Abstract:
Apparatus for securing a first bone fragment to a second bone fragment, said apparatus comprising: a fusion device, said fusion device comprising: a shaft having a first end and a second end; a first bone-engaging feature formed on said shaft at a first location, said first bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, such that said first end of said shaft may be advanced into a hole in the first bone fragment when said at least one barb is elastically constrained to a position substantially parallel to the longitudinal axis of said shaft but is prevented from being withdrawn from the hole in the first bone fragment when said at least one barb is in its unbiased condition; and a second bone-engaging feature formed on said shaft at a second location, said second bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, such that said second end of said shaft may be advanced into a hole in the second bone fragment when said at least one barb is elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, but is prevented from being withdrawn from the hole in the second bone fragment when said at least one barb is in its unbiased condition.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATIONS 
       [0001]    This patent application: 
         [0002]    (1) is a continuation-in-part of pending prior U.S. patent application Ser. No. 13/624,643, filed Sep. 21, 2012 by MX Orthopedics, Corp. and Matthew Fonte for OSTEOSYNTHETIC SHAPE MEMORY MATERIAL INTRAMEDULLARY BONE STENT AND METHOD FOR TREATING A BONE FRACTURE USING THE SAME (Attorney&#39;s Docket No. FONTE-091014), which patent application:
       (a) claims benefit of prior U.S. Provisional Patent Application Ser. No. 61/537,766, filed Sep. 22, 2011 by Matthew Fonte for OSTEOSYNTHETIC SHAPE MEMORY MATERIAL INTRAMEDULLARY BONE STENTS AND METHODS FOR TREATING BONE FRACTURES USING THE SAME (Attorney&#39;s Docket No. FONTE-10 PROV); and   (b) claims benefit of prior U.S. Provisional Patent Application Ser. No. 61/570,091, filed Dec. 13, 2011 by Matthew Fonte for OSTEOSYNTHETIC SHAPE MEMORY MATERIAL INTRAMEDULLARY BONE STENTS AND METHODS FOR TREATING BONE FRACTURES USING THE SAME (Attorney&#39;s Docket No. FONTE-14 PROV); and       
 
         [0005]    (2) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 61/903,820, filed Nov. 13, 2013 by MX Orthopedics, Corp. and Matthew Palmer et al. for BONE STAPLES, INTRAMEDULLARY FIXATION DEVICES, SOFT TISSUE ANCHORS AND PINS SCREWS CAN BE SHORTENED IN VIVO TO IMPROVE FRACTURE REDUCTION BY USING SUPERELASTIC OR SHAPE MEMORY EFFECT CHARACTERISTICS OF NITINOL (Attorney&#39;s Docket No. FONTE-34 PROV). 
         [0006]    The four (4) above-identified patent applications are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0007]    The present invention relates to devices and methods for generating, applying, and maintaining compression to a site in a human or animal body in order to effect healing of diseased or damaged tissue. The invention finds particular utility in the field of orthopedics and specifically for generating and maintaining compression between bone fragments that are to be fused. While the invention has application throughout the body, its utility will be illustrated herein in the context of the repair of injured bone tissue, such as the proximal and distal interphalangeal joint of the second, third, or fourth toe and/or fingers. Additionally, the invention has application to aid in the fusion of broken ribs, etc. 
       BACKGROUND OF THE INVENTION 
       [0008]    In the field of orthopedic surgery it is common to rejoin broken bones. The success of the surgical procedure often depends on the successful re-approximation of the bone fragments, the amount of compression achieved between the bone fragments, and the ability to maintain that compression between the bone fragments. If the surgeon is unable to bring the bone fragments into close contact, a gap will exist between the bone fragments and the bone tissue will need to fill that gap before complete healing can take place. Furthermore, gaps between bone fragments that are too large allow motion to occur between the bone fragments, disrupting the healing tissue and thus slowing the healing process. Optimal healing requires that the bone fragments be in close contact with each other, and for a compressive load to be applied and maintained between the bone fragments. Compressive strain between bone fragments has been found to accelerate the healing process in accordance with Wolf&#39;s Law. 
         [0009]    Broken bones can be rejoined using screws, staples, plates, pins, intramedullary devices, and other devices known in the art. These devices are designed to assist the surgeon with reducing the fracture and creating a compressive load between the bone fragments. Intramedullary devices are often used for fractures of the long bones; however, they are also frequently used in the phalanges and specifically for the treatment of “hammer toe”, which is a deformity of the proximal interphalangeal joint of the second, third, or fourth toe causing the toe to be permanently bent. Typical intramedullary devices used in the phalanges have opposing ends that are adapted to grip against the wall of the intramedullary canal. These intramedullary devices are typically made of titanium alloys, stainless steel alloys, Nitinol and other materials, e.g., PEEK. The titanium alloy devices and stainless steel alloy devices often have barbs or threaded regions at their opposing ends to grip the wall of the intramedullary canal. The Nitinol devices typically have a pair of radially extending “legs” at their opposing ends that expand outward when warmed to body temperature, with the pair of legs at each end being disposed in a common plane. 
