Abstract:
The present invention relates to a tissue grafting material that includes a first tissue element harvested from an individual, a carrier, and a therapeutic substance contained in the carrier. The carrier and therapeutic substance are incorporated into the first tissue element and the first tissue element, carrier, and therapeutic substance form a unitary implant.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Tissue grafting, including bone grafting, is well known. Tissue such as bone is removed from one part of a body (the donor site) and inserted into tissue in another (the host site) part of the same (or another) body. It is desirable to be able to remove a piece of tissue graft material which is the exact size and shape needed for the host site where it will be implanted. However, it is rarely possible to do this.  
           [0002]    Accordingly, various tissue grafting techniques have been tried to solve this problem. For example, U.S. Pat. No. 4,678,470, granted to Nashef, discloses a method of creating bone graft material by machining a block of bone to a particular shape, or by pulverizing and milling it. The graft material is then tanned with glutaraldehyde to sterilize it. This process can produce bone plugs of a desired shape.  
           [0003]    In the Nashef process, the exogenic bone material selected for the graft is presumably dead at the beginning of the process. The process of pulverizing or milling the bone materials destroys the structure of the bone tissue. The step of tanning it with glutaraldehyde then renders the graft material completely sterile. This condition is not conducive to graft healing and ingrowth. Specifically, applicant has found that it is desirable to maintain graft tissue in a living state during the grafting process. The use of living tissue in a graft will promote bone healing.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention is a tissue press for shaping or compressing a piece of tissue. The press includes first and second members movable relative to each other. First and second forming elements of different predetermined shapes are positionable on the first and second members on opposite sides of the piece of tissue. The first and second members are moved toward each other to shape or compress the tissue between the first and second forming elements.  
           [0005]    Means are preferably provided for monitoring and controlling the amount of force or pressure applied to the piece of tissue, in order to maintain the tissue in a viable living condition. Means may also be provided for draining off fluid from compressed tissue, so that the tissue can be implanted in a compressed state and imbibe fluid from the host site.  
           [0006]    The present invention is also a method of reshaping tissue for use as graft material comprising the steps of determining the shape which the tissue should possess as graft material; providing a tissue press, selecting forming elements adapted to press tissue approximately to the desired shape, and placing the forming elements on the tissue press; placing the tissue in the tissue press between the forming elements; operating the tissue press to shape the tissue between the forming elements to give the tissue the desired shape; and controlling the pressure on the tissue during the shaping step to minimize damage to or necrosis of the tissue.  
           [0007]    The present invention is also a method which includes the steps of determining the shape and size which the tissue should possess as graft material; placing the tissue in a tissue press having forming elements adapted to press tissue approximately to the shape and size desired; compressing the tissue in the tissue press to give the tissue the shape and size desired; controlling the pressure on the tissue during the compressing step to minimize damage to or necrosis of the tissue; and draining off fluid expressed during the compressing step.  
           [0008]    In accordance with another feature of the present invention, a retainer is provided for retaining tissue graft material in its compressed state. The retainer is placed around the compressed graft. The retainer can help to maintain the graft in a compressed configuration or in a specific shape for a period of time long enough to be placed in the body. The retainer may be made of a material which expands after it is placed in the body, to mechanically interlock the graft to the body.  
         GENERAL DESCRIPTION OF THE INVENTION  
         [0009]    With the apparatus and method of the present invention, bone or other tissue can be compressed or reshaped or both, while preserving the tissue alive.  
           [0010]    Often, reshaping of graft tissue is necessary to obtain the best possible graft. For example, in an anterior cruciate ligament reconstruction, the graft material which is removed usually has a triangular cross-sectional configuration. This graft material is placed in an opening in bone formed by drilling with a round drill. When the triangular graft material is placed in the round opening, there is minimal physical contact between the graft material and the surrounding bone. This reduces the holding power of the graft and also reduces the ingrowth ability of the graft.  
           [0011]    Thus, an important feature of the present invention is that bone or other tissue such as ligament is reshaped while still leaving it in a whole condition and without substantial tissue damage. The tissue is placed in the tissue press of the present invention and sufficient force is applied to reshape the tissue to the desired shape—for example, a cylindrical shape as needed for an anterior cruciate ligament reconstruction. Excessive pressure on the tissue, which can damage or kill the tissue, is avoided as described below. A properly shaped graft is thus provided which is still in a living condition.  
           [0012]    Compression of graft tissue is also sometimes desirable. Generally, tissue is stronger when it is denser. Compressing graft tissue increases its density and thus strengthens the graft tissue. The graft tissue also stays together better.  
           [0013]    For example, a tendon is made of a plurality of fibers. The individual fibers are weak when separated or unraveled. If a tendon graft is implanted with the fibers in a loose condition, the graft is weak. On the other hand, if prior to implantation the tendon graft is compressed to orient and pack the fibers tightly, then the entire group of fibers acts as one whole unit and the graft is much stronger. Therefore, compressing the tendon graft gives it more mechanical integrity—making a smaller tendon graft much stronger.  
