Abstract:
A method and apparatus for reducing the incident of infection proximate to an exit site where a prosthesis fastener, e.g., a bone screw, percutaneously penetrates a patient&#39;s skin/soft tissue. Infection reduction is achieved in accordance with the invention by growing a tissue integrating seal around the fastener proximate to the exit site.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to medical technology and more particularly to a method and apparatus for minimizing marsupialization, i.e., pouch formation, and infections associated with the percutaneous projection of a fastener, e.g., bone screw, used to attach a prosthesis. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various techniques are known for attaching a prosthesis to a patient&#39;s natural bone. Such techniques often involve the use of a screw having a head end adapted to be secured to the prosthesis and a point end adapted to be threaded or otherwise fixed into the patient&#39;s bone. To achieve reliable initial and long term fixation, it has been proposed, e.g., U.S. Pat. No. 5,360,448, to apply a porous coating to the screw shaft designed to accept bone ingrowth. 
         [0003]    In an exemplary application, the screw head can be secured to a leg prosthesis and the screw point can be threaded into the patient&#39;s thigh bone, or femur. In such, and similar applications, the screw shaft projects percutaneously through the patient&#39;s outer skin layers and subcutaneous soft tissue. Over time, these skin layers grow over the natural bone but frequently produce sinus tracts around the screw shaft which are prone to marsupialization and infection, and which can lead to serious cases of osteomyelitis. 
         [0004]    U.S. application US2004/0204686 A1 published 14 Oct. 2004, incorporated herein by reference, describes a use of porous material carried on a percutaneously projecting stud of an implantable device for promoting tissue ingrowth to avoid the formation of sinus tracts, form an infection and marsupialization resistant barrier, and enhance device anchoring. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is directed to a method and apparatus for reducing the incidence of marsupialization and infection proximate to an exit site where a prosthesis fastener, e.g., a bone screw, percutaneously penetrates the patient&#39;s skin/soft tissue. Marsupialization and infection reduction is achieved in accordance with the invention by growing a tissue integrating seal around the fastener proximate to the exit site. 
         [0006]    An apparatus in accordance with the invention includes a fastener having a shaft extending between a first end adapted to be secured to a patient&#39;s bone and a second end adapted to be secured to an external prosthesis. In accordance with the invention, one or more bands, or layers, of porous material are mounted around the fastener shaft in a position to contact the patient&#39;s outer skin layers and subcutaneous soft tissue when the fastener is installed in the patient&#39;s bone. The band of porous material, preferably titanium mesh, promotes skin/soft tissue ingrowth which, over time, forms a seal, or barrier, to prevent the incursion of pathogens around the shaft into the patient&#39;s body. The seal thus serves to minimize the risk of infections. 
         [0007]    A porous band in accordance with the invention preferably comprises multiple fibrous layers of biocompatible materials such as metals, including titanium, nitinol, silver, and stainless steel, etc. or polyolefins including Teflon, nylon, Dacron, and silicone, etc. The fibrous layers can be directly wound onto a portion of the fastener shaft and attached thereto by a suitable technique, e.g., adhesives, welding, brazing, etc. Alternatively, the porous band can be separately fabricated, e.g., of fibrous or sintered metallic or polymeric material, and then attached to the fastener shaft either mechanically or by a suitable adhesion technique including adhesives, welding, brazing, etc. To properly promote skin/soft tissue ingrowth, the porous band preferably has a porosity within a range of 60 to 95% and contains pores within a size range of 50 to 200 microns. 
         [0008]    In a preferred embodiment of the invention, the material forming the porous band incorporates appropriate substances to promote healing, reduce inflammation, and resist infection. These functions are particularly important in the initial period after fastener implantation to promote tissue ingrowth into the porous band. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]      FIG. 1  is a side sectional view showing a bone screw carrying a porous band in accordance with a first embodiment of the invention for attaching a limb prosthesis to a patient&#39;s bone, e.g., femur; 
           [0010]      FIG. 2  is a side sectional view showing a second embodiment of the invention including a flange carried on the screw shaft for positioning the porous band; 
           [0011]      FIG. 3  is a side sectional view showing a third embodiment in which the band is configured to provide perpendicular porous surfaces for enhancing tissue integration; 
           [0012]      FIG. 4  is a side sectional view showing a fourth embodiment in which the bone screw major shaft transitions into multiple minor shafts for attaching a limb prosthesis to a patient&#39;s bone; 
           [0013]      FIG. 5  is a bottom sectional view taken substantially along the plane  5 - 5  of  FIG. 4 ; and 
           [0014]      FIG. 6  is a side sectional view showing a fifth embodiment incorporating bone support members. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Attention is initially directed to  FIG. 1  which illustrates a fastener  10  for attaching an external prosthesis  12  to a patient&#39;s natural bone  14 . Although the teachings of the invention are applicable to a variety of external limb prosthesis devices, in the exemplary application assumed herein, the prosthesis  12  comprises a leg prosthesis and the patient&#39;s bone  14  comprises a thigh bone, or femur. 
