Abstract:
Custom moldable cushions for covering medical implants fastened to a bone are provided. Such cushions inhibit irritation of the surrounding soft tissue by covering the bone-implant interface and by reducing friction caused by movement of soft tissue adjacent the implant. Such cushions may also be employed to spread and absorb forces reducing patient discomfort and risk of injury and infection associated with such implants.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    I. Field of the Invention 
         [0004]    The present invention relates to implanted orthopedic, maxillofacial, spinal and plastic surgery hardware. More specifically, the present invention relates to cushions which cover such hardware to cushion the surrounding tissue from such hardware. 
         [0005]    II. Related Art 
         [0006]    Orthopedic implants made of various materials are employed to provide adequate strength to resist biologic stress across a pathologic area during bone healing. Orthopedic implants are often made of metal such as titanium which can be forged or bent in a variety of ways to allow such implants to conform to a surface of the bone to some degree while maintaining the mechanical properties of the implant. Despite new advances in low profile implant technologies, metal implants are limited in their ability to match the contour of the underlying bone. The result is often a step-off at the bone-implant interface. The existence of such a step-off can cause a variety of issues. Such a step-off is often the site of discomfort for patients, particularly when skin, muscle, or other tissues are pushed against the step-off. In addition, the surfaces of metal implants are a source of friction between such implants and surrounding soft tissues which are constantly moving. Problems arising from such friction are most extreme when a tendon is overlying the implant. Such friction can not only cause painful tendonitis, but can also eventually lead to a rupture of the tendon. 
         [0007]    Screws are sometimes used alone or to fasten plates, rods, and other orthopedic implants. Whether used alone or to fasten another orthopedic implant in place, the screws are often prominent and painful. For example, in spinal fusion surgery screws are typically placed low in the back where soft tissue coverage is thin and pressures from the waist line of clothing and belts or furniture when sitting is significant. The result is often painful irritation. In extreme cases, the implanted hardware can erode through the skin causing dangerous ulceration requiring prolonged wound care and increased risk of infection. All too often secondary surgeries are required to remove the hardware, reposition the hardware or graft skin to treat such an ulcer. 
         [0008]    Issues of the type described above are not uniquely associated with orthopedic implants. Dental and maxillofacial implants used to treat or repair bones of the jaw and face and similar implants used in plastic surgery often result in a step-off at the bone-implant interface or in friction between the implant and soft tissue covering the implant. Such a step-off or friction can lead to substantial discomfort and in severe cases ulceration, infection, and damage to the surrounding tissue. 
         [0009]    Various attempts have been made to address step-offs at the bone-implant interface of plates used to treat bone fractures. Most involve efforts to alter the contours of the implant to reduce the step-off. Pre-formed plastic covers with tapered edges have also been employed to cover such plates. Neither of these two approaches have successfully or universally solved the significant problems which exist. The significant limitations of these two approaches becomes apparent after a surgeon modifies the plate shape to fit the affected bone. Bones have significant individual differences in shape. Failing to properly modify the shape of the plate to fit the bone can cause poor fracture healing if the bone fragments are not properly opposed. When the surgeon modifies the shape of the hardware to provide proper opposition of the bone fragments, the prominence of the hardware and the bone-implant step-off often becomes exaggerated. Also, once the shape of the plate has been modified, preformed plastic covers often no longer fit the implant. The application of a preformed plastic cover after the surgeon modifies the shape of the plate so the bone fragments are properly opposed is often not possible or the result is an unacceptably loose, insecure fit between the plate and the cover. For such a cover to be used and provide any benefit it must not only be securely locked to the plate so it does not float within the body, but it must also reduce the bone-implant step-off. While such covers can be designed to fit a plate of a particular shape and reduce the step-off, any change in the shape of the plate can result in the cover exacerbating rather than reducing this step-off. 
         [0010]    Various malleable polymers have been employed to fill bone voids and as bone graft alternatives. The use of such materials has broad application in the field of dentistry to fill voids in teeth. Such materials have also been used to act as a scaffolding for bone healing. These materials have different properties to allow for varying work times, stiffness, and strength. For a variety of reasons, such materials have rarely, if ever, been used to reduce friction between an implant and surrounding tissue or a step-off at the bone-implant interface. 
         [0011]    Other bio-absorbable polymers have been employed in the design of internal bandages to allow for guided tissue regeneration or to prevent adhesion of tissue at a surgical site during healing. These materials have not been employed to either reduce friction or to reduce a bone-implant step-off. The use of a bio-absorbable material would likely only temporarily reduce friction. Also, generation of tissue around an implant could make explantation more difficult and give rise to a number of other anatomical issues making the use of such materials undesirable. 
