Abstract:
Apparatus is described for creating a direct mechanical connection between skeletal bone and a prosthetic device located outside of the body. The apparatus provides a means for creating an effective biologic seal to prevent the transmission of microbiologic particles into the body.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to the medical field, and more particularly to apparatus for creating a direct mechanical connection between skeletal bone and a structure located outside overlying soft tissues, wherein the mechanical connection passes through the overlying soft tissues and creates a biologic barrier between the inside and the outside of the overlying tissues, or between the bone and the outside of the body. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the treatment of certain medical conditions it is desirable to provide a mechanical connection between an external structure and the patient&#39;s skeletal bone. Examples include external prostheses, such as artificial limbs or teeth, and external fixation pins used in reconstructive orthopedics. In each of these cases, it is desirable to create a direct mechanical linkage between a device external to the body and bone inside the body. This linkage can be characterized as having three functional components: a first part that is anchored to the bone; a second part that passes through the mucosal or cutaneous surface of the body; and a third part that is mechanically linked to the prosthesis or other device outside of the body. 
         [0003]    Various devices intended for fixation to bone are known in the art, including artificial joint replacements, metal plates used to repair bone, anchors for attaching ligaments or tendons to bone, and dental implants. Typical examples of such devices are the OmniFit™ EON® hip joint replacement (Stryker Orthopedics, Mahwah N.J.), the NexGen® knee joint replacement, the Statak® soft tissue attachment anchor, (Zimmer Inc, Warsaw Ind.), and various dental implants (Nobel Biocare AB, Gothenburg Sweden; Institut Straumann AG, Waldenberg Switzerland; Zimmer Dental Inc, Carlsbad Calif.; Dentsply International Inc., York, Pa.; Biohorizons, Birmingham Ala.). 
         [0004]    Three different techniques are commonly used to fix devices to bone: cement; mechanical interconnections using friction (screws, staples); and materials that promote growth of bone into the device. The Duracon® knee joint (Stryker) is an example of a device that is fixed to bone with cement, wherein the cement interlocks with the surface structure of the bone and with the surface structure of the implant to provide fixation. The NaturalHip T™ (Zimmer Inc) is an example of a device that has a surface intended to promote bone growth into the implant, creating a direct connection between the bone and the implant. Most dental implants and soft tissue anchors use some variation of screw intended to thread directly into the bone. 
         [0005]    All of these techniques, when properly used, provide a reliable mechanical connection between the prosthetic device and bone. 
         [0006]    There are many techniques suitable for connecting a prosthetic to an implant, including interference fits, tapers, threads, cements or adhesives. Dental implants typically use threads or cement to attach the ceramic or metal and ceramic to the part of the implant fixed to the bone. 
         [0007]    Implants that pass through the mucosal or cutaneous tissues (such as dental implants, percutaneous access devices or orthopedic external fixation pins) offer a significant risk of infection. Tissue in contact with the implant can usually be disrupted with minimal force, providing a channel through which infectious microorganisms may pass into the body. 
         [0008]    It is well known that the stimulation of fibrous in-growth into an implant where the implant passes through soft tissue to the outside of the body can reduce infection. The presence of attached tissue, (such as gingival tissue in the case of a dental implant) significantly improves the biologic seal around the implant, thereby inhibiting the access of microorganisms to the tissue and bone around the implant. It is also recognized that increased blood supply to the tissue near the implant leads to increased white blood cell concentration and a commensurate reduction of infection around the implant site. Furthermore, in the case of dental implants, promotion of fibrous in-growth to the implant more closely mimics the natural connection between the tooth and jaw. 
         [0009]    A suitable means for promoting the in-growth into that portion of an implant that passes through mucosal or cutaneous tissue is to construct the implant, or at least that part of the implant intended to be in contact with tissue, of a material that promotes such in-growth. It is known in the art to provide such a means in the case of percutaneous access devices (e.g. U.S. Pat. No. 4,897,081), through the use of biocompatible polymer material. Although this approach is suitable for relatively short-term applications, such polymers do not offer the high strength and long life required in the case of implants that provide a mechanical linkage between the skeleton and a device outside the body. 
         [0010]    It is also known in the art to provide an implant with a region of porous metal into which soft tissue can grow. For example, U.S. Pat. No. 3,855,638 discloses a porous region comprising small, discrete particles of metallic material bound together at their points of contact for promoting such tissue ingrowth. Examples of porous metallic materials described in this patent include austenitic stainless steel, titanium, titanium alloys and cobalt alloys, and the cobalt alloy called Vitallium™. 
         [0011]    There remains a need for improved materials and apparatus for promoting tissue ingrowth in connection with implant devices. 
       SUMMARY OF THE INVENTION 
       [0012]    The subject invention provides an implant intended for connecting a device located outside the body to skeletal bone, including a means for promoting the in-growth of tissue into that portion of the implant that passes through mucosal or cutaneous tissues, in order to provide a biologic seal around the implant. The means for promoting in-growth consists of providing a region of trabecular metal, such as titanium or tantalum, in that portion of the surface of the implant that is intended to come into contact with mucosal or cutaneous tissues. 
