Patent Publication Number: US-2010114097-A1

Title: Implant Devices Constructed with Metallic and Polymeric Components

Description:
BACKGROUND 
     Various implants are used in the orthopedic field to stabilize portions of bone after a fracture, following an osteotomy procedure, or prophylactically to prevent fractures of bone weakened due to tumor, disease, etc. These implants include, for example, fixation plates and intramedullary nails. Such plates and nails typically are constructed of either biocompatible metallic materials or biocompatible polymeric materials. Purely metallic devices constructed, for example, of titanium alloy, have the advantage of increased strength but require mechanical fixation means such as screws while polymeric devices are sometimes difficult to clearly visualize under fluoroscopy. 
     SUMMARY OF THE INVENTION 
     A device according to the present invention is directed to treating a bone, the device comprising rigid body including a polymeric material extending over at least a target portion thereof. The device further comprises a locking element extending into the bone and attached to the device by forming a permanent bond therebetween by melting a portion of an outer surface of the locking element and the polymeric material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a view of an exemplary fixation apparatus according to the present invention inserted within a bone; 
         FIG. 2  shows a perspective view of an intramedullary nail of the apparatus of  FIG. 1 ; 
         FIG. 3  shows a cross-sectional view of a distal tip of the intramedullary nail of  FIG. 2 ; 
         FIG. 4  shows a perspective view of a locking element of the apparatus of  FIG. 1 ; 
         FIG. 5  shows a first perspective view of the fixation apparatus of  FIG. 1  partially inserted into the bone; 
         FIG. 6  shows a second perspective view of the fixation apparatus of  FIG. 1  partially inserted into the bone and rotated about an axis of the bone relative to  FIG. 5 ; 
         FIG. 7  shows a perspective view of an end cap according to the present invention; 
         FIG. 8  shows a cross-sectional view of a bore for receiving the end cap of  FIG. 7 ; 
         FIG. 9  shows a cross-sectional view of a bone plate according to another exemplary embodiment of the invention; 
         FIG. 10  shows a cross-sectional view of a further embodiment of a bone plate according to the invention; 
         FIG. 11  shows a cross-sectional view of a still further embodiment of a bone plate according to the invention; 
         FIG. 12  shows a cross-sectional view of an additional embodiment of a bone plate according to the invention; 
         FIG. 13  shows a cross-sectional view of an additional embodiment of a bone plate according to the invention; and 
         FIG. 14  shows a perspective view of the bone plate of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to devices for stabilizing portions of bone which may be employed either after a fracture or prophylactically to prevent fractures of weakened portions of bone (i.e., due to tumor or disease). A device according to the present invention comprises an implantable device (e.g., an intramedullary or extramedullary nail, bone plate, etc.) including both metallic and polymeric components and adapted to fix portions of bone in a living body. The present invention also teaches locking elements adapted to lock the device to the bone by passing through holes in the device into the bone. Specifically, a device according to the present invention is placed within or on a bone according to methods known in the art and coupled to the bone via fixation elements inserted either through the device into the bone or through the bone into the device. A core of the device is formed of a material with a stiffness greater than that of the polymeric portion. Specifically, the core may be metallic, carbon fiber or other polymeric material with substantially rigid properties designed to withstand pressures exerted theretagainst during insertion and retention in the bone. The fixation elements may then be permanently secured to the device (e.g., via adhesive, ultrasonic heating, etc.). Specifically, energy (e.g., heat, ultrasonic vibration) may be applied to a polymeric material of the locking element to permanently bond a polymeric portion of the device thereto. It is noted that although the embodiments of the present invention are described herein with respect to specific procedures and specific portions of the anatomy, they are not intended to limit the scope of the present invention, which may be used in any of a number of procedures such as, for example, treatment of pediatric fractures of long bones. 
