Abstract:
The present invention relates to a femoral component for use in connection with knee anthroplasty. The implant includes a support having a contoured inner bone engaging surface, and a shell affixed to the support. The shell has an outer surface spaced so as to provide an articulation surface for engaging the tibia that substantially replicates the shape of a femoral condyle, and an inner surface for receiving an outer surface of the support. The support bone engaging surface is structured to mate with a prepared surface of the distal femur and the support spaces the shell outer surface at a predetermined distance from the prepared surface.

Description:
BACKGROUND OF THE INVENTION 
     Total knee replacement (“TKR”) is a commonly-used procedure for correcting deformities and repairing damage to the knee joint. The procedure used for TKR is generally known in the art and includes many variations. Generally, such a procedure includes exposing the knee joint by forming at least one incision through the soft tissue in the knee area and retracting the wound. The joint is then resected, which includes removing the damaged portions of the joint. This typically includes removing one or both of the femoral condyles and/or the tibial plateau, which is typically accomplished by forming a series of cuts according to any one of various patterns. The cuts are typically made so that the bone can accept an artificial replacement for the resected portions of the joint. As the precise anatomy of the knee on which TKR is preformed varies significantly among patients, it is necessary to provide artificial replacements for the knee components in various shapes and sizes. It is also necessary to form the cuts in the bones of the knee joint to appropriately accept the implant that best suites the anatomy of the individual joint as best suited for the patient. 
     In order to facilitate the appropriate joint resection and artificial joint selection, various trial implants have been developed are used in “trial reduction” of the resected joint. To assist in trial reductions, a number of differently sized “trial” joint implants (which are also referred to as “provisional” implants) are supplied. After a preliminary estimate of the appropriately sized implant is made, trial implants are inserted into the resected joint, usually on both the femur and on the tibia. The implant is then examined for proper fit, and the joint is tested for proper kinematics. If the fit of the trial is improper, different trials are selected in succession until proper fit is achieved. Selection of differently sized trials may require further joint resection. Once a proper size determination has been made, a permanent joint implant of a size which corresponds to that of the appropriately-sized trial is affixed within the joint. In TKR this typically includes affixing permanent implants into both the femoral and the tibial components of the knee. A similar trial reduction procedure is used to determine proper implant fit in a total hip replacement (THR) procedure. 
     Trial femoral components must accurately match the geometry of the permanent implant to be used in TKR. Further, femoral trials must be sufficiently rigid to replicate proper joint kinematics. Costs associated with manufacturing such trial components has lead to known trial components being made so as to be reusable throughout multiple procedures. Reuse of trials requires that the trials be sterilized prior to each use, which is typically done using an autoclave procedure. Such a procedure is somewhat rigorous with respect to the items subjected thereto, which further requires robust construction of the trials. In response to these requirements, known trial components have been manufactured from cast cobalt-chromium (CoCr) or stainless steel (“SS”), both of which can withstand multiple autoclave cycles and are sufficiently rigid to provide accurate trial reduction. However, the processing required to impart the necessary geometry onto these materials requires many secondary operations, such as CNC grinding or polishing. The material properties of CoCr and of SS are such that these secondary operations require relatively low feed and tool speed rates to properly create the complex geometries that are part of the trial. Each of these secondary operations is, thus, costly and time consuming, leading to a large overall cost increase of trial components. 
     In addition to the cost associated with processing the cast materials of typical trials, the density of the material can be quite high, resulting in a relatively heavy component. Each trial component may weigh approximately 1-1.5 pounds, a weight which becomes problematic due to the methods employed during TKR and THR procedures. Currently, validated sterilization methods require each component that may potentially enter the sterile field to be steam-sterilized prior to surgery (typically via an autoclave process) As a result, all surgical tools that may potentially be used during TKR and THR procedures are kitted and held in sterilization trays. The kitting of instruments is based on the surgical steps for which they are required as part of a particular procedure. As a result, all instruments required to complete a step are preferably stored in one tray or case. Multiple trays are then placed into a sterilization case and the case is processed through the sterilization process and brought into the operating room. In the case of femoral trials, because final determination of femoral size is made interoperatively, all such devices for a given TKR system are housed on a single tray and brought into the operating room together. A typical TKR system can have eight differently sized trials for both the left and right femoral components, resulting in sixteen femoral trials being stored in a single sterilization tray. Based on the average trial weight, the fully-loaded tray may twenty pounds or more. When combined with the other trays contained in the sterilization case, total case weight is significant. The same problem applies for THR procedures: as with femoral sizing, proximal stem sizing must be performed interoperatively. Therefore, a fully-loaded THR tray may also weigh upwards of twenty pounds. 