           [0014]    Similarly, bone tissue is stronger and better able to bear force when it is denser and more compact. Compressing bone graft tissue prior to implantation produces a stronger graft. Compression of bone or other tissue also allows a surgeon to convert a larger irregular shape into a smaller specific shape. Thus, the surgeon when removing the graft material from the donor Site is not limited by the conditions at the host site but can remove the graft material in the best way possible from the donor site. Similarly, the surgeon when implanting the graft material at the host site is not limited by the shape of the material removed (as dictated by the conditions at the donor site) but can implant the graft material in the best way possible to fit the conditions at the host site.  
           [0015]    The anterior cruciate ligament, for example, attaches to the femur and tibia at specific isometric locations. When the ligament is being replaced in an anterior cruciate ligament reconstruction, typical uncompressed graft material can be many times the size of those locations. In such a case it is necessary to drill openings much larger than desired in the bone to attach the new ligament. The graft tendon then tends to fall eccentrically in this larger opening, the functional anatomy of the ligament can not be recreated, and the functioning of the knee joint is compromised.  
           [0016]    However, if the graft material for the new ligament is compressed in accordance with present invention, its size can be reduced substantially. This allows the surgeon to drill a substantially smaller opening in the bone to attach the new ligament/graft structure, so as to recreate the functional anatomy of the ligament.  
           [0017]    With the present invention, it is also possible to make a composite graft. For example, the graft material for an anterior cruciate ligament reconstruction is preferably tendon in the middle with bone at both ends. In accordance with the apparatus and method of the present invention, bone tissue can be compressed around the ends of tendon tissue to form a subtitute anterior cruciate ligament more closely approximating the original.  
           [0018]    It should also be noted that tissues other than bone and tendon can be worked with the tissue press. For example, a surgeon can harvest liver cells or pancreas cells and then compress them into a particular shape. They can then optionally be placed into a sack or some type of structural support which can be introduced into the body.  
           [0019]    With the present invention, graft material can be formed into almost any shape. A specific pair of forming (mold) parts, having a desired predetermined shape, are positioned on the tissue press, and the tissue is shaped or compressed between the forming elements. In addition to three-dimensional shaped parts, it is also possible to make a flat piece of graft material. For example, shaved skin can be placed on a flat plate, perhaps on a retaining mesh. The cells are then subjected to pressure to adhere them together. A flat, even, piece of graft material is formed which is suitable for skin grafting.  
           [0020]    Compressing graft material in accordance with the present invention also allows the surgeon to build up a larger piece of graft material out of several smaller parts. Sometimes a relatively large piece of graft material is needed for a particular host site. It is often not feasible to take such a large piece of graft material without damaging the donor area. To avoid this problem, several smaller pieces of graft material are placed in the tissue press and pressure is used to at least temporarily form the smaller parts together as one larger whole. The larger graft piece is then inserted into the host site.  
           [0021]    Compressing graft material in accordance with the present invention also aids in introducing additional materials to the graft material. These additional materials could be antibiotics, bone growth enhancers, tri-calcium phosphate, fibrin, allograft or autograft material, etc. When added to the graft material under pressure, the added materials adhere to and become a part of the graft material and not merely something added to the surface of it. By combining physiologic solutions or a carrier such as a gelatin, polysaccharides, antibiotics or synthetic bone materials to the compressed bone, for example, it is possible to create a plug of living bone with the other materials added into it. This plug has the graft properties of the bone tissue in the plug, as well as the properties of the added material.  
           [0022]    When tissue is compressed, fluid may be forced out of (expressed from) the tissue. If tissue in this compressed and defluidized state is laid back in a tissue pouch or in a bone hole, body fluids from the host site are absorbed by the graft material. This imbibition causes swelling of the graft material and thus creates a mechanical interlock between the graft and the host. Such a mechanical interlock is not produced with a typical implantation process in which graft tissue is not compressed. Further, the swelling (enlarging) of the graft material allows the graft material to fill an opening of any given shape with a perfect fit of the graft material therein.  
           [0023]    As noted above, applicant has found that it is desirable to maintain bone graft tissue in a living state during the grafting process. It is important not to kill tissue used in grafting because the living graft cells provide a superior substrate for grafting and graft viability and improved tissue healing. There is significantly faster incorporation of living tissue than of dead tissue. The cells and the tissue that are implanted into the body therefore need to be maintained in a viable condition.  
           [0024]    Excess pressure on tissue can cause destruction of the tissue, disorganization of the tissue fibers and irregular mechanical structure which can damage the tissue graft. Thus, a feature of the present invention is that the pressure or force on the tissue being reshaped or compressed is monitored and controlled. Pressure can be monitored by suitable pressure sensors and readouts such as a pressure gauge. Pressure can be controlled by force limiting means such as a torque wrench or similar device.  
           [0025]    The desired pressure levels may vary. For example, it may be desirable to provide a higher compressive force for cortical bone than for cancellous bone. Similarly, it may be desirable to provide a higher compressive force for bone than for tendon tissue. The appropriate level of pressure or force is selectively available with the tissue press.  