         [0012]    In view of the foregoing, there continues to exist a real, significant and long-felt need for an apparatus which successfully and nearly universally addresses the problems associated with a bone-implant step-off and at the same time reduces friction between the implant and surrounding tissue. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention provides various medical implant cushions comprising a bio-compatible, non-absorbable polymer having a first state and a second state. When the polymer is in its first state, the polymer is malleable in vivo so it can be formed into a desired shape to easily and securely seat the cushion onto the implant and provide a cushioning layer between an implant and the surrounding tissue which may otherwise be irritated by the contours of the implant. When the polymer is in its second state, the polymer retains its desired shape. The polymeric material of such an implant cushion can be shaped by the surgeon to cover any bone-implant step-off and provide a smooth, custom taper between the cover and the bone. In this way, a cushion made in accordance with the present invention minimized discomfort and the risk of irritation, injury, or infection caused by a prominent bone-implant step-off. 
         [0014]    The polymeric material(s) used in the construction of such an implant cushion can be lubricious or impregnated with a lubricant to reduce friction between the surfaces of the implant and surrounding tissue thereby reducing the risk that such tissue (such as a tendon or other soft tissue) will rupture or become irritated or ulcerated. The polymeric material used in constructing the cushion, when the polymer is in its second polymerization state, is also preferably mildly visco-elastic. As such, there is a more natural transition between the implant and surrounding natural tissue when felt through the skin from above and the cushion will absorb contact stress by deforming under applied pressure and then regaining its shape as the pressure is removed. 
         [0015]    Sometimes it will be desirable to remove the implant. Removal of the implant may be required to address infection, hardware breakage, or a subsequent injury adjacent the implant site. Thus, the implant cushion should be easily removable from the implant. Likewise, a radio-opaque strip or other identification codes can be included in the cushion to enhance patient safety and to enable accounting for the cushion during routine radiographic follow-up via X-ray or the like. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a perspective view showing two orthopedic implant plates used to repair a bone fracture. 
           [0017]      FIG. 2  is a perspective view showing an intramedullary nail device implanted in a bone with plurality of interlocking screws. 
           [0018]      FIG. 3  is a perspective view of an augmentation implant being employed to augment the bone structure of a chin. 
           [0019]      FIG. 4  is a perspective view of a first type of spinal fusion hardware. 
           [0020]      FIG. 5  is a perspective view of a second type of spinal fusion hardware. 
           [0021]      FIG. 6  is a perspective view of an implant cushion covering an orthopedic implant plate and an adjacent portion of the bone to which the implant plate has been attached. 
           [0022]      FIG. 7  is a cross-sectional view showing an implant attached to a bone using a screw and covered by an implant cushion. 
           [0023]      FIG. 8  is a perspective view showing the negative inner surface of the implant cushion of  FIG. 7 . 
           [0024]      FIG. 9  is a top plan view of another implant cushion. 
           [0025]      FIG. 10  is a bottom plan view of the implant cushion shown in  FIG. 9 . 
           [0026]      FIG. 11  is an exploded cross-sectional view showing an implant and an implant cushion. 
           [0027]      FIG. 12  is a cross-sectional view showing the implant and implant cushion of  FIG. 11 . 
           [0028]      FIG. 13  is a view showing a tool being used to create a tunnel between a bone and surrounding soft tissue. 
           [0029]      FIG. 14  shows a bone plate attached to the tool of  FIG. 13 . 
           [0030]      FIG. 15  shows the tool of  FIG. 13  being used to position the bone plate within the tunnel. 
           [0031]      FIG. 16  shows a cushion attached to the tool of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]      FIGS. 1-5  show representative examples of the types of implants employed by surgeons today.  FIG. 1  shows a bone  1  which has been fractured and a pair of orthopedic implant plates  10  and  12  employed for fixation of the fracture. Each plate has a plurality of holes  13  surrounded by a recessed flange. The plates are made of a suitable metal such as titanium. Screws  14  are used to fasten the plates  10  and  12  to the bone  1 . The screws  14  include a threaded shaft having a diameter smaller than the hole  13  and a head which engages the recessed flange surrounding the hole  13  to fix the plate  10  or  12  to the bone  1 . The bone-implant step-off associated with plate  10  is shown at  16 . The bone-implant step-off associated with plate  12  is shown at  18 . As shown, the shape of each of plates  10  and  12  has been modified to match the contours of the bone  1  and provide a proper alignment between the bone fragments on opposite sides of the fracture. 
