method of locking the meniscus implant  12  within the bone, other short and/or long-term fixation methods may be used. Some of these methods include the use of bone screws, nails, staples, sutures, and/or hooks. In addition, porous metal or poly pegs, sheets, rectangles or other geometric shapes could be used by itself or in combination to fix the implant in place to allow for bone ingrowth. Further, bone cement could also be used to hold the implant to the bone; and ultrasonic waves may be used to cause protrusions on the backside of the implant to melt while being driven into the underlying bone. This would force the molten material to infuse and harden into the surrounding bone. The protrusions could be made of a homogenic material or other porous or solid metal reinforced plastic. Any additional property of the plastic protrusions could have the ability to be reabsorbed into the body and allow soft tissue or bone to grow into the implant for fixation. The metal reinforcements could be hollow, slotted or porous. 
         [0036]    Another option may be to use small metal tubes or pegs that are slotted or porous and filled with a polymer material that could be melted or forced outside of the metal tubes after insertion into the bone. UV curing adhesives could be injected through openings in the implant after it is inserted into the bone and allowed to harden by inserting fiberoptic cables into the openings and activating the adhesive. A mesh could also be disposed on the tibia facing surface  16  to help secure the articulation portion  20  of the meniscus implant  12  to the tibial plateau. In yet another alternate embodiment as shown in  FIGS. 7 and 8 , a meniscal implant  112  is similarly structured as meniscal implant  12  and therefore includes a femoral facing surface  14  and an oppositely-facing tibial surface  16 . These surfaces may be constructed similarly to the surfaces of meniscal implant  12  and form an articulation portion  120  of the implant  112 . 
         [0037]    The meniscal implant  112  also includes a keel  130 , similar to keel  30  of meniscal implant  12 . Although the keel  130  may have many different geometrical configurations as may keel  30 , keel  130  is shown in  FIGS. 7 and 8  has a circular bone-anchoring portion  132  and a rectangular intermediate portion  134 . The bone anchoring portion  132  is substantially circular in side view but as shown in  FIG. 8  is actually substantially cylindrical. The intermediate portion  134 , although not shown in the figure, may extend into the cylindrical shape of the bone anchoring portion  32  such that the respective porosity of each portion is not limited to exactly being within the bone anchoring portion  132  or intermediate portion  134 . Of course, when using a cylindrically shaped bone anchoring portion  132 , any trough that is cut within the tibial plateau should have a similar geometric configuration or allow for the cylindrical bone anchoring portion  132  to be placed within the trough. 
         [0038]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.