           [0026]    A separate device or structure can be used to maintain graft tissue in the compressed state prior to and during implantation. This separate device or structure can be a mesh sack, a ring around a cylindrically shaped graft material, etc. This additional retainer structure can assist the surgeon in introducing the graft tissue into the body in the compacted condition, to provide a denser stronger graft and to allow imbibition for creating a mechanical interlock. The retainer can be made of a material which expands when placed in the body, to provide a mechanical interlock for the graft tissue.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The foregoing and other features of the present invention will become apparent to one skilled in the art upon a consideration of the following description of the invention with reference to the accompanying drawings, wherein:  
         [0028]    [0028]FIG. 1 is a top plan view of a tissue press embodying the present invention;  
         [0029]    [0029]FIG. 2 is a top plan view of a tissue press similar to the tissue press of FIG. 1 and having a pressure sensing and monitoring mechanism;  
         [0030]    [0030]FIG. 3 is an elevational view of the tissue press of FIG. 2;  
         [0031]    [0031]FIGS. 3A and 3B illustrate a tissue press having means for removing excess tissue after a compression;  
         [0032]    FIGS.  4 A- 4 F illustrate a plurality of different forming elements for use in the tissue press of FIGS.  1 - 3 ;  
         [0033]    [0033]FIG. 5 is a view of a tissue press in accordance with a second embodiment of the invention;  
         [0034]    [0034]FIG. 6 is a view of a tissue press in accordance with a third embodiment of the invention;  
         [0035]    [0035]FIG. 6A illustrates a tissue press having means for limiting the amount of pressure applied to the tissue;  
         [0036]    [0036]FIGS. 7 and 7A illustrate a composite tissue graft such as compression of bone around tendon;  
         [0037]    [0037]FIG. 8 is a view of a tissue press in accordance with a fourth embodiment of the invention illustrating extrusion of tissue graft material;  
         [0038]    FIGS.  9 A- 9 F illustrate a plurality of different retainers for compressed tissue;  
         [0039]    [0039]FIGS. 10A and 10B illustrate an expanding tissue retainer;  
         [0040]    [0040]FIGS. 11, 11A and  11 B illustrate an expanding surgical implant; and  
         [0041]    [0041]FIGS. 12 and 12A illustrate expanding surgical stabilization devices. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0042]    The present invention relates to a tissue press and particularly to a tissue press for shaping or compressing bone or other tissue. The present invention is applicable to various constructions of tissue shaping or compressing apparatus. As representative of the present invention, FIG. 1 illustrates a tissue press  10 .  
         [0043]    The tissue press  10  includes a base  12 . The base  12  has a support surface  14  for supporting the base  10  and thus the press  10  on a table or other support (not shown). The base  12  has the same longitudinal cross-section throughout its width as is seen in elevation in FIG. 1. Two cylindrical pins  16  protect upwardly from the base  12 . A U-shaped saddle  18  is secured to the base  12  by screws  20 . A pin opening  22  extends transversely through the saddle  18 .  
         [0044]    The tissue press  10  also includes a handle  30 . The handle  30  has a connector portion  32  received between the upwardly projecting side portions of the saddle  18  of the base  12 . A pin opening  34  extends transversely through the connector portion  32  of the handle  30 . The opening  34  is aligned with the opening  22  in the saddle  18 . A pivot pin  36  extends through the pin openings  34  and  22  in the handle  30  and base  12 , respectively, and pivotally connects the handle  30  to the base  12 .  
         [0045]    Extending from the connector portion  32  of the handle  30  is a form-supporting portion  40  of the handle  30 . A channel  42  is defined on the lower side of the form-supporting portion  40  between a pair of channel surfaces  44 . The channel  42  extends transversely across the width of the form-supporting portion  40 . Extending from the form-supporting portion  40  is a circular portion  46  of the handle  30 . The circular portion  46  has a manually engageable surface  48  for the application of force to the handle  30 .  
         [0046]    A first forming element  50  is slidably received on the base  12 . The first forming element  50  has two vertically extending pin openings  52  which receive the pins  16  of the base  12 . The first forming element  50  has an upper surface  54  which includes a forming surface  56 . The forming surface  56  has a cylindrical surface portion  58 . The first forming element  50  has the same longitudinal cross-section throughout its width as is seen in elevation in FIG. 1. It should be understood that the first forming element  50  can be connected with the base  12  in any other suitable manner.  
         [0047]    A second forming element  60  is slidably received in the channel  42  of the handle  30 . The second forming element  60  has two projecting edges  62  which engage the channel surfaces  44  of the handle  30 . The second forming element  60  has a lower surface  64  which includes a forming surface  66 . The forming surface  66  includes a cylindrical surface portion  68 . The second forming element  60  has the same longitudinal cross-section throughout its width as is seen in elevation in FIG. 1. It should be understood that the second forming element  60  can be connected with the handle  30  in any other suitable manner.  