         [0010]    While these intramedullary devices are designed to bring the bone fragments into close contact and to generate a compressive load between the bone fragments, these devices do not always succeed in accomplishing this objective. It is widely reported that the compressive load dissipates rapidly as the bone relaxes and remodels. Furthermore, gripping the bone with only a pair of co-planar legs does not provide significant torsional stability to the fusion site. 
         [0011]    Thus there exists a clinical need for intramedullary devices that are better able to bring bone fragments into close proximity with each other, generate a compressive load, and maintain that compressive load for a prolonged period of time while healing occurs. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention comprises the provision and use of novel intramedullary devices that are better able to bring bone fragments into close proximity with each other, generate a compressive load, and maintain that compressive load for a prolonged period of time while healing occurs. 
         [0013]    In one preferred form of the invention, there is provided apparatus for securing a first bone fragment to a second bone fragment, said apparatus comprising: 
         [0014]    a fusion device, said fusion device comprising:
       a shaft having a first end and a second end;   a first bone-engaging feature formed on said shaft at a first location, said first bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, such that said first end of said shaft may be advanced into a hole in the first bone fragment when said at least one barb is elastically constrained to a position substantially parallel to the longitudinal axis of said shaft but is prevented from being withdrawn from the hole in the first bone fragment when said at least one barb is in its unbiased condition; and   a second bone-engaging feature formed on said shaft at a second location, said second bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, such that said second end of said shaft may be advanced into a hole in the second bone fragment when said at least one barb is elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, but is prevented from being withdrawn from the hole in the second bone fragment when said at least one barb is in its unbiased condition.       
 
         [0018]    In another preferred form of the invention, there is provided a method for securing a first bone fragment to a second bone fragment, said method comprising: 
         [0019]    providing a fusion device, said fusion device comprising:
       a shaft having a first end and a second end;   a first bone-engaging feature formed on said shaft at a first location, said first bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, such that said first end of said shaft may be advanced into a hole in the first bone fragment when said at least one barb is elastically constrained to a position substantially parallel to the longitudinal axis of said shaft but is prevented from being withdrawn from the hole in the first bone fragment when said at least one barb is in its unbiased condition; and   a second bone-engaging feature formed on said shaft at a second location, said second bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, such that said second end of said shaft may be advanced into a hole in the second bone fragment when said at least one barb is elastically constrained to a position substantially parallel to the longitudinal axis of said shaft but is prevented from being withdrawn from the hole in the second bone fragment when said at least one barb is in its unbiased condition;       
 
         [0023]    elastically constraining said at least one barb of said first bone-engaging feature to a position substantially parallel to the longitudinal axis of said shaft, and elastically constraining said at least one barb of said second bone-engaging feature to a position substantially parallel to the longitudinal axis of said shaft; 
         [0024]    advancing said first bone-engaging feature into a hole in the first bone fragment, and advancing said second bone-engaging feature into a hole in the second bone fragment; and 
         [0025]    releasing the constraint on said at least one barb of said first bone- 1  engaging feature and releasing the constraint on said at least one barb of said second bone-engaging feature, whereby to generate and maintain compression between the first bone fragment and the second bone fragment. 
         [0026]    In another preferred form of the invention, there is provided apparatus for securing a first bone fragment to a second bone fragment, said apparatus comprising: 
         [0027]    a fusion device, said fusion device comprising:
       a shaft having a first end and a second end;   a first bone-engaging feature formed on said shaft at a first location, said first bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, said at least one barb being configured so that said first end of said shaft may be advanced into a hole in the first bone fragment but prevents said first end of said shaft from being withdrawn from the hole in the first bone fragment; and   a second bone-engaging feature formed on said shaft at a second location, said second bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, said at least one barb being configured so that said second end of said shaft may be advanced into a hole formed in the second bone fragment but prevents said second end of said shaft from being withdrawn from the hole in said second bone fragment;   wherein at least a portion of said shaft disposed between said first bone-engaging feature and said second bone-engaging feature is capable of being elastically stretched; and       
 
         [0032]    a holding element connectable to said fusion device for releasably holding said at least a portion of said shaft in a stretched condition. 