         [0013]    In another aspect of the invention, means is provided to remove and replace that portion of the implant disposed to promote the in-growth of tissue, such that the in-grown portion can be removed and replaced should it become infected, without the need to remove the entire implant. 
         [0014]    An implant in accordance with the invention may take several forms. A dental implant in accordance with the invention provides a separate, removable section of trabecular metal connected to a first portion intended for connection to bone and removeably connected to a second portion intended for connection to a tooth prosthesis. An orthopedic external fixation pin in accordance with the invention provides a region of trabecular metal where the pin passes through the cutaneous tissue. An implant for connecting a limb prosthesis to a person&#39;s body in accordance with the invention provides a portion disposed for connection to bone, a second portion disposed for connection to a limb prosthesis, and a region of trabecular metal where the implant passes through the cutaneous tissue. 
         [0015]    In an alternative embodiment, a dental implant in accordance with the invention provides a section of trabecular metal intended for connection to gingival and connective tissue, connected to a portion intended for connection to a tooth prosthesis. In this alternative embodiment, the dental implant does not connect directly with the bone, but instead is held in place by attachment of the underlying tissues, advantageously mimicking the fixation of natural teeth. 
         [0016]    Trabecular metals are utilized in accordance with the current invention to promote tissue ingrowth. Trabecular metals provide an approximately dodecahedral pore structure, having highly connected (“open cell”) porosity that promotes a high degree of tissue linkage through the pores, intertwining the ingrown tissue with the trabecular metal structure. This provides a strong connection between the soft tissue and the metallic structure and promotes vascularization of the ingrown tissues. 
         [0017]    An implant that disposes a material a trabecular metal in the region of the implant that passes through gingival, mucosal or cutaneous tissue promotes strongly attached fibrous in-growth of the tissue, thereby reducing the risk of infection of the underlying tissue and bone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The foregoing and other objects, features and advantages of the present invention will become apparent upon reference to the following detailed description of the preferred embodiments and to the drawings, wherein: 
           [0019]      FIG. 1  is a representation of a dental implant in accordance with the invention. 
           [0020]      FIG. 2  is a cross-section illustration of the dental implant of  FIG. 1  as it would appear when implanted. 
           [0021]      FIG. 2A  is a cross-section illustration of an alternate embodiment of the dental implant of  FIG. 1  and  FIG. 2 . 
           [0022]      FIG. 3  is a cross-section illustration of an another alternate embodiment of the dental implant of  FIG. 1  and  FIG. 2 . 
           [0023]      FIG. 3A  is an alternate embodiment of the dental implant of  FIG. 3 . 
           [0024]      FIG. 4  is an illustration of an orthopaedic external fixation pin in accordance with the invention. 
           [0025]      FIG. 5  is an illustration of an implant intended for connecting an artificial limb to a person&#39;s skeleton. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    With reference to  FIG. 1 , an embodiment of the invention is illustrated in the form of a dental implant  10 . Dental implant  10  is a generally cylindrical object made of a metal such as stainless steel, titanium, tantalum or an alloy of titanium and tantalum and similar metals. Proximal portion  12  of implant  10  provides features to promote secure attachment to bone. In  FIG. 1 , bone attachment is facilitated with screw thread  14 ; however, other means for attaching implant  10  to bone are possible without departing from the current invention. Other means for attaching implant  10  to bone include the provision of a convoluted region for attachment of cement, or provision of a region of trabecular metal for promoting the in-growth of bone. 
         [0027]    Distal portion  16  of implant  10  provides for attachment of a prosthetic tooth. Such attachment is achieved with the use of cement, although other attachment means may be used, such as an interference fit, taper fit, threading or other means. 
         [0028]    Intermediate portion  18  of implant  10  is made of trabecular metal, preferably titanium or tantalum, having pore sizes in the range of 100 to 800 microns. In the preferred embodiment, the trabecular metal is formed directly on implant  10  using a chemical vapour deposition technique that allows deposition of trabecular metal on selected regions of a substrate. (e.g. Hedrocel™ Implex Corporation, Allendale, N.J.), Other techniques for forming trabecular metal are known in the art, such as carbon dioxide injection (U.S. Pat. No. 6,759,004), moulding (e.g. U.S. Pat. No. 6,221,447 or U.S. Pat. No. 5,958,314) or sintering (U.S. Pat. No. 6,674,042). As noted earlier, trabecular metals such as those advantageously used herein provide an approximately and substantially dodecahedral pore structure, having highly connected open cell porosity that promotes a high degree of tissue linkage through the pores, intertwining the ingrown tissue with the trabecular metal structure. This provides a strong connection between the soft tissue and the metallic structure and promotes vascularization of the ingrown tissues. 
