Abstract:
A device for treating fractures, comprises a plate receiving structure including on a bone facing side thereof a recess sized and shaped to receive a fixation plate anchored in a desired position on a bone and a mating structure sized and located to engage a corresponding structure of the fixation plate to prevent relative movement between the fixation plate and the device in combination with at least one leg projecting laterally away from the recess, a first one of the at least one legs including a first fixation element receiving hole extending therethrough, the first leg being positioned and oriented so that, when the device is received on a fixation plate anchored to the bone in the desired position, the first fixation element receiving hole is aligned to pass a fixation element inserted therethrough into the bone without passing through a longitudinal axis of the medullary canal.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims priority to the U.S. Provisional Application No. 60/987,560 entitled “Periprosthetic Fracture Repair,” filed Nov. 13, 2007. The specification of the above-identified application is incorporated herewith by reference. 
     
    
     BACKGROUND 
       [0002]    The use of fixation plates to treat periprosthetic fractures has been limited by the interference of a prosthetic within the medullary canal poses to the insertion of screws, pegs, nails or other fixation devices therethrough. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention is directed to a device for treating fractures, comprising a plate receiving structure including on a bone facing side thereof a recess sized and shaped to receive a fixation plate and a mating structure sized and located to engage a corresponding structure of the fixation plate to prevent relative movement between the fixation plate and the device in combination with one or more legs projecting laterally away from the recess, at least a first one of the legs including a fixation element receiving hole extending therethrough, the first leg being positioned and oriented so that, when the device is received on a fixation plate anchored to the bone, the fixation element receiving hole is aligned to pass a fixation element inserted therethrough into the bone without passing through a longitudinal axis of the medullary canal. Those skilled in the art will understand that medullary canals are generally neither straight nor concentric with the bone. Thus the axis of the medullary canal, as that term is used in this application, refers to a curve connecting points in the center of the medullary canal along the length of the bone. 
         [0004]    The present invention is further directed to a method for treating fractures, comprising coupling a first end of a longitudinal fixation device to a bone on a first side of a fracture so that the longitudinal fixation device extends along the bone substantially parallel to a longitudinal axis of the medullary canal and coupling a first lateral fixation device to a portion of the longitudinal fixation device extending over a portion of the bone on a second side of the fracture by mating the longitudinal fixation device within a recess of the first lateral fixation device in combination with coupling the first lateral fixation device to the bone by inserting a fixation element through a hole formed in a portion thereof separated from the longitudinal fixation device by a selected distance around a portion of a circumference of the bone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a side view of a fracture located on a distal end of a hip prosthesis; 
           [0006]      FIG. 2  is a side view of a fracture located distal to a hip prosthesis; 
           [0007]      FIG. 3  is a side view of a fracture located near to a knee prosthesis; 
           [0008]      FIG. 4  is a perspective view of an embodiment of a fixation kit according to the invention; 
           [0009]      FIG. 5  is a side view of the fixation kit of  FIG. 4 ; 
           [0010]      FIG. 6  is a front view of the fixation kit of  FIG. 4 ; 
           [0011]      FIG. 7  is a perspective view of a first embodiment of an attachment plate according to the invention; 
           [0012]      FIG. 8  is a top view of the attachment plate of  FIG. 7 ; 
           [0013]      FIG. 9  is a front view of the attachment plate of  FIG. 7 ; 
           [0014]      FIG. 10  is a perspective view of a second embodiment of an attachment plate according to the invention; 
           [0015]      FIG. 11  is cross-sectional view of the fixation kit of  FIG. 4 ; 
           [0016]      FIG. 12  is a perspective view of an embodiment of an aiming block according to the invention; 
           [0017]      FIG. 13A  is a side view of an embodiment of a fixation device according to the invention; 
           [0018]      FIG. 13B  is a cross-sectional view of the fixation device of  FIG. 13A ; 
           [0019]      FIG. 13C  is a top view of the fixation device of  FIG. 13A ; 
           [0020]      FIG. 13D  is a front view of a distal portion of the fixation device of  FIG. 13A ; 
           [0021]      FIG. 14A  is a side view of a second embodiment of a fixation device according to the invention; 
           [0022]      FIG. 14B  is a cross-sectional view of the fixation device of  FIG. 14A ; 
           [0023]      FIG. 14C  is a top view of the fixation device of  FIG. 14A ; 