         [0033]      FIG. 2  shows a rod  20  which has been placed within bone  1  to stabilize the bone. A plurality of screws  14  are used to secure the rod  20  in place. The screws pass through the bone  1  and the rod  20  at a variety of angles. The head of each screw  14 , and in some cases a portion of the threaded shaft of a screw  14 , projects from the surface of the bone providing a bone-implant step-off and an unnatural exposed surface. 
         [0034]      FIG. 3  shows a chin implant  24  attached to the bone of the chin  22  by a suitable adhesive. The chin implant  24  can be made of a suitable metal such as titanium or a medical grade plastic. As shown, there is a considerable step-off  26  at the bone-implant interface. 
         [0035]      FIGS. 4 and 5  show two styles of implants used in performing a spinal fusion. The implant shown in  FIG. 4  comprises two sets of plates  100  and  102  attached to bone using pedicle screws  103 . The implant shown in  FIG. 5  comprises a pair of rods  110  and  112 . Each rod is secured to bone using hooks  114  and pedicle screws  116 . Implants of the type shown in  FIGS. 4 and 5  are often positioned at the lower aspect of the spine near the patient&#39;s waist. This area of the body is typically subjected to a number of external forces such as by the waste band of clothing or furniture when sitting. Such forces cause the a soft tissue surrounding the implant to constantly rub across the surfaces and edges of the implant. Irritation, infection and ulceration of the surrounding tissue can result. 
         [0036]      FIG. 6  shows a first embodiment of a cushion  30  being employed to cover an orthopedic implant plate such as  10  or  12  in  FIG. 1 . The implant cushion  30  shown in  FIG. 3  has a main body  32  covering the implant (not visible) and the heads of the screws  14 . The implant cushion  30  also has an edge portion  34  which covers the step-off of the bone-implant interface and is feathered into the bone  1  to virtually eliminate any step-off at the cushion-bone interface  36 . 
         [0037]    The cushion  30  shown in  FIG. 1  is preferably made of a biocompatible polymer which is not absorbable by the human body. The polymer has a first state in which the polymer is malleable within the body (in vivo) so it can be formed into a desired shape such as that shown in  FIG. 6 . The polymer also has a second polymerization state in which the cushion retains its desired shape. Cushion  30  is shaped to provide a cushioning layer between the implant and the surrounding tissue which may otherwise be irritated by the contours of the implant. 
         [0038]    How the polymer used to construct cushion  30  transitions from its first state to its second state will depend on the polymer employed. Some polymers will make this state change at body temperature or when exposed to body fluids. Others will change from the first to the second state when exposed to light of predetermined wavelengths. The polymer employed to construct cushion  30  should either be lubricious or impregnated with a biocompatible lubricant. Suitable lubricants include, but are not limited to, lubricin and hyaluronic acid. When in its second state, the polymer selected should be somewhat visco-elastic. Using a visco-elastic material minimizes palpable differences between an implant covered with the cushion  30  and the native soft tissue adjacent to the implant when felt through the overlying skin. The use of a visco-elastic material will also absorb some of the shock when an external force is applied to the implant site thus offering some protection not only to the implant covered by cushion  30 , but also to the surrounding tissue. 
         [0039]    Various polymers can be employed in forming the cushion  30  and the invention is not limited to any particular polymer. The polymer could be any of a number of foam, hydrogel or malleable solid plastic materials. The polymer could also consist of a hydrolytic and oxidative stable thermoplastic polyurethane like a poly (dimethyl siloxane) derivative, or a cross-linked hyaluronan of adequate durability. 
         [0040]    To permit removal of the cushion  30  from the implant, the implant can be formed with one or more scored channels on the under surface of the cushion  30  and corresponding markings identifiable on the outer surface which allow the cushion  30  to be split for removal. For alignment, location, or identification purposes, a radio-opaque material may be embedded in the implant cushion  30 . 
         [0041]      FIGS. 6 and 7  are provided to illustrate the cushion  30  being employed in conjunction with an implant plate  12  which has been attached to a bone  1  with screws  14 . As shown, the cushion  30  has a first portion  32  covering the plate  12 , a second portion  34  covering adjacent portions of the bone  1 , and a cushion-bone interface  36  providing a smooth, feathered transition between the bone  1  and the cushion  30 . 
         [0042]      FIG. 7  shows the molded negative under surface  37  of the cushion  30 . As shown, the molded negative under surface  37  has a contour  38  matching the contours of the head of the screw  14  used to attach the plate  12  to the bone  1 . The molded negative under surface  37  also has a main body portion  40  matching the shape of the plate  12 , and contours  42  matching the shape of the adjacent portions of bone  1 . These contours assist in securing the cushion  30  to the implant. 