     It is therefore desired to provide a trial component that has a reduced weight, and which reduces costly process steps, while retaining the desired characteristics for such a component. 
     As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a femoral component for use in connection with knee anthroplasty. The implant includes a support having a contoured inner bone engaging surface, and a shell affixed to the support. The shell has an outer surface spaced so as to provide an articulation surface for engaging the tibia that substantially replicates the shape of a femoral condyle, and an inner surface for receiving an outer surface of the support. The support bone engaging surface is structured to mate with a prepared surface of the distal femur and the support spaces the shell outer surface at a predetermined distance from the prepared surface. 
     The femoral component of the present invention may have a support that is formed from a plastic. Further, the femoral component may have a shell that is made from a metal, such as stainless steel or cobalt chrome, which may be formed using a hydroform process. Preferably, the shell is further shaped so as to provide an outer profile having a rib extending therefrom in a direction substantially away from the articulation surface. Further preferably, the support is made from a polymeric material and wherein the shell further includes a folded portion extending orthogonally away from the rib into a portion of the support. 
     In an alternative embodiment, the shell is made from carbon fiber, which can include either long or short fibers. Further, the shell may include a layer of polymer overmolded on the carbon fiber. 
     A further embodiment of the present invention relates to a femoral component for use in connection with a joint replacement for a patient. The femoral component includes a support and a shell affixed to the support. The shell is shaped so as to provide an articulation surface for the joint and the support is structured to mate with a prepared surface of the joint and to space apart the shell at a predetermined distance therefrom. 
     In a preferred embodiment, the prepared joint is the knee, and the articulation surface is formed so as to replicate the anatomy of an articulation surface of a femoral condyle. In such an embodiment, the support bone engaging surface is structured to mate with a prepared surface of the distal femur. 
     In an alternative embodiment, the prepared joint is the hip and the articulation surface is formed so as to replicate the anatomy an articulation surface of a femoral head. In such an embodiment, the support surface forms a stem being adapted to mate with the inside surface of a prepared femoral canal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood on reading the following detailed description of nonlimiting embodiments thereof, and on examining the accompanying drawings, in which: 
         FIG. 1  is an isometric view of the trial implant according to an embodiment of the present invention; 
         FIG. 2  is an assembly view of the trial implant according to an embodiment of the present invention; 
         FIG. 3  is a distal to proximal view of an implant according to an embodiment of the present invention; 
         FIG. 4  is a posterior to anterior view of an implant according to an embodiment of the present invention; 
         FIG. 5  is a proximal to distal view of an implant according to an embodiment of the present invention; 
         FIG. 6  is a lateral view of an implant according to an embodiment of the present invention; 
         FIG. 7  is an isometric view of the outer surface of an implant according to a further embodiment of the present invention; 
         FIG. 8  is an isometric view of a bone engaging surface of an implant according to a further embodiment of the present invention; 
         FIG. 9  is a cross section view taken along line  9 - 9  in  FIG. 5 ; 
         FIG. 10  is a hip implant according to an alternative embodiment of the present invention; and 
         FIG. 11  is a cross section view taken along line  11 - 11  in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     In describing the preferred embodiments of the subject matter illustrated and to be described with respect to the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Referring to the drawings, wherein like reference numerals represent like elements, there is shown in  FIGS. 1-6 , in accordance with a preferred embodiment of the present invention, more particularly, a femoral implant  10  used during a TKR procedure. The particular implant shown is preferably used as a trial implant; however it may be used as any type of femoral implant. Generally, the implant of the present invention has two primary surfaces thereof, including an articulating surface  12 , and a bone engaging surface  14 . Preferably, articulating surface  12  is shaped so as to approximately replicate the shape of the distal femur and, in particular, the articulating surfaces of the femoral condyles. It is not necessary that articulating surface  12  match the particular anatomy of the knee of the particular patient. Further, articulating surface is preferably designed to engage an artificial tibial implant (not shown). The desired general shape and design for articulating surfaces of femoral implants is known in the art. 