           [0024]      FIG. 14D  is a front view of a distal portion of the fixation device of  FIG. 14A ; 
           [0025]      FIG. 15A  is a cross-sectional view of a third embodiment of a fixation device according to the invention; 
           [0026]      FIG. 15B  is a top view of the fixation device of  FIG. 15A ; 
           [0027]      FIG. 16A  is a cross-sectional view of a fourth embodiment of a fixation device according to the invention; 
           [0028]      FIG. 16B  is a top view of the fixation device of  FIG. 16A ; 
           [0029]      FIG. 17A  is a cross-sectional view of a fifth embodiment of a fixation device according to the invention; 
           [0030]      FIG. 17B  is a top view of the fixation device of  FIG. 17A ; 
           [0031]      FIG. 18A  is a cross-sectional view of a sixth embodiment of a fixation device according to the invention; and 
           [0032]      FIG. 18B  is a top view of the fixation device of  FIG. 18A . 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    The present invention may be further understood with reference to the following description and to the appended drawings, wherein like elements are referred to with the same reference numerals. The present invention relates to devices for treatment of fractures. In particular, the invention relates to improved methods and systems for repairing periprosthetic fractures. Although exemplary embodiments of the present invention will be discussed with reference to knee and hip prostheses, the present invention may be successfully implemented in any long bone including a prosthetic device inserted into its medullary canal. In addition, as would be understood by those skilled in the art, the present invention may be used for the treatment of fractures around nails and for “conventional” fractures in patients with poor bone quality. The present invention allows the user to apply standard fixation plates while placing screws and/or pins therethrough into the bone along paths selected to: 1) avoid any prosthesis in the medullary canal; 2) align the screws/pins non-parallel to one another to improve purchase in the bone; and/or 3) maximize the length of cancellous bone through which the screws/pins pass. 
         [0034]    Periprosthetic fractures may occur intraoperatively (during implantation or replacement of a prosthetic), or postoperatively (e.g., as a result of stress or trauma to the bone in which the prosthetic was previously implanted). As would be understood by those skilled in the art, fractures have been effectively treated by stabilizing the bone using fixation plates (e.g., dynamic compression plates (DCPs), locking compression plates (LCPs), etc.), which typically comprise a metal plate including a plurality of holes through which anchoring of screws or other fixation devices are inserted into underlying bone tissue. Periprosthetic fractures are more difficult to treat than ordinary fractures because a prosthesis extending within the medullary canal may interfere with the proper coupling of a fixation plate across the fracture by preventing the fixation devices from being inserted through the bone across the medullary canal. For example, hip prostheses may interfere with certain fractures of the femur. These hip prostheses often include a ball joint and a stem which is inserted into the medullary canal of the femur. As the femur absorbs significant stresses with each step, to adequately couple a fixation plate thereto, it is desired to maximize the purchase of the fixation devices in the femur. 
         [0035]    In determining a course of treatment, the needs of the patient must be considered. For example, an active 55 year old with a periprosthetic fracture will likely have functional demands different from those of a sedentary 85 year old. Important factors to consider include the location of the fracture, how well-fixed the prosthesis is, and the quality of the femoral bone stock. The Vancouver classification divides periprosthetic hip fractures into three categories: Type A fractures are trochanteric (i.e., disposed at or near the greater or lesser trochanters); Type B fractures occur around the stem of the prosthesis; and Type C fractures occur so far from the stem that the fracture may be treated as a general fracture (i.e., the prosthesis may be ignored). Of these fractures, Type B is the most common. As used in the following descriptions of exemplary embodiments of the invention, the term “distal” refers to a direction away from the end of the bone through which the prosthesis is inserted into the medullary canal. Thus, the distal end of a hip prosthesis is that which is located furthest from the pelvis and the proximal end is that which is nearest to the pelvis.  FIG. 1  shows an example of a Type B fracture  50  located along a distal portion  110  of a hip prosthesis  100 . As discussed above, the present invention may also be used to treat general fractures (e.g., Type C fractures). An example of a Type C fracture is shown in  FIG. 2 . In particular,  FIG. 2  shows a fracture  52  located distal of the hip prosthetic  100 . However, those skilled in the art will understand that the apparatus according to the present invention may also be used to treat Type A fractures as well as similar fractures of other bones. For example,  FIG. 3  shows a treatable fracture  54  located near a knee prosthetic  130 . 