         [0033]    In another preferred form of the invention, there is provided a method for securing a first bone fragment to a second bone fragment, said method comprising: 
         [0034]    providing a fusion device, said fusion device comprising:
       a shaft having a first end and a second end;   a first bone-engaging feature formed on said shaft at a first location, said first bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, said at least one barb being configured so that said first end of said shaft may be advanced into a hole in the first bone fragment but prevents said first end of said shaft from being withdrawn from the hole in the first bone fragment; and   a second bone-engaging feature formed on said shaft at a second location, said second bone-engaging feature comprising at least one barb which, in its unbiased condition, flares outwardly from the longitudinal axis of said shaft and which is capable of being elastically constrained to a position substantially parallel to the longitudinal axis of said shaft, said at least one barb being configured so that said second end of said shaft may be advanced into a hole formed in the second bone fragment but prevents said second end of said shaft from being withdrawn from the hole in said second bone fragment;   wherein at least a portion of said shaft disposed between said first bone-engaging feature and said second bone-engaging feature is capable of being elastically stretched;       
 
         [0039]    longitudinally stretching said fusion device so that said fusion device is in a longitudinally stretched condition; 
         [0040]    holding said fusion device in its longitudinally stretched condition; 
         [0041]    inserting said fusion device into a hole in the first bone fragment while said fusion device is in its longitudinally stretched condition, and inserting said fusion device into a hole in the second bone fragment while said fusion device is in its longitudinally stretched condition; and 
         [0042]    releasing said fusion device from its longitudinally stretched condition so as to apply compression between the first bone fragment and the second bone fragment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
           [0044]      FIGS. 1-4  are schematic views showing an intramedullary fusion device formed in accordance with the present invention; 
           [0045]      FIG. 4A  is a schematic view showing removable retaining tabs which may be used to hold the first and second barbed end regions of the intramedullary fusion device of  FIGS. 1-4  in their radially constrained condition; 
           [0046]      FIGS. 5-15  are schematic views showing the novel intramedullary fusion device of  FIGS. 1-4  being used to treat a hammer toe condition; 
           [0047]      FIG. 15A  is a schematic view showing another intramedullary fusion device formed in accordance with the present invention; 
           [0048]      FIG. 15B  is a schematic view showing another intramedullary fusion device formed in accordance with the present invention; 
           [0049]      FIGS. 16-18  are schematic views showing another intramedullary fusion device formed in accordance with the present invention; 
           [0050]      FIGS. 19-25  are schematic views showing an intramedullary fusion system formed in accordance with the present invention; 
           [0051]      FIGS. 26-36  are schematic views showing the novel intramedullary fusion system of  FIGS. 19-25  being used to treat a hammer toe condition; and 
           [0052]      FIG. 37  is a schematic view showing another intramedullary fusion device formed in accordance with the present invention and which may be used with the intramedullary fusion system shown in  FIGS. 19-25 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]    The present invention comprises the provision and use of novel intramedullary devices that are better able to bring bone fragments into close proximity with each other, generate a compressive load, and maintain that compressive load for a prolonged period of time while healing occurs. 
         [0054]    Looking first at  FIGS. 1-4 , there is shown an intermedullary fusion device  5  manufactured from a shape memory material (e.g., a material capable of exhibiting superelasticity and/or a temperature-induced shape change). The shape memory material may be a metal alloy (e.g., Nitinol) or a polymer (e.g., appropriately processed PEEK). Intramedullary fusion device  5  comprises a first barbed end region  10 , a second barbed end region  15 , and a central bridge region  20  connecting first barbed end region  10  to second barbed end region  15 . Intramedullary fusion device  5  is preferably cannulated so as to allow the intramedullary fusion device to be installed over a k-wire if desired, while also allowing a k-wire to be passed through the intramedullary fusion device following implantation if the surgeon desires to fuse a distal or proximal joint. The first and second barbed regions flare outward in multiple planes, preferably engaging the surrounding bone about the full circumference of the intramedullary fusion device, thereby providing excellent torsional stability to the fusion site. 
         [0055]    First barbed end region  10  comprises a plurality of barbs  25  which, in their unbiased condition, flare outward from the longitudinal axis of intramedullary fusion device  5  in the manner shown in  FIG. 1 . The flare may increase linearly over the length of the barb, or it may increase non-linearly over the length of the barb to enable first barbed end region  10  to better engage the “hourglass-shaped” intramedullary canal. The better that first barbed end region  10  engages the intramedullary canal, the more even the pressure distribution will be. However, barbs  25  can be strained to a position parallel to the longitudinal axis of intramedullary fusion device  5  and constrained in that position (e.g., via a removable retaining tab  30 ,  FIGS. 2 and 4A ) so as to reduce the cross-sectional profile of first barbed end region  10 , whereby to allow for insertion into a drilled hole in bone, as will hereinafter be discussed. Barbs  25  can be constrained in a state where they partially occupy the cannulation of intramedullary fusion device  5  (i.e., where barbs  25  are strained to a point past parallel to the longitudinal axis of intramedullary fusion device  5 ), thereby further reducing the cross-sectional area of first barbed end region  10 . This is beneficial for accessing the intramedullary canal through a small drilled hole. 
         [0056]    While  FIG. 1  illustrates a device with four barbs  25  on first barbed end region  10 , it should be appreciated that first barbed end region  10  can be made with more or fewer barbs. Upon removing retaining tab  30 , barbs  25  are allowed to flare outward again, whereby to grip the side wall of the drilled hole and intramedullary canal receiving first barbed end region  10 , whereby to lock first barbed end region  10  to a bone fragment via an expansive force on the intramedullary canal, as will also hereinafter be discussed. 