     As shown in  FIGS. 1-4 , an intramedullary nail  100  according to a first embodiment of the invention is sized and shaped to be received within the medullary cavity of a bone  10  (e.g., the ulna). As would be understood by those skilled in the art, dimensions of the intramedullary nail  100  may be modified to conform to the dimensions of any long bone in the body (e.g., the forearm, the fibula, the clavicle, etc.). The intramedullary nail  100  comprises a core  102  comprised of any biocompatible metal such as, for example, a titanium alloy. It is noted that, although exemplary embodiments of the intramedullary nail  100  are described with a core  102 , any material of a comparable rigidity may be employed without deviating from the scope of the present invention. For example, a metallic alloy, carbon fiber or another polymeric material may form the core  102 . The core  102  is formed as an elongated substantially cylindrical core extending along substantially the entire longitudinal length of the intramedullary nail  100  and providing structural rigidity needed to stabilize a bone  10  which has been weakened or which includes a fracture such as the mid-shaft ulna fracture shown in  FIG. 1 . Those skilled in the art will understand that the nail  100  will not likely extend along a straight line and that, therefore, the term cylindrical is only a loose approximation for the shape of the core  102 . More specifically, although a cross-section of the core  102  in a plane substantially perpendicular to a longitudinal axis of the nail  100  may be substantially circular, the true shape of the core  102  will be formed substantially as a series of circular sections extending along the curved path of the longitudinal axis of the nail  100 . Alternatively, the core  102  may be substantially elliptical or otherwise non-circular with a similarly complex shape defined by a series of these cross-sectional shapes arranged along the curved path of the longitudinal axis of the nail  100 . In a further embodiment of the invention, the shape of the core  102  may be specifically formed to match the anatomy of a bone into which it is to be inserted. Specifically, a proximal end thereof may be flared to fill a metaphyseal area, as those skilled in the art will understand. Alternatively, the core  102  and the intramedullary nail  100  may be formed with a non-circular cross-section to improve bony purchase thereof. For example, the cross-section may be formed with a star-shaped cross-section. Furthermore, the cross-section may be rectangular, as will be described in greater detail below with respect to the bone plates of  FIGS. 9-12 . 
     When deployed in a medullary canal of a target bone, the core  102  further serves as a visual indicator of the location of the intramedullary nail  100  under fluoroscopy providing a clearer image than non-metallic portions of the nail  100 . Accordingly, fluoroscopy may be used to guide the intramedullary nail  100  into the bone  10 . Furthermore, the core  102  provides a substantial coupling for any known instrument (not shown) for inserting and/or removing the intramedullary nail  100  to or from the bone  10 . Specifically, by engaging the rigid core  102 , such an implantation/explantation instrument can exert the required axial and/or torsional forces to the nail  102  without exceeding the strength of the nail  100 . 
     A non-metallic casing  104  surrounds at least a portion of the core  102 . As would be understood by those skilled in the art, the casing  104  may be formed as a polymeric shroud, covering or coating extending over at least a portion of the intramedullary nail  100  formed of a biocompatible material such as, for example, polyetheretherketone (PEEK), polylactide or UHMWPE. However, those skilled in the art will understand that the casing  104  is required only in areas to which it is desired to permanently bond a locking element  106 . For example, it may be desirable to form the casing only over target areas to be contacted by the locking elements  106  while in other areas, the core  102  forms an outer surface of the nail  100 . 
     In a preferred embodiment, as shown in  FIGS. 1-3 , the casing  104  covers the entire length of the core  102  and is permanently secured thereto. The casing  104  may, for example, be insert molded onto the core  102  or formed via an extrusion process, as those skilled in the art will understand. Alternatively, the casing  104  may be heat sealed to the core  102 . The casing  104  preferably extends distally past a distal end of the core  102  to form a non-metallic distal tip  124 , as shown in  FIG. 1B . Specifically, the casing  104  extends past the core  102  by a distance X 1 , preferably assuming a tapered shape to facilitate insertion of the nail  100  into the medullary canal. In a preferred embodiment, the casing  104  tapers at an angle α of approximately between 10° and 30° and, more preferably, approximately 20°. It is further submitted that the value of X 1  and α are directly related to one another to prevent the tapered portion from exceeding a minimum thickness X 2 . Furthermore, it is noted that the values for X 1  and X 2  may vary with respect to the anatomy of the bone  10 . An intramedullary nail according to an alternate embodiment of the present invention (not shown) may be formed with a core  102  that extends distally past the casing  104 . 