         [0036]    Regardless of how the fracture is classified, complications common to each of the fracture types may make proper treatment critical while creating difficulties, rendering aspects of standard fracture treatment unsuitable. For example, standard fixation plates are typically fixed by inserting one or more fixation devices (e.g., bone screws) substantially diametrically through the bone. Thus, these fixation devices pass through the periosteum and compact bone adjacent to the fixation plate, through the medullary canal and then into the compact bone on the opposite side of the medullary canal. When a prosthesis occupies the medullary canal, inserting a screw directly therethrough is no longer possible and inserting the screw through only that portion of compact bone between the fixation plate and the medullary canal often does not provide sufficient anchorage. Thus, it is desirable to maximize the length of the screw within the compact bone. Accordingly, exemplary embodiments of fracture repair devices according to the present invention, as will be described below, enable the anchoring of screws or other fixation devices along extended paths through compact bone without contacting the prosthesis occupying the medullary canal. Furthermore, the devices and methods according to the present invention may also allow the user to customize the configuration and location of the fixation plate to achieve a desired fracture treatment. 
         [0037]    Exemplary embodiments of a fixation kit according to the present invention will now be described with reference to fracture repair devices designed to work in conjunction with any conventional fixation plate such as, for example, locking compression plates (LCPs). Exemplary embodiments of the fracture repair devices may be utilized in conjunction with any number of different types of LCPs or other fixation plates including, for example, an LCP broad curved plate, an LCP broad plate, an LCP Distal Femur (DF) plate, an LCP Less Invasive Stablization System (LISS) plate, an LCP proximal femur plate, an LCP proximal femur with hook plate, an LCP condylar plate, etc. As will be understood by those skilled in the art, the exemplary fracture repair devices may also be used with other conventional fixation plates in addition to LCPs. Thus, the fixation plate may be selected to fit a specific situation in the same manner as would be done if there were no prosthesis or other reason for avoiding the insertion of fixation devices through the axis of the medullary canal. 
         [0038]      FIGS. 4-6  show a fixation kit including an LCP  500  and two fracture repair devices  200  in place on a femur  10 . An intermedullary prosthesis  100  (shown in hidden view) has been inserted into the medullary canal of the femur  10  and the LCP  500  extends proximally from a distal end  510  on a portion of the femur  10  distal of a distal end of the prosthesis  100  across a fracture to a proximal end  520 . Two devices  200  are received over proximal and medial portions of the LCP  500  as these portions of the LCP  500  overlay the prosthesis  100 . As the distal end  510  extends distally beyond the distal end of the prosthesis  100 , it may be secured to the femur in any conventional manner (e.g., by one or more bone screws inserted straight through the medullary canal and the compact bone on either side thereof). As would be understood by those skilled in the art, the size, configuration and/or location of the devices  200  may vary depending on anatomy, fracture location and the position and/or size of a prosthesis relative to the LCP  500 . For example, devices  200  may be placed only where the LCP overlays a prosthesis, on both sides of a fracture regardless of location of the fracture relative to the LCP  500  or in any other desired arrangement so long as the required bond between the LCP  500  and the underlying bone is established. Thus, one or more additional devices  200  may be attached to the distal end  510  of the LCP  500  or at any other locations to provide further stabilization. The LCP  500  of  FIGS. 4-6  extends distally beyond the distal end of the prosthesis  100 . However, in other embodiments a shorter LCP may be selected with devices  200  providing support at all points along the length thereof. Those skilled in the art will understand that the number and the location of the devices  200  may be determined according to physician preference. Therefore, in some embodiments the LCP  500  may be coupled to only a single device  200  (e.g., by centering the device  200  over the fracture) supplemented as desired by additional fixation devices including, for example, screws inserted only through the portion of compact bone adjacent to the LCP  500 . Each device  200  is coupled to the LCP  500  via a screw  227  and includes one or more screws  257  or other fixation devices that anchor the device  200  to the bone  10 . 