         [0057]    In one preferred form of the invention, barbs  25  of first barbed end region  10  are separated from one another by relatively small longitudinal gaps when barbs  25  are strained to a position parallel to the longitudinal axis of intramedullary fusion device  5 , such that barbs  25  collectively provide a substantially full circumferential structure for first barbed end region  10  (i.e., when barbs  25  are strained to a position parallel to the longitudinal axis of intramedullary fusion device  5 , barbs  25  collectively provide a substantially continuous extension of central bridge region  20  of intramedullary fusion device  5 ). See  FIGS. 3 and 4 . 
         [0058]    Second barbed end region  15  comprises a plurality of barbs  35  which, in their unbiased condition, flare outward from the longitudinal axis of intramedullary fusion device  5  in the manner shown in  FIG. 1 . The flare may increase linearly over the length of the barb, or it may increase non-linearly over the length of the barb to enable second barbed end region  15  to better engage the “hourglass-shaped” intramedullary canal. The better that second barbed end region  15  engages the intramedullary canal, the more even the pressure distribution will be. However, barbs  35  can be strained to a position parallel to the longitudinal axis of intramedullary fusion device  5  and constrained in that position (e.g., via a removable retaining tab  40 ,  FIGS. 2 and 4A ) so as to reduce the cross-sectional profile of second barbed end region  15 , whereby to allow for insertion into a drilled hole in bone, as will hereinafter be discussed. Barbs  35  can be constrained in a state where they partially occupy the cannulation of intramedullary fusion device  5  (i.e., where barbs  35  are constrained to a point past parallel to the longitudinal axis of intramedullary fusion device  5 ), thereby further reducing the cross-sectional area of second barbed region  15 . This is beneficial for accessing the intramedullary canal through a small drilled hole. 
         [0059]    While  FIG. 1  illustrates a device with four barbs  35  on the second barbed end region  15 , it should be appreciated that second barbed end region  15  can be made with more or fewer barbs. Upon removing retaining tab  40 , barbs  35  are allowed to flare outward, whereby to grip the side wall of the drilled hole and intramedullary canal receiving second barbed end region  15 , whereby to lock second barbed end region  15  to a bone fragment via an expansive force on the intramedullary canal, as will also hereinafter be discussed. 
         [0060]    In one preferred form of the invention, barbs  35  of second barbed end region  15  are separated from one another by relatively small longitudinal gaps when barbs  35  are strained to a position parallel to the longitudinal axis of intramedullary fusion device  5 , such that barbs  35  collectively provide a substantially full circumferential structure for second barbed end region  15  (i.e., when barbs  35  are strained to a position parallel to the longitudinal axis of intramedullary fusion device  5 , barbs  35  collectively provide a substantially continuous extension of central bridge region  20  of intramedullary fusion device  5 ). See  FIGS. 3 and 4 . 
         [0061]    It should be appreciated that first barbed end region  10  and second barbed end region  15  may have different numbers of barbs, e.g., first barbed end region  10  may comprise four barbs  25  and second barbed end region  15  may comprise three barbs  35 . However, it should be appreciated that regardless of the number of barbs  25  provided on first barbed end region  10 , and regardless of the number of barbs  35  provided on second barbed end region  15 , the barbs  25  of first barbed end region  10  preferably engage the surrounding bone about the full circumference of the intramedullary fusion device, and the barbs  35  of second barbed end region  15  preferably engage the surrounding bone about the full circumference of the intramedullary fusion device. 
         [0062]    Central bridge region  20  preferably comprises a generally cylindrical shape and is preferably sized so as to have an outer diameter somewhat less than the major diameters of first barbed end region  10  and second barbed end region  15  when their barbs  25 ,  35 , respectively, have been strained to a position parallel to the longitudinal axis of intramedullary fusion device  5 . 
         [0063]    Intramedullary fusion device  5  may be used to secure together two bone fragments under compression. By way of example but not limitation, and looking now at  FIGS. 5-15 , intramedullary fusion device  5  may be used to treat a hammer toe deformity ( FIG. 5 ). 
         [0064]    First the distal end of the metatarsal  50  is cut off to correct the deformity and create a bone face  55  suitable for fusion ( FIGS. 6 and 7 ). Then the proximal end of the phalange  60  is removed to correct the deformity and create a bone face  65  suitable for fusion ( FIGS. 7 and 8 ). With these two cuts complete, the bones of the metatarsal-phalange joint can be properly aligned ( FIG. 8 ) for subsequent fusion, as will hereinafter be discussed. 
         [0065]    Next, the surgeon inserts a k-wire  70  through the distal end of the toe, phalange  60  and into the metatarsal  50  ( FIG. 9 ). Preferably k-wire  70  passes down the intramedullary canals of phalange  60  and metatarsal  50 . Removal of k-wire  70  leaves a canal  75  (i.e., the opened intramedullary canal) in phalange  60  and a canal  80  in metatarsal  50 , with canal  75  in phalange  60  being aligned with canal  80  in metatarsal  50  when phalange  60  is aligned with metatarsal  50  ( FIG. 10 ). Canals  75  and  80  receive intramedullary fusion device  5  as will hereinafter be discussed. 
         [0066]    Following removal of k-wire  70 , phalange  60  is flexed downward so as to expose the prepared metatarsal face  55  ( FIG. 11 ). 
