Patent Document

FIELD OF INVENTION  
         [0001]    This invention relates to a patella replacement apparatus and more particularly to an improved patella prosthesis allowing improved biological fixation to the patella knee area of a patient user.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various prosthetic devices for replacement of the articulating surface of the patella or knee cap have been designed for implantation in cases where replacement of the natural articulating surface of the patella is indicated. Generally, the posterior surface of the patella is resected and replaced by an artificial articulating surface. These devices have been designed for implantation in many cases where replacement of the articulating surface of the patella is indicated. There are many examples of such devices in the prior art and there are many devices which are commercially available to surgeons. Examples of such devices are disclosed in U.S. Pat. No. 5,019,104 to Whiteside, et al., U.S. Pat. No. 4,007,495 to Frazier and U.S. Pat. No. 5,314,489 to Elloy et al. One common patella replacement device has two basic components, a metal portion that is attached opposite the femur and a plastic portion that fits onto the metal portion. Frazier discloses a patella femoral prosthesis having a patella prosthesis and a femoral prosthesis which are connected to each other. The femoral prosthesis is formed of steel and is attached onto the femur by bone cement. The patella prosthesis is formed of polyethylene or a similar plastic material and is mounted on the patella. The two prostheses are connected together through a pin and slot arrangement. The device disclosed by Elloy et al. in U.S. Pat. No. 5,314,480 has a femoral component and a tibial component including a metal disk imbedded within a high molecular polyethylene disk. The polyethylene disk surrounds the edge of the metal disk so that the metal disk is flush with the polyethylene disk on one surface. The metal disk is provided with serrations which fit into the polyethylene disk to prevent rotation of the metal disk relative to the polyethylene disk.  
           [0003]    Patella replacement devices must be able to fix biologically to remnant bone or soft tissue such as muscle or tendons. Additionally, the devices must articulate and function within the confines of the cartilage on the femoral bone. The survivorship rate of devices that attempted to replace the patella has been poor and surgeons often choose to remove the total patellar bone, leaving the patient with a compromised knee joint. Removal of the patellar bone creates about a 20% loss of the quadriceps mechanism muscle strength. Research has concluded that many patients only regain about 75% of their knee normal strength. For the case of removal of a failed prosthetic patellar component from a total knee joint replacement procedure, the amount of cancellous bone remaining is minimal and offers the surgeon a challenge for the new implant stability and long term fixation, while attempting to provide biomechancial articulation against the existing femoral component. The incidence of pain, lack of full function, and rate of component loosening has been unacceptably high.  
           [0004]    It is therefore an object of the present invention to provide an improved patella replacement device that can restore or mimic the biomedical function of the normal patella bone. In this manner, the replacement by the patella replacement device of this invention results in an improved operation and improved muscle use for the patient. The device is simple to install as compared to prior art devices and offers advantages both to the surgeon and to the patient.  
         SUMMARY OF THE INVENTION  
         [0005]    A patella replacement device for use in repairing or replacing the destroyed natural patella of a patient has two generally hemispherical members. One of the two members has a porous surface and is attached to the other member during implantation. The second member need not have a porous surface and is attached to the first member with bone cement or by mechanical means. Both members are fabricated from biocompatible materials. One member preferably is a plastic and the other member preferably is metal and has a porous surface. The porous surface allows biological fixation of the device to the patella region of the patient. The first member has a rounded fixation surface for implantation within the patella region of a patient and a relatively flat surface opposite the rounded surface. The second member has a top rounded surface and an opposing surface. At least one projection extends from a flat surface in one member into a mating aperture in the flat surface of the other member to enable the first member to couple to the second member with the second member operative to allow articulation against the femoral area of the patient. A ring having a greater diameter than the diameters of the first and second members and suture holes about the periphery of the ring can be positioned between that first member and the second member. Alternatively, an annular collar containing suture holes can be provided on either or both of the first member and the second member. Both members can be made of a ceramic or other biocompatible material rather than one component being metal and the second component being plastic.  
           [0006]    An alternative embodiment of the patella replacement apparatus also has two members similar in shape to those in the first embodiment, but having no collar. Suture holes are preferably provided through the circumference of the second member. This embodiment does not have an annular shoulder or intermediate ring.  
           [0007]    In yet another embodiment the metal member or ceramic member is not itself porous, but is coated with a plasma spray hydroxyapatite coating. That coating is porous. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]    [0008]FIG. 1 is a side view of a first embodiment of a patella replacement device according to the present invention and shows the two members of the device connected together.  
         [0009]    [0009]FIG. 2 is an exploded view of the embodiment shown in FIG. 1.  
         [0010]    [0010]FIG. 3 is a plan view of the flat surface of one member of the embodiment of FIG. 1.  
         [0011]    [0011]FIG. 4 is a plan view of the flat surface of the second member of the embodiment of FIG. 1.  
         [0012]    [0012]FIG. 5 is an exploded view similar to FIG. 2 of a second embodiment of the invention utilizing a patella ring.  