     The casing  104  is adapted to accept at least one polymeric locking element  106 , as shown in  FIG. 4 , to retain the intramedullary nail  100  in the bone  10 . In use, the locking element  106  is permanently bonded or welded to the casing  104 . The locking element  106  may also be formed of any suitable biocompatible polymeric material such as, for example polyetheretherketone (PEEK). The locking element  106  can be constructed solely from the polymeric material or, alternatively, may have a substrate of another material (i.e., metal, etc.) encased in the polymeric material. For example, the locking element  106  may include a metal core to provide structural rigidity thereto and to aid in location thereof using fluoroscopy in a manner similar to that described above for the nail  100 . As would be understood by those skilled in the art, the locking element  106  may be formed as a locking tack with a head  108  having a diameter greater than that of a shaft  110  thereof. A distal end of the locking element  106  comprises two faces  112  angled to extend proximally from outer, distal-most ends toward a centrally located abutment  114 . The faces  112  and the abutment  114  increase a surface area of the locking element  106  engaging a surface of the casing  104  of the nail  100  to enhance the bonding therebetween. 
     An exemplary method of use of the intramedullary nail  100  comprises inserting the intramedullary nail  100  into a medullary cavity of a designated long bone in the same manner as a conventional intramedullary nails. As shown in  FIGS. 5 and 6 , as the nail  100  is moved further into the medullary canal, its position is monitored (e.g. through fluoroscopic observation of the core  102 ) and, as the distal tip  124  nears a location at which it is desired to insert a locking element  106  (i.e., when a distal end of the core  102  has reached the location), the user may use the fluoroscopic image of the core  102  to ensure that a drill bit  122  of a drill (not shown) is aimed directly toward a portion of the nail  100  to which the locking element  106  is to be bonded. The drill is then operated to form a hole  116  through which the locking element  106  is to be inserted. As would be understood by those skilled in the art, designated hole locations may be calculated during preoperative planning and distributed along the length of the bone to provide the desired locking force holding the intramedullary nail  100  in a desired position within the medullary canal. Each of the holes  116  is drilled just before the intramedullary nail  100  passes the hole location. In this manner, a tip of the intramedullary nail  100  is used as a reference to ensure that the locking element  106  is coaxial with the intramedullary nail to ensure proper bonding while, at the same time, avoiding any potential damage to the casing  104  by the drill. 
     When all of the holes  116  have been drilled at the desired locations and the nail  100  has been inserted into the medullary canal to the desired position therein, a locking element  106  is inserted into one of the holes  116  until the angular faces  112  and the abutment  114  of the locking element  106  contact the casing  104  of the intramedullary nail  100 . Application of pressure to the head  108  forces the locking element  106  against the casing and a source of energy (e.g., ultrasound vibration from an ultrasonic generator) is applied to the head  108  generating heat between the locking element  106  and the casing  104  and melting the polymeric materials thereof. These molten polymeric materials bond to one another, as those skilled in the art will understand to form a permanent connection between the locking element  106  and the casing  104 . This process is then repeated to bond a locking element  106  to the casing via each of the holes  116 . For example, a plurality of locking elements  106  may be disposed along all or a portion of the length of the nail  100  and at any desired angular orientations with respect to a longitudinal axis of the nail  100 . Once bonded to the bone  10 , any outlying portion of the head  108  is cut flush with the outer cortex of the bone so that no portion of the locking element  106  projects out of the bone  10 . In this manner, the present invention offers substantially unlimited locking options for the intramedullary nail  100  (i.e., locking elements  106  may be placed at any desired locations), wherein any plurality of locking elements  106  may be employed depending on the requirements for a particular procedure. 
     The intramedullary nail  100  may also be provided with an optional end cap to provide an additional means for preventing rotation thereof. As shown in  FIGS. 7-8 , an end cap  118  may provided over one or both ends of the intramedullary nail  100 . An exemplary end cap  118  according to the present invention is non-circular in shape and is formed either of a biocompatible polymer known in the art or as a combination of a metal and a polymer material as disclosed earlier in regard to the nail  100  and the locking elements  106 .  FIG. 7  shows an end cap  118  in the shape of a figure eight, with two curved elements joined together. It is noted, however, that any non-circular shape is permissible, including, but not limited to, oval, rectangular, triangular, etc. After insertion of the intramedullary nail  100  into the medullary canal, an end cap  118  may be attached to the proximal end thereof. Specifically, an opening  120  is drilled into the end of the long bone, as shown in  FIG. 8  just prior to the insertion of the intramedullary nail  100 , providing the added benefit of easing the insertion of the intramedullary nail  100  into the bone. The depth and width of the opening  120  may be sized to match up with the dimensions of the end cap. Once inserted, the polymeric material of the end cap  118  can be bonded to the casing  104  via the application of heat thereto, as discussed with respect to  FIGS. 1-6 . 