         [0067]    Next, with retaining tabs  30 ,  40  constraining barbs  25 ,  35 , respectively, of first barbed end region  10  and second barbed end region  15 , respectively, to their “inboard” position (i.e., as shown in  FIG. 2 ), intramedullary fusion device  5  has its first barbed end region  10  advanced into canal  80  of metatarsal  50  ( FIG. 12 ). Then removable retaining tab  30  is removed, allowing barbs  25  of first barbed end region  10  to expand outwardly and grip the side wall of canal  80 , whereby to securely fasten intramedullary fusion device  5  to metatarsal  50  through an expansive force against the intramedullary surface ( FIG. 13 ). Phalange  60  is then pressed over second barbed region  15  of intramedullary fusion device  5 , with second barbed region  15  being received in canal  75  of phalange  60  ( FIG. 14 ). This action brings face  65  of phalange  60  against face  55  of metatarsal  50 , with just enough room being left for retaining tab  40  to extend from intramedullary device  5  to a region outside of the bone. 
         [0068]    Next, retaining tab  40  is removed so that barbs  35  of second barbed end region  15  are allowed to expand outwardly and grip the side wall of canal  75  of phalange  60  ( FIG. 15 ). At this point barbs  25  of first barbed end region  10  are securely engaging metatarsal  50 , and barbs  35  of second barbed end region  15  are securely engaging phalange  60 , with central bridge region  20  extending across the fracture line. A force can be applied to reduce any gap left after removing retaining tab  40 . 
         [0069]    It should be appreciated that novel intramedullary fusion device  5  can first be implanted into phalange  60  and then implanted into metatarsal  50  if the surgeon so chooses. 
         [0070]    If desired, and looking now at  FIG. 15A , central bridge region  20  may have an enlargement  83  intermediate its length to act as a stop, limiting how far intramedullary fusion device  5  can be pushed into either side of the intramedullary canal during implantation. 
         [0071]    In  FIGS. 1-4 , barbs  25  of first barbed end region  10  are shown as being circumferentially offset from barbs  35  of second barbed end region  15 , i.e., barbs  25  and barbs  35  are not axially aligned with one another. However, if desired, and looking now at  FIG. 15B , barbs  25  of first barbed end region  10  may not be circumferentially offset from barbs  35  of second barbed end region  15 , i.e., barbs  25  and barbs  35  may be axially aligned with one another in the manner shown in  FIG. 15B . 
         [0072]    If desired, and looking now at  FIGS. 16-18 , intramedullary fusion device  5  can have a slight bend from central bridge region  20  to one or both of its first barbed end region  10  and second barbed end region  15 . By way of example but not limitation, in the metatarsal-phalange fusion shown in  FIGS. 5-15 , it may be desirable to provide a slight bend to second barbed end region  15  so as to facilitate the restoration of the normal anatomy. In this form of the invention, intramedullary fusion device  5  may be bent after machining and during the working of the shape memory material, e.g., it may be shape-set at the desired angulation through heat treatment. 
         [0073]    It should also be appreciated that the central bridge region  20  can be processed so as to be malleable (i.e., to take a set). At body temperature, the barb regions  10  and  15  can be superelastic while central bridge region  20  can be fully annealed Nitinol or martensitic Nitinol, such that central bridge region  20  is malleable and can take a set. This allows the surgeon to deform central bridge region  20  at the time of surgery so that it assumes the bend desired. 
         [0074]    Looking next at  FIG. 19 , there is shown an intramedullary fusion system  85  which generally comprises an intramedullary fusion device  90 , an internal restrainer  95  and a locking pin  100 . 
         [0075]    Intramedullary fusion device  90  is manufactured from a shape memory material (e.g., a material capable of exhibiting superelasticity and/or a temperature-induced shape change). The shape memory material may be a metal alloy (e.g., Nitinol) or a polymer (e.g., appropriately processed PEEK). Looking now at  FIGS. 20 and 21 , intramedullary fusion device  90  comprises a first barbed end region  105 , a second barbed end region  110 , and a central bridge region  115  connecting first barbed end region  105  and second barbed end region  110 . Intramedullary fusion device  90  is preferably cannulated so as to allow the intramedullary fusion device to be installed over a k-wire if desired, while also allowing a k-wire to be passed through the device following implantation if the surgeon desires to fuse a distal or proximal joint. The first and second barbed regions flare outward in multiple planes, preferably engaging the surrounding bone about the full circumference of the intramedullary fusion device, thereby providing excellent torsional stability to the fusion site. 