         [0013]    [0013]FIG. 6 is a plan view of the ring in the embodiment shown in FIG. 5.  
         [0014]    [0014]FIG. 7 is a plan view of the assembled ring and patella insert shown in FIG. 5.  
         [0015]    [0015]FIG. 8 is an exploded view of a third embodiment of the patella replacement apparatus of the present invention.  
         [0016]    [0016]FIG. 9 is a cross-sectional view taken along the line IX-IX of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring to FIGS. 1 through 4 there is shown a present preferred embodiment of the patella knee replacement device  10  which consists of two members or components  11  and  12 . One member  11  is a bearing component and has a rounded top surface and a flat bottom surface giving the component a generally hemispherical shape. The flat bottom surface may be circular or elliptical depending upon the patient for whom the device is used. The second member  12  is similar in shape to the first component. The first and second members are made from a biocompatible material such as plastic, particularly polyethylene, biocompatible metals, such as titanium or cobalt chrome alloys, or a ceramic. I prefer to make one component from a metal and the second component from a plastic. The projections preferably are plastic. At least a portion of the second component  12  has a porous surface. The surface can be formed through use of a porous metal, by roughening the surface, by providing a beaded surface or other conventional means. In the first embodiment both components  11  and  12  have an annular collar  13  or  14  surrounding the flat surface of the component. There are a series of suture holes  15  provided in the collars. The edges of the suture holes should be beveled and smooth so as not to cut the sutures. When the first and second components are assembled together the suture holes in the collars will be aligned. In this embodiment the bottom surface of the polyethylene bearing component  11  has three upstanding posts  17 . I prefer that the posts be plastic. The posts can extend from either the first or second component. The posts  17  are inserted into apertures  18  of the second component  12 . The apertures may be holes or slots. Any number of posts and corresponding holes or slots could be provided. The posts and apertures are preferably sized and configured to provide a snap fit. Preferably a cement is applied to the mating flat surfaces. A pair of transverse holes  16  is provided in component  12  that can also be used for sutures. The porous surface in the second component  12  allows for bone or soft tissue in-growth. The surgeon actually receives as a patella replacement device the base unit  11  and the bearing component  12  separated from one another. These components are secured together during the surgical procedure by the surgeon.  
         [0018]    All individuals are not the same size and not the same weight. Consequently, a single patella replacement device will not work for all people. In order to provide a complete range of patella replacement devices, one needs to accommodate men, women, and children of various heights and weights. The diameter and thickness of the devices will vary accordingly. Indeed, the devices may have an elliptical cross-section with the major and minor diameters being different among the various sizes of the device. For a small size, the minor diameter of the flat surface of each member may be 0.958 inches (24.3 mm) and the major diameter may be 1.099 inches (28 mm). The height or thickness of each member may vary from 0.276 inches (7 mm) to 0.709 inches (18 mm) to allow for different knee configurations. For a medium individual, the minor diameter may be 1.063 inches (27 mm), the major diameter 1.218 inches (31 mm), and the height may vary from between 0.276 to 0.799 inches (7 mm to 20 mm). For a large individual, the minor diameter may be 0.555 inches (14 mm) and the major diameter may be 1.336 inches (34 mm). For an extra large individual, the minor diameter may be 0.607 inches (15 mm) and the major diameter may be 1.454 inches (37 mm). The component height for both the large and extra large person varies between 0.276 inches (7 mm) to 0.709 inches (18 mm) which essentially is the variation in height of the size for the small individual. The position of the posts  17 , holes  18  and their diameters is the same for all sizes. The replacement device has a shape to conform to the patella of a typical user. A surgeon would measure the patient&#39;s knee area and select a patella insert having suitable dimensions for that patient.  
         [0019]    A second preferred embodiment is shown in FIGS. 5, 6 and  7 . In this embodiment  20  there is a first component  21  and a second component  22  similar to the two components  11  and  12  of the first embodiment shown in FIGS. 1 through 4. However, components  21  and  22  do not have a collar. Instead there is ring  24  that is positioned between components  21  and  22 . Ring  24  is can be fabricated from titanium, cobalt chrome or tantalum and contains suture holes  25  about its periphery. However, I prefer to make the ring of plastic because plastic will not cut the sutures or damage surrounding tissue as can happen with a metal ring. In this embodiment the first component  21  is polyethylene or other bio-compatible material and the second component  22  is a metal having a porous surface. Posts  27  associated with the polyethylene bearing unit  22  pass through apertures  26  in the ring  24  and are inserted into apertures (not shown) which are formed in the base unit  22 . These apertures could be holes or slots.  
         [0020]    The patella ring  24  could have a central aperture (not shown) which is of a diameter slightly less than a central flange portion (not shown) extending from member  21  or  22  similar to the collars  13  and  14  in the first embodiment. The ring could be force-fit over that region of the patella assembly by means of a press or a clamping system to provide an interference bond or fit. To provide an interference fit, the diameter of the aperture may be between one and four thousandths of an inch less than the diameter of the flange portion. The apertures  25  enable a physician to apply sutures to secure the ring and the patella insert to the knee area of a patient. Again, because people vary in height and weight, members  21  and  22  as well as the ring should be available in different sizes.  