     Bone engaging surface  14  is formed to match the surface of the distal femur once the bone has been resected. Resection of the distal femur may vary by application, but is generally performed so as to remove one or both of the femoral condyles. This is generally done by making a series of cuts in the distal femur, the positioning and formation of which is known in the art. The femoral implant bone engaging surface shown in  FIGS. 1-6  has a profile that matches one known shape for the resected distal femur; however, other shapes may be now known or later contemplated and corresponding shapes for bone engaging surface  14  would be understood by one having reasonable skill in the art. 
     The geometry of both articulating surface  12  and bone engaging surface  14  lead to bone engaging surface  14  being spaced proximally of articulating surface  12  and being spaced apart at a distance therebetween. Accordingly, implant  10  has a thickness that is appropriate to provide the preferred spacing between articulating surface  12  and bone engaging surface  14 . Preferably, the general shape of implant  10  is similar to that of implants known in the art. In particular, when implant  10  is to be used as a femoral trial, it is preferred that implant  10  matches the shape of a corresponding permanent implant as closely as possible. 
     As best shown in  FIG. 2 , implant  10  is preferably formed from two separate parts. Support  16  is interposed within shell  18  and forms bone engaging surface  14  therein. The outside surface  20  of support  16  is designed to substantially mate with inside surface  22  of shell  18 . Shell  18  forms articulating surface  12 , and preferably has a thin, substantially uniform thickness such that the shape of inside surface  22  substantially matches that of articulating surface  12 . Accordingly, support  16  provides a majority of the appropriate spacing between articulating surface  12  and bone engaging surface  14 . 
     Various materials can be used in formation of shell  18  and support  16 . Acceptable materials for shell  18  include various metals, such as CoCr, SS and aluminum alloys, or polymeric material, such as polyetheretherketone (PEEK). If a polymeric material is used to form shell  18 , the polymer may be reinforced with carbon fiber, including long short or micro fibers, as they are known in the art. Preferably, shell is formed from a metal, such as CoCr or SS having a thickness between about 0.015 inches and about 0.065 inches, or aluminum alloy having a thickness between about 0.030 inches and about 0.080 inches. In a preferred embodiment, shell  18  is formed from SS and has a thickness of about 0.040 inches. 
     Various materials may also be used in the formation of support  16 . Acceptable materials for support  16 , include metal and polymeric material. Metals may include CoCr, aluminum alloys and SS, and polymeric materials may include ULTEX®, PEEK, polycarbonate, polysulphone, XYLAR®, and LEXAN®. In an embodiment of the present invention, support  16  can be made from a fiber-reinforced polymeric material. Such materials may include PEEK reinforced with carbon fibers, which may comprise long, short or micro fibers. Further, support  16  is preferably formed with a series of recesses  24  therein. The inclusion of recesses  24  within support  16  reduces the amount of material used to form support  16 , which may reduce the overall cost of implant  10  and/or the weight thereof. Further, the formation of recesses  24  in support  16  results in the formation of a number of ribs  26  within the structure of support  16 . Ribs may increase the overall strength of support  16  and, thus, of implant  10 , allowing for less-rigid and, possibly, less expensive materials to be used. Still further, the inclusion of recesses  24  allows the material from which support  16  is formed to have a more uniform thickness. This is advantageous when forming support  16  using an injection molding process because uniform material thickness allows the material throughout the entire part to cool (and thus, shrink) uniformly. This helps prevent the part from warping during cooling. 
     In a preferred embodiment of implant  10 , shell  18  is formed from a metal, preferably CoCr or SS and support  16  is formed from a polymeric material, preferably XYLAR®. In such an arrangement, shell  18  is more preferably formed using a hydroform process. Hydroform is a process that is generally known in the art and is useful for imparting complex, three-dimensional (“3D”) shapes into metal. Preferably, shell  18  is formed using a vertical hydraulic hydroforming press. Such a process can be carried out by Aero Trades Manufacturing, located at 65 Jericho Turnpike, Mineola, N.Y. It is preferred that a metal subjected to a hydroform process is thin enough to be accurately formed by the process. It is also preferred that the material be thick enough to retain the shape imparted therein. The ideal thickness for shell in this embodiment will vary by the material and specific geometry used and will be known by those having reasonable skill in the art. The use of a hydroform process to form shell  18  reduces the need for the additional process steps of CNC grinding or polishing, as are needed with a casting process. 