     As shown in  FIG. 9 , an intramedullary nail  200  according to a further embodiment of the invention includes one or more holes  212  each for engaging a corresponding locking element  106 . Specifically, the hole  212  of the nail  200  includes a polymeric insert  218  therein obviating the need for a casing  104 . Accordingly, the intramedullary nail  200  may be formed entirely of a biocompatible metallic material with polymeric inserts  218  in the holes  212  thereof so that the polymeric inserts  218  may be employed to permanently bond the locking elements  106  to the nail  200  at the respective holes  212 . That is, as the locking elements  106  may be permanently bonded to the inserts  218 , they need not be bonded to a polymeric casing of the nail  200 . However, as would be understood by those skilled in the art such a coating may be included if desired for any reason. The metallic portion  202  of the nail  200  may be formed of a material similar to that of the core  102  of the nail  100  described above in regard to  FIGS. 1-3 . The holes  212  are preferably formed in an hourglass shape with flared ends defined by angled faces  214 ,  216  at either end thereof as disclosed, for example, in International Application No. WO2004/110291 entitled “Surgical Nail” filed on Jun. 12, 2003 to Schlienger et al., the entire contents of which are incorporated herein by reference. This shape aids in maintaining the insert  218  constructed, for example, of a polymeric material suitable for bonding to a locking element  106  as described above, within the locking hole  212  even when subjected to forces along the axis of the hole  212  (e.g., by a locking element  106  inserted therethrough). The polymeric insert  218  preferably completely fills the void of the transverse locking hole  218 . In an alternate embodiment, the polymeric inserts  218  may be formed as coatings covering at least a portion or preferably the entire surfaces of the angled faces  214 ,  216  and may, optionally extend out of the hole  212  along a portion of an outer surface of the nail  200 . That is, the polymeric inserts  218  may be formed to be solid or alternatively may include a bore formed therethrough (not shown), the bore being longitudinally aligned with a longitudinal axis of the transverse locking hole  212  to receive a locking element  106  therethrough. 
     The polymeric inserts  218  are adapted to accept polymeric locking elements  106  that may be bonded or welded thereto to in the same manner described above in regard to the bonding between the casing  104  and the locking elements  106 . Accordingly, once an intramedullary nail  200  has been implanted within a bone (not shown) in the same manner described above in regard to the nail  100 , locking elements  106  may be fitted through preformed holes in the bore, as described earlier, so that angled faces  112  and abutment  114  lie in contact with the polymeric inserts  218 . A permanent bond is then formed by causing a heating therebetween, as also disclosed earlier with respect to the embodiment of  FIGS. 1-6 . 
     As shown in  FIG. 10 , an exemplary bone fixation apparatus according to the present invention may also be formed as a bone fixation plate  300  comprising at least one locking element receiving aperture  320  therein. A wall of the aperture  320  is formed with a polymeric bushing  318  pre-installed and permanently bonded to the plate  300 . As would be understood by those skilled in the art, the plate  300  may be constructed from any suitable material such as, for example, stainless steel, a titanium alloy, or a rigid core with a polymeric casing as described above in regard to the nail  100 . The plate  300  may be further be constructed in any known fashion including apertures  320  for receiving any bone fixation elements (e.g., bone screws, pins, etc.) in the manner, for example, of any of the plates disclosed in U.S. Pat. No. 5,976,141 entitled “Threaded Insert for Bone Plate Screw Hole” filed on Feb. 23, 1995 to Haag et al., the entire contents of which are incorporated herein by reference. In an exemplary embodiment of the present invention, as shown in  FIG. 10 , the polymeric bushing  318 , constructed from any of the materials described above, may include a bore extending therethrough for receiving the locking element  306  in the same manner described above. The aperture  320  receiving the polymeric bushing  318  is formed in a substantially hourglass shape to increase a surface bonding area with the polymeric bushing  318 , thus ensuring a rigid bond therebetween. A proximal portion  316  of the polymeric bushing  318  is formed in a substantially semi-spherical shape, transitioning to a outwardly tapered shape at a distal portion  314  thereof. Furthermore, the polymeric bushing  318  may be formed with a greater diameter on a proximal side thereof, the diameter tapering to a reduced diameter at a central portion. 