         [0076]    First barbed end region  105  comprises a plurality of barbs  120  which, in their unbiased condition, flare outward from the longitudinal axis of intramedullary fusion device  90  in the manner shown in  FIG. 20 . The flare may increase linearly over the length of the barb, or it may increase non-linearly over the length of the barb to enable first barbed end region  105  to better engage the “hourglass-shaped” intramedullary canal. The better that first barbed region  105  engages the intramedullary canal, the more even the pressure distribution will be. While  FIG. 20  illustrates a device with four barbs  120  in first barbed end region  105 , it should be appreciated that the device can have more or fewer barbs. Barbs  120  can be strained to a position parallel to the longitudinal axis of intramedullary fusion device  90 , e.g., during insertion into a hole drilled in bone. Once inserted into the intramedullary canal, barbs  120  flare outwardly so as to engage with the side wall of the drilled hole receiving first barbed end region  105 . By angling barbs  120  relative to the longitudinal axis of intramedullary fusion device  90 , i.e., with an arrowhead configuration, barbs  120  can ensure that first barbed end region  105  is insertable into a hole in a bone but not withdrawable from the hole in the bone. In this way barbs  120  can selectively lock first barbed end region  105  to a bone fragment, as will hereinafter be discussed. 
         [0077]    Second barbed end region  110  comprises a plurality of barbs  125 , which, in their unbiased condition, flare outward from the longitudinal axis of intramedullary fusion device  90  in the manner shown in  FIG. 20 . The flare may increase linearly over the length of the barb, or it may increase non-linearly over the length of the barb to enable second barbed end region  110  to better engage the “hourglass-shaped” intramedullary canal. The better that second barbed region  110  engages the intramedullary canal, the more even the pressure distribution will be. While  FIG. 20  illustrates a device with four barbs  125  in second barbed end region  125 , it should be appreciated that the device can have more or fewer barbs. Barbs  125  can be strained to a position parallel to the longitudinal axis of intramedullary fusion device  90 , e.g., during insertion into a hole drilled in bone, with barbs  125  flaring outwardly so as to remain in constant engagement with the side wall of the drilled hole receiving second barbed end region  110 . By angling barbs  125  relative to the longitudinal axis of intramedullary fusion device  90 , i.e., with an arrowhead configuration, barbs  125  can ensure that second barbed end region  110  is insertable into a hole in a bone but not withdrawable from the hole in the bone. In this way barbs  125  can selectively lock second barbed end region  110  in the intramedullary canal of a bone fragment, as will hereinafter be discussed. 
         [0078]    Note that barbs  120  of first barbed end region  105  are flared in a direction which is opposite to that of barbs  125  of second barbed end region  110 . 
         [0079]    Central bridge region  115  preferably comprises a generally cylindrical shape and is configured so that it can be selectively strained (i.e., stretched) longitudinally and constrained in that position (e.g., via the aforementioned internal restrainer  95  and locking pin  100 ). As will hereinafter be discussed, upon removing locking pin  100 , internal restrainer  95  will release the constraint on central bridge region  115 , whereupon central bridge region  115  will attempt to foreshorten. As will also hereinafter be discussed, this foreshortening can be harnessed to apply compression between two bone fragments. 
         [0080]    In order to allow central bridge region  115  to be constrained in its longitudinally stretched state, intramedullary fusion device  90  preferably comprises cutouts  130 , disposed in first barbed end region  105  and second barbed end region  110 , that allow central bridge region  115  to be constrained in its longitudinally stretched state by internal restrainer  95 , as will hereinafter be discussed. 
         [0081]    Internal restrainer  95  is shown in further detail in  FIG. 22 . Internal restrainer  95  may comprise a shape memory material if desired. Internal restrainer  95  generally comprises a cannulated cylindrical body  140  terminating in a pair of fingers  145  at each end of cylindrical body  140 . Each pair of fingers  145  are normally biased together, however, they may be elastically forced apart so that they extend outboard beyond the circumference of cylindrical body  140 , whereby to allow fingers  145  of internal restrainer  90  to lock to cutouts  130  of intramedullary fusion device  90  when intramedullary fusion device  90  is in its longitudinally stretched state, as will hereinafter be discussed. 
         [0082]    As noted above, central bridge region  115  of intramedullary fusion device  90  can be strained (i.e., longitudinally stretched), locked in that position via internal restrainer  95  and locking pin  100  and, upon removing locking pin  100 , internal restrainer  95  will release the constraint on central bridge region  115  of intramedullary fusion device  90 , whereupon central bridge region  115  will attempt to foreshorten. More particularly, and looking now at  FIGS. 23-25 , central bridge region  115  is stretched using a stretching mechanism (not shown) of the sort which will be apparent to those skilled in the art in view of the present disclosure, and internal restrainer  95  is inserted into intramedullary fusion device  90 . As internal restrainer  95  is inserted into intramedullary fusion device  90 , the pairs of fingers  145  disposed at each end of internal restrainer  95  are aligned with cutouts  130  in first barbed end region  105  and second barbed end region  110 . Locking pin  100  is then inserted into internal restrainer  95 , whereby to cause the pairs of fingers  145  disposed at each end of internal restrainer  95  to project through cutouts  140  of intramedullary fusion device  90 , whereby to lock central bridge region  115  of intramedullary fusion device  90  in its strained (i.e., longitudinally stretched) state. The external load stretching central bridge region  105  (i.e., via the aforementioned stretching mechanism, not shown) can now be removed, and central bridge region  115  of intramedullary fusion device  90  will remain in its strained (i.e., stretched) state due to the action of internal restrainer  95  and locking pin  100 . However, when locking pin  100  is thereafter removed from the interior of internal restrainer  95 , the pairs of fingers  145  disposed at each end of internal restrainer  95  retract from cutouts  130  by virtue of their inward bias, allowing central bridge region  115  of intramedullary fusion device  90  to foreshorten, whereby to generate compression between the bone fragments, as will hereinafter be discussed. 