         [0048]    In operation of the press  10 , a pair of forming elements  50  and  60  are selected which when brought together will shape a piece of living tissue  70  to the desired shape. For example, the forming elements  50  and  60  illustrated in FIG. 1 have cylindrical surface portions which will shape tissue into a cylindrical shape suitable for insertion into a round drill hole in bone. The first forming element  50  is slid onto the pins  16  on the base  12 . The second forming element  60  is slid into the channel  42  in the handle  30 . A set screw  72  is tightened to secure the second forming element  60  to the handle  30 .  
         [0049]    The piece of tissue  70  to be shaped is placed between the first and second forming elements  50  and  60 . The tissue  70  is preferably positioned on the forming surface  56  of the first forming element  50  as illustrated in FIG. 1. The handle  30  is then brought toward the base  12  in the direction indicated by the arrow  74 , by the application of force to the handle  30 . The forming surface  66  with its cylindrical surface portion  68  engages the tissue  70 , and forces the tissue  70  against the forming surface  56  with its cylindrical surface portion  58 . The tissue  70  is thereby formed to the desired shape.  
         [0050]    It should be understood that with the tissue press  10 , tissue can be reshaped only, or reshaped and compressed. The amount and type of work performed on the tissue depends on the particular application, and is controlled by (i) the particular forming elements selected by the operator of the tissue press, and (ii) the amount of pressure or force applied to the tissue. For example, the forming elements can be selected so that they define between them a forming chamber having the same initial volume as the piece of tissue to be shaped. In this case, the piece of tissue is not compressed (that is, reduced in volume), but is merely reshaped. Alternatively, forming elements can be selected which will not merely reshape the piece of tissue but also will compress it, as described above. Because the first and second forming elements  50  and  60  are removably received on the base  12  and handle  30 , respectively, the surgeon can easily select first and second forming elements  50  and  60  to reshape or compress the tissue  70  as desired.  
         [0051]    As discussed above, it is important to control the pressure on the tissue in the tissue press  10 . There are many ways to do this. For example, in the press  80  shown in FIG. 2 and  3 , a second forming element  84  has an opening  82  which extends between its upper surface  86  and its forming surface  88 . An opening  90  in the handle  92  is aligned with and in fluid communication with the opening  82 . A pressure monitor device indicated schematically at  94  is in fluid communication with the opening  90  in the handle  92 , and thus with the forming surface  88  on the second forming element  84 . The pressure monitor device  94  can be any known device for displaying pressure, such as a dial readout, a bar movable axially in the openings  82  and  90  in response to fluid pressure, etc. The pressure monitor device  94  displays the pressure at the forming surface  88  of the second forming element  84 . This is the pressure applied to the tissue being pressed in the press  80 . Thus, an operator of the press  80  can observe this pressure monitor device  94  and limit the applied force accordingly, in order to avoid tissue damage or necrosis.  
         [0052]    Alternatively, a strain gauge of a known construction, indicated schematically at  96 , can be placed on the forming surface  88  of the second forming element  84 . Electric wires (not shown) transmit signals from the strain gauge  96  to an electric readout device indicated schematically at  94 . The electric readout indicates electrically the pressure applied to the tissue in the press  80 . Again, one can observe this pressure monitor device and limit the amount of force applied, in order to avoid tissue damage or necrosis. It should be understood that similar ways of monitoring the pressure on the tissue being pressed are the equivalent and are thus included within the scope of the invention.  
         [0053]    Any of the tissue presses of the present invention can include means for cutting off excess tissue. As an example, FIG. 3A and 3B illustrate a tissue press  91  having an independent cutoff arm  93  pivotally mounted adjacent the handle  95 . The cutoff arm  93  has a blade portion  97  operable to cut off excess tissue compressed by the tissue press  91 . The cutoff arm  93  is, of course, merely illustrative of the many equivalent structures usable to remove excess tissue after the compressing or shaping operation. Thus, the length of the tissue graft material, as well as its compressed diameter, can be selectively controlled by operation of the tissue press. It should be noted that this cutting off process can be affected with the edges of the forming elements themselves, as illustrated in the apparatus in FIG. 5.  
         [0054]    As noted above, the first forming element  50  and the second forming element  60  are removably received on the base  12  and handle  30 , respectively. Thus, forming elements having forming surfaces with other shapes can be easily placed in the tissue press  10 , in order to shape or compress tissue into other shapes. A few of the many shapes obtainable are illustrated in FIG. 4.  
         [0055]    [0055]FIG. 4A, for example, illustrates the forming elements  50  and  60  of FIG. 1- 3  which shape tissue into a cylindrical cross-sectional shape. FIG. 4B illustrates forming elements  100  and  102  which shape tissue into an oblong cross-sectional shape, between a forming surface  104  on the first forming element  100  and a forming surface  106  on the second forming element  102 . FIG. 4C illustrates forming elements  108  and  110  which shape tissue into a square cross-sectional shape, between a forming surface  112  on the first forming element  108  and a forming surface  114  on the second forming element  110 .  