         [0043]    While  FIGS. 6-7  illustrate how the cushion  30  forms a protective layer over an implant comprising a plate  12  and screws  14 , the implant can take other forms and the cushion  30  can be molded in a similar fashion to such implants. For example, the implant can simply comprise screws such as  14  or an implant such as the implant shown in  FIG. 2 . In such cases, the molded negative under surface of  37  may be as shown in  FIG. 8 . As illustrated in  FIG. 8 , the molded inner surface  37  has contours  38  and  39  matching the contours of the heads of two screws, a main body portion  40  covering the area of bone separating the screw heads and an edge portion  42  forming a smooth, feathered transition between the cushion  30  and the bone. The cushion  30  forms a protective (e.g., lubricious) layer not only over the implant, but also over an edge of the implant and the adjacent exposed portion of the bone, irrespective of the type of implant, to reduce any trauma associated with the bone-implant interface. Likewise, the implant can be any augmentation implant. One example of an augmentation implant is the implant  24  shown in  FIG. 3 . When used with an augmentation implant, the contours of the cushion will be dictated by the shape of the implant  24 , the adjacent portions of bone  22 , the bone-implant interface  26 , and often cosmetic considerations. 
         [0044]    From the foregoing, those skilled in the art will appreciate the thickness of the cushion  30  will not be uniform. The surgeon can mold the implant cushion over the implant after the implant has been installed as the needs of the patient dictate. The implant will tend to be thinner across flat surfaces with no changes in elevation and tend to be thicker in areas where the contours of the implant or bone-implant interface present elevational changes. In some cases, the surgeon may elect to thicken the cushion for cosmetic purposes or to provide extra cushioning in areas subjected to substantial external pressure. By way of example, spinal fusion surgery implants such as those shown in  FIGS. 4 and 5  typically include at least one plate rod screwed in place near the base of the spine. In this area, the surgeon may elect to increase the thickness of the cushion to absorb and spread over a larger area outside forces associated with sitting. 
         [0045]      FIGS. 9-11  illustrate an alternative embodiment of a cushion  60  made in accordance with the present invention. 
         [0046]    Cushion  60  is ideally suited for application to an orthopedic implant such as plate  12 . The cushion  60  has a center portion  62 . Surrounding the center portion  62  is an edge portion  63 . The center portion  62  and the edge portion  63  can be constructed of the same material or different materials. The center portion  62  and the edge portion  63  each should be made of a bio-compatible, non-absorbable polymer. The polymer used to form the edge portion  63  should have a first state in which the polymer is malleable in vivo so it can be formed into a desired shape and a second state in which the polymer retains its desired shape. When the center portion  62  is constructed of the same material as the edge portion  63 , the material of the center portion  62  (but not the material of the edge portion  63 ) is converted to the second state during manufacture or otherwise prior to implantation. The material of the edge portion  63  is preferably not converted to the second state until after the surgeon has had the opportunity to mold the outer portion over the bone-implant interface and adjacent portions of the bone. 
         [0047]    As shown in  FIG. 9 , the top of the cushion  60  is marked with an indexing indicia  65  and with identification indicia  66  and  67 . The identification indicia can include, for example, the name of the manufacturer  66  and a serial number  67 . The identification indicia  66  and  67  are preferably printed using an indelible radio-transparent material so as to not interfere with X-ray evaluation of the bone and soft tissue. The indexing indicia  65  should be radio-opaque. This permits orientation of the cushion to be ascertained via an X-ray after implantation. The indexing indicia  65  should be in the form of a thin strip extending along the length of the cushion  60  so the indicia  65  may be detected without unduly inhibiting X-ray evaluation of the bone and soft tissue. The top of cushion  60  also has a pair of orifices  68  which are engaged by a surgical tool (see  FIG. 16 ) to grip the cushion and then position the cushion  60  in vivo over the plate  12  to which the cushion  60  is then attached. 
         [0048]      FIG. 10  shows the bottom of the cushion. Again, the center portion  62  and the edge portion  63  are shown. The edge portion  63  is made of a biocompatible, non-absorbable polymer left in its first state until after the cushion  60  has been secured to the plate  12  and the surgeon has molded the edge portion  63  into the desired shape. The edge portion  63  then transforms into its second state by, for example, further polymerization of the material due to temperature, light or some other means. Projecting from the negative under surface  69  of the center portion  62  is a pair of button tabs  70  and  72 . These are used to assist in providing a secure connection between the cushion  60  and the implant plate  12 . While two such button tabs  70  and  72  are shown, those skilled in the art will recognize the advantages associated with using only one such button tab (e.g., when the shape of the plate  12  has changed substantially during implantation) or more than two button tabs (e.g., to provide a more secure fixation of the cushion to the plate or when a single cushion  60  is to be secured to a plurality of plates). The bottom of cushion  60  also includes one or more scorings or channels  64 . Two such channels are shown in  FIG. 10 . These channels assist with removal of the cushion  60  in the event removal becomes necessary. 