         [0083]    Intramedullary fusion system  85  may be used to secure together two bone fragments and apply compression to the fracture line. By way of example but not limitation, and looking now at  FIGS. 26-36 , intramedullary fusion system  85  may be used to treat a hammertoe deformity ( FIG. 26 ). 
         [0084]    First, the distal end of metatarsal  50  is cut off to correct the deformity and create a bone face  55  suitable for fusion ( FIGS. 27 and 28 ). Then the proximal end of phalange  60  is removed to correct the deformity and create a bone face  65  suitable for fusion ( FIGS. 28 and 29 ). With these two cuts complete, the bones of the metatarsal-phalange joint can be properly aligned ( FIG. 29 ) for subsequent fusion, as will hereinafter be discussed. 
         [0085]    Next, the surgeon inserts k-wire  70  through the distal end of the toe, then through phalange  60  and into metatarsal  50  ( FIG. 30 ). Preferably k-wire  70  passes down the intramedullary canals of phalange  60  and metatarsal  50 . Removal of k-wire  70  leaves a canal  75  (i.e., the opened intramedullary canal) in phalange  60  and a canal  80  in metatarsal  50 , with canal  75  in phalange  60  being aligned with canal  80  in metatarsal  50  when phalange  60  is aligned with metatarsal  50  ( FIG. 31 ). Canals  75  and  80  receive intramedullary fusion system  85  as will hereinafter be discussed. 
         [0086]    Following removal of k-wire  70 , phalange  60  is flexed downward so as to expose the prepared metatarsal face  55  ( FIG. 32 ). 
         [0087]    Intramedullary fusion device  90 , which has previously been strained (i.e., its central bridge region  115  longitudinally stretched) and locked in this state with internal restrainer  95  and locking pin  100 , is then implanted into canal  75  in phalange  60  ( FIG. 33 ), i.e., by advancing the free end of locking pin  100  and second barbed end region  110  of intramedullary fusion device  90  into canal  75  of phalange  60 . Note that the flare on barbs  125  of second barbed end region  110  is such that intramedullary fusion device  90  can be advanced into canal  75  of phalange  60  but not withdrawn. Note also that the free end of locking pin  100  extends completely out of phalange  60  and any adjacent bone structure(s) so that it is graspable by the surgeon and able to be retracted when desired through the distal end of the toe. 
         [0088]    Phalange  60  is then reoriented so that first barbed end region  105  is aligned with canal  80  in metatarsal  50 , and then phalange  60  is advanced towards metatarsal  50  so that first barbed end region  105  enters canal  80  of metatarsal  50  ( FIG. 34 ). Note that the flare on barbs  120  of first barbed end region  105  of intramedullary fusion device  90  is such that intramedullary fusion device  90  can be advanced into canal  80  of metatarsal  50  but not withdrawn. 
         [0089]    Phalange  60  is advanced towards metatarsal  50  until face  65  of phalange  60  engages face  55  of metatarsal  50 . At this point barbs  120  of first barbed end region  105  and barbs  125  of second barbed end region  110  prevent phalange  60  and metatarsal  50  from moving apart. 
         [0090]    With intramedullary fusion device  90  firmly secured to both phalange  60  and metatarsal  50 , locking pin  100  is then removed from internal restrainer  95  ( FIG. 35 ), i.e., by being retracted through the distal end of the toe. This allows the pairs of fingers  145  disposed on each end of internal restrainer  95  to retract from cutouts  130  of intramedullary fusion device  90 , which allows central bridge region  115  of intramedullary fusion device  90  to foreshorten, whereby to generate compression between metatarsal  50  and phalange  60  ( FIG. 36 ). 
         [0091]    If desired, internal restrainer  95  may be left in place within intramedullary fusion device  90  or, more preferably, internal restrainer  95  may be removed from intramedullary fusion device  90  after intramedullary fusion device  90  has been set, e.g., by grasping internal restrainer  95  with a grasping tool and drawing internal restrainer  95  longitudinally out of intramedullary fusion device  90  and then out canal  75  of phalange  60 . Alternatively, internal restrainer  95  can be automatically removed from intramedullary fusion device  90  when locking pin  100  is removed from internal restrainer  95 , e.g., by providing the distal end of locking pin  100  and the proximal end of internal restrainer  95  with an appropriate “catch mechanism” so that the retreating locking pin  100  engages internal restrainer  95  and carries internal restrainer  95  out of intramedullary fusion device  90  and then out canal  75  of phalange  60 . 