         [0039]    As shown in  FIG. 5 , a distal portion of the LCP  500  extends beyond the prosthesis  100  and is anchored directly into the bone through the medullary canal as is done in the manner of general fractures, when no prosthesis is present. The distal portion may be anchored using any number of screws  527  spaced in accordance with physician preference. Portions of the LCP  500  that extend along the length of the prosthesis  100  either contain no screws that enter the medullary canal (e.g., portions not coupled to a device  200 ) or are secured via a device  200 , which is screwed (via the screws  257 ) at an angle into the bone  10  to avoid the prosthesis  100 . 
         [0040]    The device  200  shown in  FIGS. 7-9  is shaped for use in conjunction with the LCP  500  of  FIGS. 4-6 . Of course, those skilled in the art will understand that any number of varieties of devices  200  may be formed for use in conjunction with any of a variety of different fixation plates of different sizes and shapes. The device  200  includes a locking attachment plate  202  formed of a substantially rigid biocompatible material such as, for example, plastic, medical-grade steel or titanium as would be understood by those skilled in the art. The attachment plate  202  includes a body  210  including, on a bone-facing side thereof, a recess contoured to receive the LCP  500 . Sidewalls  215  of the recess may preferably be shaped to substantially conform to the contours of the LCP  500 . Although the sidewalls  215  need not form a tight fit against corresponding sides of the LCP  500 , some embodiments may include sidewalls that snap-fit or otherwise couple to the LCP  500 . The body  210  may also include a coupling feature that corresponds to a corresponding feature on the LCP  500 . For example, the body  210  may include a centrally located screw hole  225  that corresponds to a coupling arrangement (e.g., a threaded bore) of the LCP  500 . Thus, the body  210  may be coupled to the LCP  500  by aligning the attachment plate  202  over to the LCP  500  and inserting a screw (e.g., the screw  227 ) or other fixation device through the hole  225  and into the threaded bore. The bore may pass through the entire body of the LCP  500 , enabling the screw  227  to extend past a bone-facing surface of the LCP  500 . Thus, in some embodiments, the screw  227  may be driven into the compact bone on the side of the medullary canal facing the LCP  500  without contacting the underlying intermedullary prosthesis. However, in other embodiments, the screw  227  may not extend past the LCP  500 , serving only to couple the LCP  500  and the device  200 . Alternatively, as would be understood by those skilled in the art, a fracture repair device according to the invention may include a projection aligned to mate with a corresponding recess in an LCP (e.g., the threaded bore), a recess aligned to mate with a corresponding projection of an LCP or a combination of such recess/projection matings. As discussed above, the sidewalls  215  may be snap-fit onto the LCP  500 . Other coupling arrangements, such as, for example, friction-fitting, adhesives, bolts, etc. may also be used to couple the LCP  500  and the attachment plate  202  as would be understood by those skilled in the art. 