     Generally, the combination of a shell  18  made from hydroformed metal and a support  16  made from a polymeric material allows for an implant  10  which is appropriately shaped and sufficiently rigid to provide acceptable trial joint reduction, while being lightweight and cost-effective from a manufacturing standpoint. The lightweight design of such an implant  10  allows for easy transportation of a number of such implants  10  when used in a set of trial implants. Further, the cost-effective manufacture of such implants makes it reasonable to use each of such implants in only one surgical procedure. The provision of such disposable trial implants may eliminate the need to design such an implant to withstand multiple autoclave cycles, and to withstand multiple trial reductions, further lowering the manufacturing cost thereof. 
     Shell  18  may be affixed to support  16  by a variety of methods, including using adhesives. Additionally, fixation elements such as screws, bolts or rivets may be included within implant  10  to secure shell  18  to support  16 . Further, corresponding tabs may be formed in appropriate portions of shell  18  and support  16  to achieve fixation therebetween. 
     Referring now to  FIGS. 7-8 , a further embodiment of the present invention is shown wherein implant  10  is formed from support  16  and shell  18  in a manner similar to that of implant  10  described with reference to  FIGS. 1-6 . Implant  10  of the present embodiment includes shell  18  having a generally proximally extending rib or flange  28  extending along at least a portion of the outer periphery of shell  18  and preferably the entire outer periphery. The integral formation of rib  28  within the outer periphery of shell  18  increases the rigidity of shell  16 , and accordingly of implant  10  overall. Rib  28  may be formed in a metal shell  16  by hydroforming. 
     More preferably, as shown in  FIG. 9 , shell  18  further includes folded section  30  extending inwardly from the upper surface of rib  28 . Folded section  30  further increases the rigidity of shell  18  and implant  10 , especially with respect to flexion of implant  10  in the anterior-posterior direction. Additionally, folded section  30  provides for a means of affixation between support  16  and shell  18 . In particular, in a preferred embodiment of the present invention, shell  18  is formed from hydroformed metal, preferably CoCr or SS, and support  16  is formed from a polymeric material. In this embodiment, support  16  is formed by insert molding the polymeric material onto shell  18 . In such a process, support  16  is formed by injection-molding of a polymeric material into an appropriately shaped mold into which a pre-formed shell  18  has been inserted. Because the molten polymeric material can easily flow into and around any geometry formed in the shell, including rib  28  and folded portion  30 , direct contact between the polymeric support  16  and the shell  18  may be the primary method of attachment therebetween. Incorporation of rib  28  and folded portion  30  furthers this attachment because the polymer flows into the shell, fully encasing the folded portion  30 . This direct contact between the two materials along the periphery of the shell provides sufficient purchase to fully affix the shell  18  to the support  16 . 
     Additionally, as shown in  FIG. 9 , shell  16  may have post  32  affixed to inside surface  22  thereof. Preferably, post  32  is either T-shaped, as shown, or includes a “stepped” geometry, as it is known in the art. Inclusion of this form of post  32  provides additional contact points between shell  18  and support  16 . Post  32  may be fabricated to provide geometry similar to folded portion  32  discussed above, wherein the contact between post  32  and the hardened polymer comprising support  16  creates additional purchase, further affixing shell  18  to support  16 . Post  32  may be added to inside surface  22  after formation of shell  18  and affixed thereto using welding or a similar process. In this particular embodiment, implant  10  may include a plurality of posts  32 . 
     In an alternative embodiment of the present invention, an implant  10  generally similar in structure to those discussed with respect to  FIGS. 1-9  is made from polymeric reinforced carbon fiber. Carbon fiber is a reinforcing fiber known for its lightweight, high strength and high stiffness. Carbon fiber is produced by a high-temperature stretching process of an organic precursor fiber based on polyacrylonitrile (“PAN”), rayon, or pitch in an inert atmosphere at temperatures above 1,800 degrees, Fahrenheit. Fibers can be transformed by removing more non-carbon atoms via heat treating above 3,000 degrees Fahrenheit. After these fibers are produced, they can be utilized in many different forms. They can be woven into long, dry fabric, pre-impregnated with resin, wound onto spools for use in filament winding, or braided and chopped into small fibers. There are several ways in which to produce components using carbon fiber; however, all of such processes require the use of a mold to impart the necessary geometry into the carbon fiber. The mold used in such a process defines the shape of the component. Accordingly, any component that can be molded can be formed from carbon fiber. For example, femoral trials can be created using carbon fibers. In a preferred embodiment, the femoral trial can be molded using a two-part mold; one mold to define the bone engaging surface  14  and the other to form the articulating surface  12 . 