         [0049]      FIGS. 11 and 12  show not only the cushion  60 , but also the plate  12 . As shown, the button tab  70  has a shaft  72  extending from the bottom of the center section  62  of the cushion  60 . The button tab  70  also has a projection  74  extending outwardly from the shaft to provide a flange  76 . To attach the cushion  60  to the plate  12  after the plate  12  has been fastened to a bone, the surgeon uses an elongate surgical tool (not shown) which grips the center section  60  using the orifices  68  and aligns the cushion  60  so that the button tab  70  is positioned adjacent the hole  80  in the plate  12 . A force is then applied to the cushion  60  to move the projection  74  of button tab  70  through hole  80  so that the flange  76  of the projection  74  engages the flange  82  of hole  80  to lock the cushion  60  to the plate  12 . This can be done using the surgical tool or by manually applying pressure to the cushion  60 . With the surgical tool removed and the button tab  70  inserted into hole  80 , the surgeon molds the edge portion  63  into the desired shape. Finally, the edge portion  63  is converted from its first state to its second state to fix the shape of the edge portion  63  and further secure the cushion  60  to the implant plate  12 . Causing the material of the edge portion  63  to transition to the second state can be done in a variety of ways depending on the material used. Examples include exposing the material to the light of a specific wavelength range or exposing the material to body temperature. 
         [0050]    When shaping the outside portion  63 , the surgeon should use the outside portion  63  to form a protective layer over the edge of the plate  12  and an adjacent exposed portion of the bone. In this fashion, the cushion  60  prevents irritation by surrounding tissue contacting the bone-implant interface. To prevent irritation due to friction, both the center portion  62  and the edge portion  63  can be made of a lubricious material or impregnated with a lubricant such as lubricine or hyaluronic acid. To minimize palpable differences between the plate  12  covered with the implant cushion  60  and native soft tissue adjacent to the implant when felt through the skin, the material(s) used to form the center portion  62  and edge portion  63  may be visco-elastic. Materials used having a first and second state should be visco-elastic at least when the material is in its second state. As should be clear from the foregoing, the center portion  62  may be formed into a predetermined fixed shape prior to implantation of the cushion  60  and the second edge portion  63  is formed into a desired shape after implantation. This is important to provide an overall shape that provides suitable cushioning because the shape of the implants being cushioned are often modified and adjusted during implantation to accommodate the shape and location of the skeletal anatomy to which the implant is attached and achieve the desired medical benefit. 
         [0051]      FIGS. 13-16  will now be referenced to describe a method of inserting a plate beneath the skin and muscle known as minimally invasive plate osteosynthesis (MIPO). MIPO may also be employed to fasten the cushion  60  to a plate. An advantage of MIPO is that large dissection and the associated disruption of healing potential is avoided. 
         [0052]    When MIPO is performed, a long insertion tool, such as epiperiosteal elevator  120 , is used to create a submuscular tunnel  122  adjacent a bone  124  to be treated as shown in  FIG. 13 . The plate  126  to be implanted can then be temporarily secured to the epiperiosteal elevator  120  as shown in  FIG. 14  and introduced from either the proximal or distal end of the tunnel  122  depending on the nature of the fracture being treated and the preference of the surgeon. Using the epiperiosteal elevator  120 , the plate  126  is positioned adjacent the bone at the desired location and fastened to the bone. The epiperiosteal elevator  120  is then detached from the plate  126  and retracted. When the cushion  60  is to be employed, the cushion  60  can then be attached to the epiperiosteal elevator  120  as shown in  FIG. 16 , introduced through either end of the tunnel and advanced through the tunnel to a position beneath the muscle and adjacent the top of the plate  126 . The cushion  60  is then attached to the plate  126  as described above and detached from the epiperiosteal elevator  120  which is then retracted. 
         [0053]    Those skilled in the art will recognize that various modifications and enhancements to the embodiments described above can be made without deviating from the invention. Likewise, alternative embodiments not described above incorporating the invention may be employed. Thus, the foregoing disclosure is not intended to be limiting. The scope of the invention is only limited by the claims which follow.