         [0092]    If desired, and looking now at  FIG. 37 , intramedullary fusion device  90  can have a slight bend at one or both of its first barbed end region  105  and second barbed end region  110 . By way of example but not limitation, in the metatarsal-phalange fusion shown in  FIGS. 26-36 , it may be desirable to provide a slight bend to second barbed end region  110  so as to facilitate restoration of the normal anatomy. In this form of the invention, intramedullary fusion device  90  may be bent after machining and during the working of the shape memory material, e.g., it may be shape-set at the desired angulation through heat treatment. 
         [0093]    Intramedullary fusion device  90  is specifically engineered so not to “tear through” the bone tissue when central bridge region  115  foreshortens. The compressive forces of intramedullary fusion device  90  can be controlled by modulating the material properties of the intramedullary fusion device and/or the geometry of the intramedullary fusion device. 
         [0094]    The percentage of cold work in the shape memory material forming intramedullary fusion device  90  affects the compressive force generated by the intramedullary fusion device. As the percentage of cold work increases, the compression force declines. The intramedullary fusion device should, preferably, have between about 15% and 55% cold work to control the recovery force of the intramedullary device. 
         [0095]    Another material property that affects the intramedullary fusion device&#39;s compression force is the temperature differential between the body that the intramedullary fusion device will be implanted into (assumed to be 37° C., which is the temperature of a human body) and the austenite finish temperature of the shape memory material forming intramedullary fusion device  90 . A smaller temperature differential between the two will result in the intramedullary fusion device generating a small compressive load; conversely, the larger the temperature differential between the two will result in the intramedullary device generating a larger compressive load. The shape memory material that the intramedullary fusion device is made out of should, preferably, have an austenite finish temperature of greater than about −10° C., resulting in a temperature differential of less than about 47° C. when the intramedullary fusion device is implanted (assuming that the intramedullary fusion device is implanted in a human body). 
         [0096]    The geometry of the intramedullary fusion device also affects the compression force generated. The cross-sectional area of the hollow central bridge region  115  affects the compression force. As the cross-sectional area increases, so does the compression force that the intramedullary fusion device  90  will generate. In this respect it should be appreciated that it is beneficial for the compression force generated by the foreshortening of intramedullary fusion device  90  to be constant as the bone relaxes and remodels. Thus, the cross-section of hollow central bridge region  115  of intramedullary fusion device  90  preferably has a constant cross-section over its entire length. Cross-sections that are not uniform over the length of hollow central bridge region  115  can result in an increase or decrease in compression as the intramedullary fusion device foreshortens. 
         [0097]    The barbs  120 ,  125  are important for transmitting the compression force to the bone without “tearing through” the bone. The height, width, and number of barbs  120 ,  125  on the intramedullary device  90  are all important to the intramedullary device&#39;s ability to not “tear through” the bone. 
         [0098]    It should also be appreciated that shape memory material can be processed to exhibit two-way shape memory. The intramedullary fusion device  90  can be trained to have an austenitic shape (i.e., barbs expanded) and a martensitic shape (i.e., barbs extending parallel to the longitudinal axis of intramedullary fusion device  90 ) In this case, the barbs can be in their austenitic shape at about body temperature. The barbs can be deformed via the creation of stress induced martensite to implant the intramedullary fusion device. If the intramedullary fusion device thereafter needs to be removed, the intramedullary fusion device may be cooled (e.g., with cold saline) to a temperature below the austenite start temperature of the shape memory material, and more preferably below the martensite start temperature of the shape memory material, and most preferably below the martensite finish temperature of the shape memory material. When cooled, the intramedullary fusion device  90  will take on its martensitic shape (i.e., the barbs laying parallel to the longitudinal axis of the intramedullary fusion device), and the surgeon can easily remove the intramedullary fusion device. 
         [0099]    Additionally, the intramedullary fusion device can be made such that central bridge region  115  of intramedullary fusion device  90  has one austenite start temperature, and such that barbs  120 ,  125  have a lower austenite start temperature. Thus, the intramedullary fusion device can be stretched at a temperature less than the austenite start temperature of central bridge region  115  but above the austenite start temperature of barbs  120 ,  125 . Thus barbs  120 ,  125  will be in the austenite phase and able to undergo a stress induced martensite transformation during insertion of intramedullary fusion device  90  into a bone canal. Maintaining the intramedullary fusion device at a temperature below the austenite start temperature of central bridge region  115  allows the intramedullary fusion device to remain in its elongated state. The intramedullary fusion device can then be advanced into a bone canal as discussed above. When the central bridge region  115  warms (either to body temperature, or to a temperature above body temperature, e.g., through the application of warm saline), central bridge region  115  will foreshorten, generating and maintaining compression across the fracture line. 
         [0100]    It should also be appreciated that central bridge region  115  of intramedullary fusion device  90  can be processed so as to be malleable (i.e., to take a set). At body temperature, first barbed end region  105  and second barbed end region  110  can be superelastic while central bridge region  115  can be fully annealed Nitinol or martensitic Nitinol. This allows the surgeon to deform the implant at the time of surgery to the bend desired. 
       Modifications Of The Preferred Embodiments 
       [0101]    It should be understood that many additional changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the present invention, may be made by those skilled in the art while still remaining within the principles and scope of the