     The semi-spherical shape of the proximal portion  316  of the polymeric bushing  318  is adapted to receive a locking element  306  so that a curvature of a head  308  of the locking element  306  substantially matches that of the proximal portion  316  allowing the locking element  306  to be angled as desired with respect to the plate  300 . At least a portion of the locking element  306  is provided with a polymeric coating for bonding with the polymeric bushing  318 . In the exemplary embodiment shown, only the head  308  of the locking element  306  is coated with a polymeric material while a shaft  310  thereof is metallic with no coating provided thereover. It is noted, however, that any or all portions of the locking element  306  may be provided with a polymeric coating without deviating form the scope of the present invention. In the same manner as the locking elements  106  described above, when the locking element is supplied with energy (e.g., ultrasound vibration) an outer portion of the polymeric bushing  318  is permanently bonded to the locking element  306 . 
     In use, the polymeric bushing  318  is pre-molded into corresponding apertures  320  of the plate  300  and permanently bonded thereto the plate  300  in any known manner as described above in regard to the bonding of the locking elements  106  and the nail  100 . The plate  300  which may, for example, be formed to conform to a contour of a target portion of bone to be treated, is placed over the target portion of bone and bores are drilled into the bone to receive one or more locking elements  306 . A locking element  306  is then inserted through the aperture  320  in the plate  300  into a corresponding bore by being screwed or otherwise forced past the polymeric bushing  318 . This is repeated for each locking element  306  to be inserted through the plate  300  into the bone. A permanent bond is then formed therebetween via application of energy (e.g., ultrasonic vibration) as discussed earlier. Of course, those skilled in the art will understand that, in any application, a plate  300  may receive one or more conventional fixation elements (e.g., bone screws or pins) through apertures formed in any known manner along with the one or more locking elements  306  which are permanently bonded to the inserts  318  by application of energy (e.g., ultrasound vibrations produced by an ultrasonic generator) as disclosed earlier. 
     As shown in  FIG. 11 , a bone plate  400  according to another embodiment of the invention includes a body  402  constructed of a metal as described above in regard to the core  102  of the nail  100  to provide stiffness greater than that attainable by a strictly polymer construction. Inserts  418  are then secured within apertures  410  of the body  402  providing a plurality of locations for engaging locking elements as described above (e.g., locking elements  106  and  306 ). As shown in  FIG. 11 , the inserts  418  may include a bore  420  extending therethrough to receive a locking element. Furthermore, the polymeric inserts  418  may be shaped to prevent their becoming dislodged from the apertures  410 . For example, the inserts  418  may include a reduced diameter central portion  412  between enlarged end portions  413 . When inserted into a correspondingly shaped aperture  410 , the enlarged end portions  413  will be too large to pass through the reduced diameter central portion of the aperture  410 . As shown in the cross-section of  FIG. 14 , the enlarged end portions  413  comprise angular faces  414 ,  416  flaring outward toward the outer surfaces of the plate  400 . The fixation plate  400  is implanted in substantially the same manner described above for the plate  300  except that the inserts  418  will generally be pre-placed within the apertures  410  and bonded to the plate  400  prior to the procedure. 
     In yet another alternate embodiment, as shown in  FIG. 12 , polymeric inserts  418 ′ can be formed in a solid configuration, wherein a bone screw may be screwed therethrough and subsequently permanently bonded thereto via ultrasonic welding. 
       FIGS. 13 and 14  shows another alternate embodiment of the present invention, comprising a bone plate  500  with a body  502  provided with a hole  512  formed therethrough. A proximal portion  514  of the hole  512  is substantially spherically curved substantially matching a curvature of a locking element  506  adapted to be received in the hole  512 . A distal portion  514  of the hole  512  tapers linearly outward from a central portion thereof. The wall of the proximal portion  514  of the hole  512  is formed with annular rings  504  machined into the material of the body  502  (e.g., a rigid material such as any of the above mentioned metals). Accordingly, when a locking element  506  is inserted through the hole  512  in accordance with the method disclosed with respect to earlier embodiments, a spherical head  508  of the locking element  506  engages the spherical proximal portion  514  and energy (e.g., ultrasonic vibration) is applied to the locking element  506  to melt the polymeric material of the head  508  into the annular rings  504 . This exemplary embodiment precludes the requirement of having polymeric portions formed on the bone plate  500 . 
     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, combinations 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.