         [0029]    Referring to  FIG. 2 , proximal portion  12  of implant  10  is embedded into bone  20 , such that intermediate portion  18  is in contact with gingival tissue  22 . Prosthetic tooth  24  (or other dental bridge work) is attached to distal portion  16  of implant  10 . Gingival tissue  22  grows into the trabecular metal that makes up intermediate portion  18 , creating a circumferential, neo-vascularized biologic seal around implant  10 . As used herein, the terms “biologic seal” and “biologic barrier” refer to a condition that mimics or approximates the barrier conditions to microbiological invasion that exist in normal, undisturbed tissue of a similar type. Thus for example, the term biologic seal is not meant to characterize an absolute barrier to microbiologic organisms crossing between external tissues and internal tissues. Instead, the term refers to a condition at the area of tissue ingrowth that inhibits microbiologic invasion, but which does not necessarily define an absolute barrier. 
         [0030]    As illustrated in  FIG. 2 , the intermediate portion  18  is in contact with tissue between bone  20  and the external surface of the tissues and the tissue ingrowth into the intermediate portion provides an effective biologic barrier to inhibit invasion of microbiologics into internal tissues. Stated another way, intermediate portion  18  extends through the interface between internal tissues and the external surface of overlying tissues. 
         [0031]      FIG. 2A  is an alternate embodiment of the dental implant of  FIG. 1  and  FIG. 2 , which has the advantage of mimicking the fixation of natural teeth. In this embodiment, proximal portion  12  of implant  10  is loosely fitted into socket  13  formed in bone  20 . Implant  10  is held in place by the in-growth of gingival tissue into intermediate portion  18 . This provides a fixation that permits prosthetic tooth  24  to move slightly with respect to bone  20  in response to externally applied forces. Such motion serves to transfer mechanical loads on tooth  24  to adjacent natural or prosthetic teeth, reducing the likelihood of mechanical failure. 
         [0032]      FIG. 3  illustrates a longitudinal cross section view of an alternate embodiment of dental implant  10 . In this embodiment, distal portion  12  of implant  10  is a separate piece, having a threaded socket  30 . Intermediate portion  18  has a threaded stud  32  which mates with socket  30 . It will be appreciated that the socket  30  and stud  32  may be fastened together in other ways, including for example by cementation, mechanical thread interlock or other means. The periphery of intermediate portion  18  is made of trabecular metal  34 . Intermediate portion  18  also includes socket  36 , to which stud  38  of distal component  16  may be connected and fastened in place using cement, threads or other means. This alternate embodiment provides the option for removal of distal component  16  from intermediate component  18 . This permits the detachment of component  18  from any in-grown gingival tissue with minimal gingival tissue loss, the removal of intermediate component  18  from proximal component  12 , and the replacement of intermediate component  18  with a new component or temporary spacer for subsequent tissue management. This may be required if intermediate component  18  becomes seeded with bacteria and is infected. 
         [0033]      FIG. 3A  shows a longitudinal cross section of an alternate embodiment of the dental implant of  FIG. 3 . In this embodiment, the implant is made of a first part having proximal end  92  disposed for fixation to bone and a distal part  94  disposed for connection to a prosthetic tooth or bridgework. Annular ring  90 , made of trabecular metal is removeably fitted around distal part  94 . This permits detachment of ring  90  from any in-grown gingival tissue with minimal gingival tissue loss, the removal of ring  90 , and the replacement of ring  90  with a new component or temporary spacer for subsequent tissue management. Advantageously, this alternate embodiment of the dental implant provides a single mechanical link between the prosthesis and the bone into which the dental implant is inserted. 
         [0034]      FIG. 4  illustrates an orthopaedic external fixation pin according to the invention, shown as it might be implanted for use. Fixation pin  50  is anchored into bone  52  at proximal end  54 , such that intermediate portion  56  is in contact with cutaneous and subcutaneous tissue  55  and underlying muscle tissue  53 . Intermediate portion  56  is made of trabecular metal, preferably titanium or tantalum. Distal portion  58  of fixation pin  50  is disposed for connection to external fixation device  60  with connection means  62 . 
         [0035]    Once implanted as shown, cutaneous and subcutaneous tissue  53  grows into the trabecular metal of intermediate portion  56 , creating a biological seal. 
         [0036]      FIG. 5  shows a partial para-sagital section of an implant for connecting a prosthesis to an amputated limb. Implant  70  is made up of proximal portion  72 , which is rigidly connected to bone  74  with cement or other fastener system as described above Intermediate portion  76  of implant  70  is made of trabecular metal preferably titanium or tantalum. Distal portion  78  of implant  70  is disposed for rigid connection to artificial limb  80 . 
         [0037]    When implanted as shown, cutaneous and sub-cutaneous tissue  84  and muscle tissue  82  grow into intermediate portion  76 , forming a biologic seal. 
         [0038]    Many different configurations of the implant apparatus described herein may be constructed without departing from the scope and spirit of the present invention, therefore the present invention should be limited only by the scope of the appended claims. For example, an implant in accordance with the invention could be an entirely trabecular metal component having an attachment means for the external prosthesis. In another possible embodiment, a dental implant in accordance with the invention could be constructed largely of materials other than metal, while providing that the portion of the implant passing through the gingival tissue is made of a material that promotes in-growth of gingival tissue.