         [0056]    [0056]FIG. 4D illustrates forming elements  116  and  1   
         [0057]    shape tissue into a triangular cross-sectional shape, between a forming surface  120  on the first forming element  116  and a forming surface  122  on the second forming element  118 . FIG. 4E illustrates forming elements  124  and  126  which shape tissue into a generally flat shape, between a forming surface  128  on the first forming element  124  and a forming surface  130  on the second forming element  126 . FIG. 4F illustrates forming elements  132  and  134  which shape tissue into a semi-circular cross-sectional shape, between a forming surface  136  on the first forming element  132  and a forming surface  138  on the second forming element  134 . Again, it should be understood that other shapes are conceivable and consequently are included within the scope of the invention.  
         [0058]    In addition to three-dimensional shaped parts, it is also possible to make a flat piece of graft material. Forming elements like those shown in FIG. 4E are useful in this case. For example, a skin graft may be placed on an adhesive based mesh (possibly using fibrin) on the flat forming surface  128 . The cells are then subjected to pressure to adhere them together. The cells are spread out over a finer layer. A flat, even, piece of skin graft material is formed. This can then be cut or pressed or formed into a specific shape and then used as a skin substitute on the body.  
         [0059]    Any forming element useful in the present invention can be coated with a non-stick coating to reduce adhesion of the compressed tissue to the forming elements. For example, the forming element  100  (FIG. 4B) is indicated as having a non-stick coating  105  such as a Teflon.RTM. coating which may be applied in any suitable known manner. Such a coating can resist the binding of the tissue (or tissue additive such as fibrin discussed below) to the forming element  100 . In a second embodiment of the invention, illustrated in FIG. 5, a tissue press  150  is fluid operated (pneumatic or hydraulic) rather than manually operated. The press  150  includes a base  152  having a support surface  154  for supporting the base  152  and thus the press  150  on a table  15  or other support (not shown). A first forming element  156  is attached to the base  152 . The first forming element  156  may be slidably or otherwise attached to the base  152  in any suitable manner which blocks movement of the forming element  156  relative to the base  152  during operation of  20  the press  150  and which allows for easy interchange of forming elements  156 . The first forming element  156  has an upwardly facing forming surface  158 . A fluid drain opening  160  is in fluid communication with the forming surface  158  of the first forming element  156 .  
         [0060]    An air or hydraulic cylinder  170  is fixed to the base  152  above the first forming element  156 . The cylinder  170  has a port  172  for the introduction of air or other fluid under pressure to operate a piston  174  in a known manner. A second forming element  180  is connected to the piston  174 . The second forming element  180  is attached to the piston  174  in any suitable manner which blocks movement of the forming element  180  relative to the piston  174  during operation of the press  150  and which allows for easy interchange of forming elements  180 . The second forming element  180  has a forming surface  182  facing the forming surface  158  on the first forming element  156 .  
         [0061]    A port  176  in the cylinder  170  is in fluid communication with the forming surface  182  of the second forming pat  180 . Connected to the port  176  is a pressure monitor device shown schematically at  178 .  
         [0062]    In operation of the press  150 , a pair of forming elements  156  and  180  are selected which when brought together will shape tissue to the desired shape. The first forming element  156  is attached to the base  152 . The second forming element  180  is attached to the piston  174 . A piece of tissue to be shaped (not shown) is placed between the first and second forming elements  156  and  180 . The piece of tissue is preferably positioned on the forming surface  158  of the first forming element  156 .  
         [0063]    The cylinder  170  is then actuated to move the second forming element  180  toward the first forming element  156 . The forming surface  182  on the second forming element  180  engages the tissue, and forces the tissue against the forming surface  158  on the first forming element  156 . Pressure on the tissue is controlled through observation of the monitor device  178 . The tissue is formed to the desired shape. Again, it should be understood that with the press  150  tissue can be reshaped only, or compressed also, depending on the application, the forming elements selected, and the amount of force applied.  
         [0064]    As discussed above, when tissue is compressed, fluid may be expressed from the tissue. In the press  150 , the second forming element  180  fits within the first forming element  156  to define between them a closed forming chamber in which the tissue is compressed. Expressed fluid is drained from the forming chamber through the fluid drain opening  160 . If a closed forming chamber is not formed, as for example with the open-ended forming elements shown in FIG. 1- 4 , then expressed fluid can drain outwardly from the tissue being pressed, without the need for a separate fluid drain port. Of course, a separate fluid drain port could be provided in any of the forming elements of the present invention.  
         [0065]    In a third embodiment of the invention, illustrated in FIG. 6, a tissue press  200  includes a base  202  having a support surface  204  for supporting the base  202  and thus the press  200  on a table or other support (not shown). A first forming element  206  is attached to the base  202 . The first forming element  206  may be slidably or otherwise attached to the base  202  in any suitable manner. The first forming element  206  includes a plurality of first fingers  208  which together have a forming surface  210  to progressively compress bone or other tissue into a predetermined shape.  
         [0066]    An upper arm  220  is pivotally mounted to the base  202  by a pivot pin  222 . A second forming element  224  is connected to the upper arm  220  in a suitable manner. The second forming element  224  includes a plurality of second fingers  226  which together have a forming surface  228  facing the forming surface  210  on the first forming element  206 . The second fingers  226  are interdigitable with the first fingers  208 .  