         [0041]    The attachment plate  202  includes one or more arms  220  extending laterally from the sidewalls  215 , away from the body  210 . Each of the sidewalls  215  includes an arm  220  extending from each end thereof and each of the arms  220  includes a first and a second screw hole  222 ,  224 , respectively, extending therethrough. However, those skilled in the art will understand that the number of arms per sidewall may vary. The arms  220  may be formed integrally with the body  210  or attached separately. Each of the arms  220  is preferably oriented such that a bone-facing surface of the arm  220  is generally follows the contours of a bone on which the arm  220  is to be mounted. Optionally, the arms may be formed of a material which may be bent by a user into a desired configuration to customize the arms  220  to the anatomy of each patient. As shown in  FIG. 9 , when viewed in a plane substantially perpendicular to a longitudinal axis of the medullary canal, each of the arms  220  extends along a curve substantially approximating the shape of an outer surface of a bone on which it is to be mounted. 
         [0042]    As seen in  FIGS. 7-9 , each arm  220  also extends away from the corresponding side wall  215  at an angle within a plane of the body  210 . Those skilled in the art will understand that the selection of this angle and any change in this angle between the first and second screw holes  222 ,  224  allow for the application of additional screws at different angles and/or at different locations or, for example, to increase the area over which the attachment forces are applied to the bone. As indicated above, each of the arms  220  includes a first screw hole  222  adjacent to the corresponding side wall  215  and a second screw hole  224  extending laterally away from the first screw hole  222 . Although each of the arms  220  is shown extending at substantially the same angle, those skilled in the art will understand that the arms  220  may extend at different angles to accommodate varying bone structure, LCP shapes, etc. and may include the same number or different numbers of screw holes. 
         [0043]    As described above and as shown in  FIG. 9 , the arms  220  preferably curve to enable the arms  220  to wrap around the bone  10  in a substantially conforming manner. A degree to which the arms  220  encircle the bone  10  may vary depending on the curvature and the angle of each arm  220  in relation to bone physiology. Thus, the arms  220  may produce a tighter fit when mounted to wide portions of the bone  10 , while providing a looser fit when mounted to narrow bone portions. In addition, the arms  220  may flex, allowing the arms  220  to be mounted closer to the bone  10 . 
         [0044]    Each of the first and second screw holes  222 ,  224 , respectively, may be threaded to match a threading of a locking head of a screw  257  or may be otherwise suited to receive the particular fixation device to be employed with the device  200 . As would be understood by those skilled in the art, the number of screw holes in each arm  220  may vary based on factors such as LCP shape, bone anatomy, desired degree of stabilization, etc. As shown in  FIG. 10 , an exemplary attachment plate  302  is substantially similar to the plate  202  of  FIGS. 7-9  except that each of the arms  320  has only one screw hole  250  extending therethrough. It will be understood by those of skill in the art that the attachment plate  302  may be used according to the device  200  in substantially the same manner as the attachment plate  202 . In some embodiments, one or more arms may not include any holes at all or may contain more than two screw holes. 
         [0045]    Each of the holes  250  defines an angle of insertion for the screw  257  selected so that when the attachment plate  202  is mounted onto the bone  10 , the screw  257  passes through the bone  10  without diametrically passing through the medullary canal, thereby avoiding contact with the prosthesis  100 . Those skilled in the art will understand that the angle is preferably selected to maximize the length of the screw  257  received in the bone  10 . Those skilled in the art will also understand that some or all of the screw holes for any of the attachment plates according to the invention may be variable angle locking holes allowing for locking screws to be inserted therethrough and locked to the plate at multiple angles relative to the attachment plate. For example, any or all of the screw holes  222 ,  224  and  250  may be formed substantially in accord with the description in U.S. Patent Application Publication No. 2005/0165400 filed by Fernandez, Jul. 28, 2005, the entire disclosure of which is hereby incorporated by reference in its entirety. 
         [0046]    For example, the screws may have a head shaped like a sphere and threaded with a substantially constant pitch substantially equal to a pitch of a threaded shank of the screw. In addition, an insertion/extraction hole may be cut in the head for the connection of an insertion/extraction tool. The thread cut in the screw head may have a double entry maintaining substantially the same pitch as that of the thread of the shank. Of course, as would be understood by those skilled in the art, the thread profile may vary according to the requirements and according to the mechanical properties of the material of which the screw is formed. 