         [0021]    The assembly as it reaches the physician, consists of the ring  24  together with member  22  assembled as a single unit with the ring attached to or force-fitted over member  22 . The polyethylene section  21  is separately supplied. All the units are ordered by the physician or the surgeon after measurements are made considering the size of the patient&#39;s knee and of course, considering the size and weight of the patient.  
         [0022]    Either of the first two embodiments could be modified as shown in FIGS. 2 and 5 to provide an intermediate portion  19  on the flat surface of either component. Intermediate portion  19  has a smaller diameter than the diameter of the flat surface. Consequently, there is a peripheral gap  40  formed when the two members  11  and  12  or  21  and  22  are joined together. This peripheral gap  40  enables the surgeon to wrap soft tissue in the gap to further secure the prosthetic device and to assure optimum coupling to the knee area.  
         [0023]    Although the second embodiment in a form containing the metal ring with a central aperture could be used, I have discovered that after such a device has been implanted, the ring can cut through the prosthesis. This can occur when movement of the patient causes the ring to move relative to the other members of the prosthesis. That movement creates a rubbing action that cuts into the prosthesis. Another problem that may occur when a metal ring is used happens when the knee is subjected to a radial force parallel with the flat surfaces of the ring. That force may cause the edge of the ring to cut surrounding soft tissue. Similar problems can occur with a plastic ring.  
         [0024]    The smooth top surface or dome of components  12  or  22  faces outward in the knee area of the patient. The porous surface of those components  12  and  22  enables bone and/or soft tissue growth into the device promoting better security. The plastic member  11  or  21  acts as an articulator surface against the patient&#39;s cartilage or prosthetic femoral replacement enabling the patient to resume normal activities and be pain free.  
         [0025]    The embodiments disclosed in FIGS. 1 through 7 could have a plastic component and a metal component. The second member  12  or  22  must be porous to allow bone and soft tissue to grow into the component and anchor the prosthesis. That objective cannot easily be obtained if the component is made of some metals or ceramics. Yet, I have discovered that it is possible to coat a metal or ceramic component with hydroxyapatite and thereby provide a porous structure into which bone and soft tissue can grow.  
         [0026]    A third present preferred embodiment  30  is shown in FIGS. 8 and 9. This embodiment has two components  31  and  32  that fit together in the same way as previous embodiments. That is, posts  37  in member  31  fit into apertures (not shown) in member  32 . In this embodiment, component  31  is made of a biocompatible plastic material, preferably polyethylene or a ceramic. Component  32  is metal or ceramic. A cement is applied to the flat surfaces of these components to bond them together. The second member  32  is coated with hydroxyapatite  34  to provide a porous surface that acts as a scaffold for tissue growth. After the device is implanted body fibers grow into the porous surface to secure the prosthesis. Suture holes  35  are provided around the perimeter of the second component  32  to permit the prosthesis to be initially secured in place. The sutures preferably are made of a material such as PLLA that decomposes to provide absorbable anchors. The hydroxyapatite coating is created by applying a plasma spray containing hydroxyapatite powder that bonds on impact with the surface of the plastic component. If desired, the hydroxyapatite coating could be placed on both components  31  and  32 , particularly if both components are metal or ceramic. Although I prefer to coat the entire curved surface of the component, less than the entire surface could be treated with hydroxyapatite.  
         [0027]    Those skilled in the art will recognize that hydroxyapatite is one of the several bone growth materials used by surgeons. Consequently, it should be possible to use other bone growth materials in place of or in combination with hydroxyapatite in the coating. Such other materials include human bone particles, bovine bone particles, ground coral and calcium sulfate.  
         [0028]    The present prosthesis is implanted using standard surgical techniques. The unit will function if a patella bone is completely removed from a patient or, if patella fragments that remain attached to the soft tissue post trauma are exposed and the loose fragments are removed; or if the primary patella component that failed is removed and a remaining patella bone shell is prepared to accept the device. The sutures will initially hold the first component in place enabling the component to act as a base. After the base is stable, the posts of one component are fit into the three holes in the other component. Trial articulation against the femur with the whole knee flexing and extending will be performed and patella tracking and joint tensioning is assessed. The implantable second patella component is then cemented into place. The surgeon holds the first component against the second component until the cement is cured and hardened. The joint is reduced and the incision is closed.  
         [0029]    I have implanted prototypes of the embodiment shown in FIGS. 1 through 5 in several patients. I also implanted in a patient a prototype of the embodiment of FIGS. 8 and 9 in which member  32  was titanium with a hydroxyapatite coating and the other member  31  was polyethylene. The patients were examined at regular intervals after surgery. The patients were able to regain nearly complete function of the knee joint and suffered no discomfort after the incision had fully healed.  
         [0030]    Although I have shown and described certain present preferred embodiments of my patella replacement device, it should be distinctly understood that the invention is not limited thereto, but may be variously embodied within the scope of the following claims.

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