         [0067]    In operation of the press  200 , a pair of forming elements  206  and  224  are selected which when brought together will progressively shape tissue to the desired shape. The first forming element  206  is attached to the base  202 . The second forming element  224  is attached to the upper arm  220 . A piece of tissue to be shaped (not shown) is placed between the first and second forming elements  206  and  224 . The piece of tissue is preferably positioned on the forming surface  210  of the first forming element  206 . The upper arm is pivoted toward the base to move the second forming element  224  toward the first forming element  206 . The forming surface  228  on the second forming element  224  engages the tissue, and forces the tissue against the forming surface  210  on the first forming element  206 . The tissue is formed to the desired shape.  
         [0068]    Because the second fingers  226  are interdigitable with the first fingers  208 , the press  200  is operable to compress tissue to different compressed sizes with only one pair of forming elements. As the second fingers  226  come together with the first fingers  208 , they compress the tissue to a smaller and smaller diameter (shape). This allows for one pair of forming elements to provide compression to variable diameters or sizes. This works well with soft tissue applications, specifically tendons, to compress the tendon into a smaller shape. The amount of compression is based on the amount of pressure applied and the needed finished size.  
         [0069]    As noted above, it is important to control the pressure or force applied to the tissue by the tissue press. Accordingly, the present invention provides means for limiting the amount of pressure applied to the tissue by the tissue press, that is, means for blocking application to the tissue of force in excess of a predetermined amount. Such means are schematically illustrated in FIG. 6A, which illustrates a tissue press  300  having a known torque wrench assembly included therein.  
         [0070]    The press  300  includes a base  302 . Attached to the base in the manner described above is a first forming part  304 . Also attached to the base is a saddle  306 . Received in the saddle  306  is the connector portion  308  of a handle assembly  310 . The press  300  also includes a pivot pin  312  pivotally interconnecting the handle assembly  310  and the base  302 .  
         [0071]    The handle assembly  310  includes a form-supporting portion  314  to which there is attached in the manner described above a second forming part  316 . The handle assembly  310  also includes a second portion  318  connected to the form-supporting portion  314  by a drive mechanism  320 . The second portion  318  includes a knurled section  322  which is rotatable about an axis  324 . On the second portion  318  there is a gauge  326 .  
         [0072]    The knurled section  322  is rotatable about the axis  324  to set the torque value desired and as shown on the gauge  326 . Thereafter, the handle assembly  310  can be pivoted toward the base  302  in the direction indicated by the arrow  328  only until the preset amount of torque is applied. At that point, no more torque is transferred through the drive mechanism  320  to the form-supporting portion  314 . This limits the amount of pressure applied to the tissue by the second forming part  316 , that is, blocks application to the tissue of force in excess of a predetermined amount.  
         [0073]    It should be understood that the torque wrench assembly or construction indicated in FIG. 6A is only illustrative of the many ways in which the amount of pressure applied to the tissue by the tissue press can be limited to a predetermined amount. There are other known mechanisms for performing the same function, and their use is included within the scope of the present invention.  
         [0074]    [0074]FIG. 7 and  7 A illustrate the use of a tissue press in accordance with the present invention to form a composite graft. As discussed above, with the present invention, it is also possible to make a composite graft. For example, the graft material for an anterior cruciate ligament reconstruction is preferably tendon in the middle with bone at both ends. In accordance with the apparatus and method of the present invention, bone tissue can be compressed around the ends of tendon tissue to form a substitute anterior cruciate ligament more closely approximating the original.  
         [0075]    Thus, as illustrated schematically in FIG. 7 and  7 A, the tissue press  10  of FIG. 1- 4  is being used to compress bone tissue  240  around tendon tissue  242  to form a substitute anterior cruciate ligament  244 . The tendon  242  can be harvested from one site and the bone  240  can be harvested from another site.  
         [0076]    It should be understood that the graft can be multiple tissue fragments rather than a composite material. Thus, the tissue press  10 , or indeed any tissue press in accordance with the present invention, can be used to compress, for example, multiple bone fragments into one larger piece. It should also be understood that the tissue press in accordance with the present invention can be used to add additional materials to body tissue material by pressure. For example, to bone tissue there can be added tri-calcium phosphate, an antibiotic, hydroxyapatite, allografts or autografts, or any other polymeric. This process is believed to be self-explanatory in light of the foregoing description, but for reference may be understood by referring to FIG. 7 and  7 A wherein  240  would be the bone tissue or other tissue to which material is being added (squeezed in under pressure), and  242  indicates the additional material being added to the tissue  240 .  
         [0077]    In this case, fibrin can be highly suitable for use as such an additional material. Fibrin is a blood component important in blood clotting. It can be separated or centrifuged from blood and has the nature of an adhesive gel. Fibrin can be used as an adhesive, either in a natural state or after being compressed, to hold together material such as separate tissue pieces pressed together in a tissue press of the present invention.  