         [0047]    This allows the screw to be inserted into a properly designed screw hole at any angle within a wide range without affecting the position of the thread of the screw head with respect to walls of the screw hole. 
         [0048]    Specifically, such a screw hole may be formed in a spherical shape, with edges thereof at both ends of the hole removed in a frusto-conical shape. That is, the screw hole may include two frusto-conical portions extending toward one another from opposite surfaces of the plate and connected at tips of the cones through a partial sphere. The inner wall of each screw hole has a small number of isolated protrusions such as pegs or spikes (e.g., between two and thirty) designed to lock against the threaded spherical head of the screws when the screws are driven in through the screw holes. The protrusions may, for example, be somewhat flattened with a width bigger than its length. 
         [0049]    Once such a screw has been driven into such a screw hole, the spherical shape of the screw head allows it to lock against the protrusions without regard to whether the screw extends perpendicular or at a tilt relative to an axis of the screw hole. The angle at which of the screw is locked may then be varied by as much as 20° relative to the axis of the screw hole. 
         [0050]    In use, a physician may begin treatment by selecting an LCP  500  of appropriate size and shape, taking into account the width of the bone  10 , the location of the fracture and other factors as would be understood by those skilled in the art. The LCP  500  is then aligned over the bone  10  to extend across the fracture in a position selected to stabilize the portions of the bone on both sides thereof. The physician then has the option of initially securing the distal portion of the LCP  500  to the portion of the bone not including a prosthesis within the medullary canal or of selecting one or more attachment plates  202  to achieve the desired coupling of the LCP  500  and the proximal portion of the bone. If securing of the distal portion first is desired, the physician drives the screws  527  directly into the bone  10  in the same manner as would be used for a fracture where no prosthesis was present. Thereafter, the physician may slide the selected plate  202  over the proximal portion of the LCP  500  to the desired alignment and attach the plate  202  to the LCP  500 . Alternatively, as would be understood by those skilled in the art, the physician may attach the attachment plate to the bone in a desired location before attaching the LCP  500  to either the plate  202  or any portion of the bone and then slide the LCP  500  through the recess into place between the bone  10  and the attachment plate  202 . 
         [0051]    The physician may choose to couple the attachment plate  202  to the LCP  500  before attaching the attachment plate  202  to the bone  10 . The attachment plate  202  is positioned over a desired location of the LCP  500 . As shown in  FIGS. 5 and 6 , the LCP  500  includes multiple attachment arrangements comprising attachment sites  507  located along the entire length thereof. The attachment sites  507  are spaced apart, either uniformly or at different distances. For example, certain lengths of the LCP  500  may include more attachment sites (i.e., tighter spacing) than others. The attachment sites  507  may correspond to anchoring locations of the screws  227 . That is, the attachment arrangements may be the same as the holes through which the screws  527  may be driven although, as would be understood, the screws are used to couple the attachment plate  202  to the LCP  500  will be shorter than those used to directly couple the LCP  500  to the bone so as to avoid interference with the intermedullary prosthesis. 
         [0052]    After positioning over the LCP  500  substantially flush with the bone  10 , the attachment plate  202  is coupled to the LCP  500  by either driving the screw  227  to a depth beyond the bone-facing surface of the LCP  500  (i.e., into the bone  10 ) or to a depth within the body of the LCP  500 . As an alternative to coupling the attachment plate  202  during treatment, the coupling may occur prior to introduction of the LCP  500  into the patient. The attachment plate  202  is then anchored to the bone  10  by individually driving each screw  257  into the bone  10  at an angle selected by the physician (e.g., to maximize a length of the path the screw travels through the compact bone without entering the medullary canal). The bone  10  beneath each hole  250  is drilled out to a desired depth (e.g., a maximum depth of penetration of the compact bone without contacting the prosthesis  100 ). As an alternative to drilling, the screws  257  may be self-tapping. As would be understood by those skilled in the art, the maximum depth to which the screws  257  may be driven is a function of known factors such as, for example, bone anatomy and the available insertion angles. 