         [0016]    The fastener  10  can comprise a substantially conventional bone screw  18  having a shaft  20  extending between a first, or point end,  22  and a second, or head end,  24 . The shaft  20  is appropriately threaded  26  for screwing into the bone  14 . The prosthesis  12  is schematically illustrated only to the extent necessary to represent that the head end  24  is configured for attachment to a prosthesis plate  27 , e.g., by being seated in prosthesis recess  28  or being otherwise affixed, as by welding or bolting. 
         [0017]    Note in  FIG. 1  that the fastener  10  projects percutaneously through the patient&#39;s subcutaneous soft tissue  30  and outer skin  32  layers. There is significant risk of infection when conventionally implanting a percutaneous screw into a patient&#39;s bone because the skin can grow longitudinally along the shaft such that sinus tracts and infections can develop around the percutaneous penetration site. To reduce the risk of infection, a tissue integrating seal  34  is formed in accordance with the invention at the penetration site. 
         [0018]    More particularly, in accordance with the invention, a band  40  of porous material  42  is mounted on the shaft  20  longitudinal surface  43  at a location to contact the soft tissue  30  and outer skin  32  when the fastener  10  is fully threaded into bone  14 . The porous band  40  preferably formed around a stiff core member  44 , e.g., a flange or other appendage to shaft  20 , is configured to promote the ingrowth of outer skin and/or soft tissue into its pores. In order to effectively do this, the porous material  42  preferably has a porosity in the range of 60 to 95% and pores within a size range of 50 to 200 microns. 
         [0019]    The porous band  40  is preferably formed of material wrapped around core member  44 . Alternatively, the band  40  can be separately fabricated and then mounted and affixed to the shaft. The band  40  is preferably formed of a fibrous and/or sintered metal, material, e.g., a suitable biocompatible metal such as titanium, nitinol, nickel, platinum, silver, tantalum, or stainless steel. Alternatively, a polyolefin such as Teflon, nylon, Dacron, or silicone can be used. The band  40  can be affixed to the shaft  20  by various mechanical means and/or suitable adhesion techniques such as welding, brazing, adhesives, etc. 
         [0020]      FIG. 2  illustrates an alternative embodiment in which the shaft  20  carries a mounting flange  45 . The flange  45  can be fixedly or adjustably mounted on the shaft  20  to facilitate positioning of the porous band  40  along the shaft longitudinal surface  43 . Additionally, the flange  45  provides structural support for the porous band  40 . 
         [0021]    Attention is now directed to  FIG. 3  which is similar to  FIG. 1  but shows a modified porous band  60 . The porous band  60  is wrapped around core member  61  to form a step  62  to define both a longitudinally oriented porous surface  64  and a laterally oriented porous surface  66 . The provision of perpendicular porous surfaces  64 ,  66  helps to facilitate the growth of an optimally integrated tissue seal around the shaft. The porous band  60  can of course also be advantageously used with the flange  45  of  FIG. 2 . 
         [0022]    Attention is now directed to  FIGS. 4 and 5  which illustrate a specially configured bone screw  70  having a major shaft  72  extending between a first, or bone end  74  and a lateral surface  75 . Multiple minor shafts  76  extend from the lateral surface  75  to a second, or prosthesis end  77 . The major shaft  72  is appropriately threaded at  78  for screwing into the bone  79 . By transitioning the major shaft  72  into multiple smaller shafts  76 , ( FIG. 5 ,  76   1 ,  76   2 ,  76   3 ), the screw  70  can be attached to an external prosthesis with less trauma to the surrounding tissue. In the embodiment of  FIGS. 4 and 5 , porous material  82  is preferably affixed to the lateral surface  75  and additionally porous material  83  is mounted along the longitudinal surfaces of minor shafts  76 . 
         [0023]    Attention is now directed to  FIG. 6  which shows a further alternative embodiment wherein the shaft  20  carries a bone supporting structural member  90  (e.g. a cup or multiple prongs). The structural member  90  is preferably comprised of lateral support elements  91  and/or longitudinal support elements  92  for bearing against the periphery of bone  14  to support and strengthen the bone to avoid cracking.  FIG. 6  also shows shaft  20 , affixed at its lower end  93  to a plate  94  configured for attachment to the external prosthesis. 
         [0024]    From the foregoing, it should be clear that a method and apparatus have been described for attaching a prosthesis to a patient&#39;s bone. The method and apparatus utilize a bone fastener carrying a band of porous material for promoting skin and soft tissue ingrowth to form a barrier around the fastener shaft for minimizing the risk of marsupialization and infection. The porous material preferably has a porosity in the range of 60 to 95% and pore size in the range of 50 to 200 microns. Although only a limited number of embodiments have been disclosed, it is recognized that various modifications will occur to those skilled in the art which fall within the intended scope of the appended claims.