         [0078]    In a fourth embodiment of the invention, illustrated in FIG. 8, a tissue press  250  is operated to extrude rather than press material. The press  250  includes a base  252  having a support surface  254  for supporting the base  252  and thus the press  250  on a table or other support (not shown). A die  256  is attached to the base  252 . The die  256  may be slidably or otherwise attached to the base  252  in any suitable manner which blocks movement of the die  256  relative to the base  252  during operation of the press  250  and which allows for easy interchange of forming elements  256 . The die  256  has an upwardly facing opening  258 . An extrusion opening  260  is in fluid communication with the opening  258  of the die  256 .  
         [0079]    An air or hydraulic cylinder  270  is fixed to the base  252  above the die  156 . The cylinder  270  has a port  272  for the introduction of air or other fluid under pressure to operate a piston  274  in a known manner. A ram  280  is connected to the piston  274 . The ram  280  has a surface  282  facing the opening  258  on the die  256 .  
         [0080]    In operation of the press  250 , a die  256  is selected which will extrude tissue in the desired shape. The die  256  is attached to the base  252 . A piece of tissue to be extruded (not shown) is placed in the opening  258  of the die  256 . The cylinder  270  is then actuated to move the ram  280  toward the die  256 . The surface  282  on the ram  280  engages the tissue, and forces the tissue into and through the die  256 , exiting through the opening  260 . The tissue is extruded in the desired shape. As discussed above, a fluid drain port can be provided in the press  250 .  
         [0081]    It can also be useful to heat or cool the tissue being worked in a tissue press of the present invention. Accordingly, the present invention contemplates the use of means for selectively controlling the temperature of the piece of tissue while it is being compressed or shaped. As an example, illustrated schematically in FIG. 4 is a fluid passage  284  extending from the outer surface of the forming element  124  and around the forming surface  128  thereof. Fluid which is either heated or cooled flows through the passage  284  and either cools or heats the material of the forming element  124  in the area adjacent the forming surface  128 . Thus, the tissue, when it comes in contact with the forming surface  128 , can be selectively heated or cooled during the compression or reshaping operation. Heating can be useful in holding together materials being compressed, for example, and cooling can be useful to avoid tissue damage arising from overheating of tissue being compressed. It should be understood that other means of achieving these functions are contemplated, such as electrical heating elements. Further, both forming elements can be heated or cooled rather than just one. Any such equivalent structure is to be considered within the scope of the present invention.  
         [0082]    Also in the present invention, a retainer is provided for retaining tissue graft material in its compressed state. After the graft is compressed, the retainer is placed around the graft. The retainer can help to maintain the graft in a compressed configuration or in a specific shape for a period of time long enough to be placed in the body.  
         [0083]    The retainer may be one of many different shapes. The shape of the retainer is chosen to meet the specific application. There are a number of suitable shapes, such as a ring, a cylinder, a cage, a rectangular shape, a mesh, a suture-like wrap, etc. Some of these are illustrated schematically in FIG. 9A- 9 F. It should be understood that this is not an exhaustive listing, but rather that these are merely exemplary of the principle involved, and accordingly, the invention is not limited to these particular shapes. For example, a retainer may be provided which is in the particular shape of the tissue material being compressed, which can be rectangular, cylindrical, planar, etc.  
         [0084]    [0084]FIG. 9A illustrates a plurality of bands or rings  290  used to hold together compressed tissue  292 . FIG. 9B illustrates a cage  294  which can be used to hold together the compressed tissue  292  of FIG. 9A. The cage  294  includes a plurality of crossed filaments  296  which define between them a series of openings  298  for tissue ingrowth. FIG. 9C illustrates another cage  300  which can be used to hold together the compressed tissue  292  of FIG. 9A. The cage  300  includes a plurality of longitudinally extending filaments  302  which define between them a series of openings  304  for tissue ingrowth. FIG. 9D illustrates a solid-walled cylinder  306  which can be used to hold together the compressed tissue  292 . FIG. 9E illustrates a mesh cylinder  308  can be used to hold together the compressed tissue  292 . FIG. 9F illustrates the wrapping of a cord or suture  310  around compressed tissue  312 .  
         [0085]    Any of these retainers may be made of various materials. The material of the retainer is chosen to meet the specific application. Some of the many materials which are suitable are biodegradable materials, ceramics (especially with bone-growth enhancers, hydroxyapatite, etc.); polymeric material such as Dacron or other known surgical plastics; metal; or composite materials.  
         [0086]    In use, the graft material may be pushed into the retainer structure after graft material is compressed. Alternatively, the graft material may be compressed with the retainer structure. After the graft material is compressed in the retainer, the combined structure of graft plus retainer is placed in the host site in the body. The retainer helps to maintain the graft in a compressed configuration or in the specific shape into which it was compressed for a period of time long enough to be placed in the body.  
         [0087]    If the retainer is made of a biodegradable material, then the retainer degrades and disappears after a period of time. If the retainer is not made of a biodegradable material, then the retainer remains in the body. Tissue ingrowth occurs to bind the host tissue to the graft material. Tissue ingrowth through and around the retainer, between the host tissue and the graft material, is promoted if there are openings as discussed above in the retainer.  