         [0053]    As would be understood by those skilled in the art, an aiming device such as an aiming block may be used to facilitate accurate drilling of the bone  10 .  FIG. 12  shows an exemplary embodiment of an aiming block  400  in an operative position. The aiming block  400 , which may be placed over the attachment plate  202  or over the combined attachment plate-LCP, includes a body portion  410  including a hole  425  matching the hole  225 . Although the hole  425  is not strictly required, including the hole  425  facilitates visual confirmation that the aiming block  400  has been placed correctly over the attachment plate  202 . Once the attachment plate  202  has been positioned at a desired location, the aiming block  400  is placed on top of the attachment plate  202  and aligned therewith. The aiming block  400  includes one or more shafts  450  corresponding to the first hole  222  and/or the second hole  224 . The shafts  450  are positioned at desired angles to form a drilling template. A plurality of aiming blocks  400  with different shaft configurations may be available for use, enabling the drilled holes to be oriented at any desired angle. A drilling tool is inserted through a hole  455  located at one end of the shaft  450  and guided through an opening at the opposite end of the shaft  450  into either of the first hole  222  and the second hole  224  and, subsequently, into the bone  10 . After reaching the desired drilling depth, the drilling tool is withdrawn from the shaft and additional holes may be created by inserting the drilling tool into further shafts  450 . When all the holes have been drilled, the aiming block  400  is removed and the operating site is cleared of bone debris before inserting the screws  257 . 
         [0054]    The screws  257  may then be inserted directly into the first and/or the second holes  222 ,  224  or guided through the shafts  450  of the aiming block  400 . As shown in the cross-sectional view of the fixation kit in  FIG. 11 , taken along line A-A, a substantial portion of each screw  257  occupies the bone cortex  15  without interfering with a prosthesis  100  within the medullary canal  12 . Screws of varying length may be provided as part of the fixation kit to take advantage of the maximum allowable insertion depth. Thus, the screws may be selected to extend from one side of the bone  10  to an opposing side. As seen in  FIG. 11 , the screws  257  may also occupy a portion of a medullary canal  12  without passing diametrically therethrough or contacting the prosthesis  100 . As the screws  257  travel toward their resting positions, the arms  220  may be drawn toward the bone  10  by pressure exerted by head portions of the screws  257 . Although a close fit is desired for stability, it may also be desirable not to excessively constrict the bone  10  or the blood supply thereto by drawing the arms  220  too tightly thereagainst. Thus, a small gap  60  may be left between the arms  220  and the bone  10 . The gap  60  promotes blood flow and reduces the amount of bone compressed by the attachment plate  202 . The gap  60  may be achieved by forming the arms  220  with sufficient curvature such that a bone-facing surface of the arms  220  is substantially concave. The gap  60  may also be a function of the extent to which the screws  257  are driven into the bone  10 . If a smaller gap is desired, more of the screw  257  can be driven in. Similarly, less driving will result in a larger gap. Thus, the length of a shaft portion  259  of each screw  257  that is exposed within the gap  60  is variable. Some screws  257  may be driven entirely into the bone  10  while other screws  257  may form large gaps. 
         [0055]    Exemplary embodiments of fixation devices that may be used in conjunction with the device  200  will now be described.  FIGS. 13A-13D  show an exemplary embodiment of a screw  557  according to the present invention. As shown in the side view of  FIG. 13A , the screw  557  includes a conical body comprising a head  552  and a shaft  554  including a plurality of threads  555 . The shaft  554  also includes one or more slots  559  extending substantially the entire length thereof. The conical body of the screw  557  tapers from the head  552  toward the shaft  554 .  FIG. 13B  shows a cross-section of the screw  557 , taken along line B-B. As shown, a portion of the head  552  includes a recess  532 . In the exemplary embodiment, the recess  532  is hex-shaped. The shape of the recess  532  is shown more clearly in the top view of the head  552  illustrated in  FIG. 13C . Although the exemplary embodiment utilizes a hex-shaped recess, other shapes (e.g., stars or triangles) may be utilized in other embodiments.  FIG. 13D  shows a front view of a distal tip  590  of the screw  557 . As shown in  FIG. 13D , the slots  559  are equidistantly spaced about the perimeter of the distal tip  590 . As would be understood by those skilled in the art, the screws may be formed of stainless steel, titanium or a suitable biocompatible polymer. 