     Molding processes used to form a trial from carbon fiber include autoclave molding, compression molding, bladder molding, resin transfer molding (“RTM”) roll wrapping, filament winding, and wet lay-up. Any of these methods can be used to produce knee femoral trials for TKR and hip stem trials for THR. All of these types of molding processes force the carbon and resin to conform to the desired shape using heat and/or pressure. Once the part has cured, it maintains its shape permanently and the composite construction provides sufficient rigidity to allow the implant  10  to perform equivalently to a metal trail during trial reduction. The use of micro carbon fibers reduces manufacturing costs, but also reduces material strength. Preferably, implant  10  of the present embodiment is molded from a polymer reinforced with long fiber, which is then overmolded with a “neat” polymer. 
     While robust, the composite construction of the implant  10  of the present embodiment of the invention possesses less resistance to the effects of repeated autoclave cycling than cast CoCr or SS trials. Previously known trials have been designed to survive multiple autoclave cycles and retain the rigidity they had before the first use thereof. Implant  10  of the present embodiment need only possess sufficient rigidity for a single use and needs not have the same robustness of reusable trials. Implant  10  of the current embodiment, however, has a weight that is significantly less than reusable trials, and thus alleviates many of the problems associated with the weight thereof. 
     Implant  10  of the present embodiment can be formed using a two-part structure as shown in  FIGS. 1-9 , wherein shell  18  includes articulating surface  12 , and support  16  includes bone engaging surface  14  and appropriately spaces apart articulating surface  12  from bone engaging surface  14 . In such an embodiment, shell  18  is preferably affixed to support  16  using an adhesive or an epoxy compound. Alternatively, implant  10  can be molded in a unitary form, having articulating surface  12  and bone engaging surface  14  formed therein. 
     Referring now to  FIG. 10 , an alternative embodiment of the present invention is shown in which implant  110  is in the form of a hip stem trial as is used in a THR procedure. The use of hip stem trials is similar to that of femoral trials. Generally, implant  10  replicates the shape and joint kinematics of a permanent implant and is used in trial reduction of the replacement joint. Implant  110  of the present invention includes a modular articulating surface  112 , which replicates a resected femoral head and is generally in the shape of a portion of a sphere. Further, implant  110  includes a bone engaging stem portion having surface  114 , which is appropriately shaped so as to fit within a resected proximal femoral canal. Support  116  gives shape to bone engaging surface  114  and appropriately spaces apart articulating surface  112  therefrom. Implant  110  can be fabricated using a hydroform process as discussed above by forming two half-shells with the hydroform process and then assembling the half-shells onto a plastic inner structure. Alternatively, implant  110  can be formed using a tube hydroforming process, which can be carried out by Vari-Form, which is located at, 250 Lothian Ave., Strathory, Ontario, Calif. 
     Support  116  can be formed from various materials including metal. In one form of the present embodiment, support  116  is made from a metal tube, which is subjected to pressure to impart the appropriate shape therefor. In an alternative embodiment, support  116  is made from a molded polymeric material, which may be fiber reinforced in a manner similar to other embodiments of the present invention discussed above. The general shape of the femoral head may be provided within support  116 . In such an arrangement, shell  118  may be affixed thereto to provide implant  110  with articulating surface  112 . Shell  118  can be formed from various metals including CoCr and SS or molded polymeric material, which may be fiber reinforced. A metal shell  118  may be formed by hydroforming, as discussed above. Alternatively, articulating surface  112  may be provided on support  116  in a unitary fashion. 
     Although the various embodiments of the present invention have been discussed as they apply either to the human knee and hip joints, one having reasonable skill in the art upon reading this disclosure would understand that the present invention can be used to form other joints of human or animal bodies. Such joints may include the elbow, wrist, shoulder, etc. 
     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.