         [0088]    The invention, the retainer may, if desired, be made of a material which expands after it is placed in the body, to mechanically interlock the graft to the body. The expansion can take place in one of two ways. First, the retainer can itself be compressed, as with the tissue, and then expand when placed in the body. Second, the retainer can be made of a material which expands when it comes in contact with water or other bodily fluids. (It should be noted that the tissue can itself be compressed then expand when contacted by water. As an example, a tendon can be compressed in a desiccated state, and as it imbibes water it expands and creates a firmer lock or tighter fit in the host site.)  
         [0089]    The expandable material can first be compressed with the tissue being grafted, and which then expands when placed in the body. The retainer is preferably made of a material which has more structural stability than the tissue being grafted, and provides mechanical integrity and structural support for the graft tissue. A retainer made of a solid polymeric material, for example, is useful to retain in a compressed state a tendon or bone tissue graft.  
         [0090]    These expandable materials can be used not only to retain graft material, but for any shape required for stabilization surgery, such as a wedge, screw, rivet, retaining ring, or spacer, an intramedullary rod, a joint replacement part such as a femoral component of acetabular cup, an expandable sleeve, or another mechanical structure. The expandable materials thus can be used both as a carrier or retainer for another material (e.g. tissue graft material) and on their own as a prosthetic element.  
         [0091]    There are a number of suitable materials which expand when they come in contact with water or other fluids. One is PEEK (polyether-etherketone). A desiccated biodegradable material, or a desiccated allograft may also be  
         [0092]    As a simple example, an expandable retainer  330  (FIG. 10A) with graft tissue  332  therein is placed into a tissue or bone space  334  defined by an edge  336  in host tissue  38 . As the retainer  330  imbibes body fluids or water from the host tissue  338 , it expands radially outwardly into the tissue or bone space  334  and creates a mechanical interlock (FIG. 10A). It also expands radially inwardly and clamps on the graft tissue  332 . Therefore, the graft tissue  332  is locked into the host site, without the necessity of damaging the tissue further through some other kind of attachment means.  
         [0093]    For example, a hip replacement (femoral head) is typically made of metal. To implant the replacement, the softer, inner cancellous bone of the femur is first removed. The inner surface of the cortical bone is then machined to provide a close fit between the external surface of the replacement and the hard outer cortical bone material. All this requires a substantial opening in the femur and still does not guarantee a close enough fit for the implant.  
         [0094]    If, instead, the implant is made of an expanding material such as PEEK, only a smaller opening is needed, thus reducing trauma to the bone. Although it is best to lock against the cortical bone, it is possible to implant solely in the cancellous bone, which because of the expansion of the implant provides a better fit than a metal implant. A benefit of implanting in the cancellous bone is reduction of the danger of putting the implant in so tightly that the cortical bone is split (wedged open). Further, if the opening in the bone is not exactly the same shape as the outer surface of the implant, the implant expands to provide a custom contoured fit to the bone and provide immediate mechanical stability. Thus, less machining of the bone is needed, while at the same time obtaining a closer fit.  
         [0095]    Thus, as illustrated in FIG. 11- 11 B, a hip replacement (femoral head)  340  is made of PEEK or another expandable material. The replacement  340  is inserted into an intramedullary channel  342  cut into a femur  344 . The replacement  340  is smaller in diameter than the channel  342 . The replacement  340  absorbs body fluids and expands to lock itself into the channel  342  in the femur  344 . (It should be understood that the scale shown in FIG. 11- 11 B is exaggerated as to the amount by which the replacement  340  expands.)  
         [0096]    Similarly, a bone plate or other structure or tissue can be secured to a bone with a fastener made of such an expandable material. As illustrated schematically in FIG. 12 and  12 A, a bone plate  350  is secured to a bone  352 . In FIG. 12, a fastener  354  is used which has an unthreaded portion  356  extending into the bone  352 . The fastener  354 , or at least the unthreaded portion  356 , is made of PEEK or another suitable expandable material. The portion  356  imbibes fluid from the bone  352  and expands radially outwardly, from an unexpanded condition as shown in phantom at  358  to an expanded condition as shown in solid lines at  360 , to lock the fastener  354  into the bone  352 . This enables the securing of the plate  350  to the bone  352  without cutting threads into the bone  352  as is usually done.  
         [0097]    In FIG. 12A, a fastener  362  has a threaded portion  364  extending into the bone  352 . The threaded portion  364  is made of PEEK or another suitable expandable material. The threaded portion  364  imbibes fluid from the bone  352  and expands radially outwardly to additionally lock the fastener  362  into the bone  352 . Alternatively, the fastener  362  of FIG. 12A may have a coating  366  on its portion threaded into the bone  352 . The coating  366  is made of PEEK or another suitable expandable material. The coating  366  imbibes fluid from the bone  352  and expands radially outwardly to additionally lock the fastener into the bone  352 .  
         [0098]    When such a fastener is made of a non-metal expandable material, removal of the fastener simply entails drilling out the center thereof. This is much easier than with a typical metal bone screw.  
         [0099]    From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.