         [0056]      FIGS. 14A-14D  show an exemplary embodiment of a screw  657  according to the present invention. The screw  657  includes a cylindrical body comprising a head  652  and a shaft  654  including a plurality of threads  655 . The shaft  654  also includes one or more slots  659  extending substantially the entire length thereof.  FIG. 14B  shows a cross-section of the screw  657 . A portion of the head  652  includes a recess  632 . As shown in the top view of the head  652  illustrated in  FIG. 13C , the recess  632  is hex-shaped. However, other shapes are also possible.  FIG. 14D  shows a front view of a distal tip  690  of the screw  657 . As shown in  FIG. 13D , the slots  659  are equidistantly spaced about the perimeter of the distal tip  690 . 
         [0057]      FIGS. 15A and 15B  show an exemplary embodiment of a screw  757  according to the present invention. The screw  757  includes an outer member  752  and an inner member  754  that couples to the outer member  752 . The inner member  754  extends substantially the entire length of the outer member  752  and includes a hex-shaped recess  732 . An inner wall  710  of the outer member  752  defines an interface shaped to mate with the inner member  754  using friction-fitting. However, the inner member  754  may be coupled to the outer member  752  in any number of ways including, for example, screwing in a direction opposite to that of threads  75  running along an outer surface of the outer member  752 . 
         [0058]      FIGS. 16A and 16B  show an exemplary embodiment of a peg  857  according to the present invention. The peg  857  includes an outer member  852  and an inner member  854  that couples to the outer member  852 . The inner member  854  extends substantially the entire length of a head portion  856  of the outer member  852  and includes a hex-shaped recess  832 . A distal end  834  of the inner member  854  is shaped to conform to the contours of the head  856 . An inner wall  810  of the outer member  852  defines an interface shaped to mate with the inner member  854  using friction-fitting. However, the inner member  854  may be coupled to the outer member  852  in any number of ways including, for example, screwing. 
         [0059]      FIGS. 17A and 17B  show an exemplary embodiment of a peg  957  according to the present invention. The peg  957  includes an outer member  952  and an inner member  954  that couples to the outer member  952 . The inner member  954  extends substantially the entire length of the outer member  952  and includes a hex-shaped recess  932 . The outer member  952  includes a head portion  956  and a shaft portion  958  having a diameter less than that of the head  956 . An inner wall  910  of the outer member  952  defines an interface shaped to mate with the inner member  954  using friction-fitting. However, the inner member  954  may be coupled to the outer member  952  in any number of ways including, for example, screwing. 
         [0060]      FIGS. 18A and 18B  show an exemplary embodiment of a screw  1057  according to the present invention. The screw  1057  includes an outer member  1052  and an inner member  1054  that couples to the outer member  1052 . The inner member  1054  extends substantially the entire length of a head portion  1056  of the outer member  1052  and includes a hex-shaped recess  1032 . The outer member  1052  includes a head portion  1056  and a shaft portion  1058  having a diameter greater than that of the head  1056 . A distal end  1034  of the inner member  1054  is conically shaped. An inner wall  1010  of the outer member  1052  defines an interface shaped to mate with the inner member  1054  using friction-fitting. However, the inner member  1054  may be coupled to the outer member  1052  in any number of ways including, for example, screwing in a direction opposite to that of threads  85  running along an outer surface of the outer member  1052 . 
         [0061]    The present invention has been described with reference to specific exemplary embodiments. Those skilled in the art will understand that changes may be made in details, particularly in matters of shape, size, material and arrangement of parts. Accordingly, various modifications and changes may be made to the embodiments. The specifications and drawings are, therefore, to be regarded in an illustrative rather than a restrictive sense.