Patent Publication Number: US-2005143734-A1

Title: Bone fixation system with radially extendable anchor

Description:
RELATED APPLICATIONS  
      This application claims the priority benefit under 35 U.S.C. § 119(e) of Provisional Application 60/440,828, filed Jan. 16, 2003 and is a continuation-in-part of Ser. No. 10/714,819, filed Nov. 17, 2003, which is a continuation of Ser. No. 09/832,289, filed Apr. 10, 2001 now U.S. Pat. No. 6,648,890, which is a continuation-in-part of Ser. No. 09/558,057, filed on Apr. 26, 2000, which is a continuation-in-part of Ser. No. 09/266,138 filed on Mar. 10, 1999 which is a divisional of Ser. No. 08/745,652 filed on Nov. 12, 1996, now U.S. Pat. No. 5,893,850. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to bone fixation systems and, more particularly, absorbable or nonabsorbable bone fixation pins of the type for fixing soft tissue or tendons to bone or for securing two or more adjacent bone fragments or bones together.  
      2. Description of the Related Art  
      Bones which have been fractured, either by accident or severed by surgical procedure, must be kept together for lengthy periods of time in order to permit the recalcification and bonding of the severed parts. Accordingly, adjoining parts of a severed or fractured bone are typically clamped together or attached to one another by means of a pin or a screw driven through the rejoined parts. Movement of the pertinent part of the body may then be kept at a minimum, such as by application of a cast, brace, splint, or other conventional technique, in order to promote healing and avoid mechanical stresses that may cause the bone parts to separate during bodily activity.  
      The surgical procedure of attaching two or more parts of a bone with a pin-like device requires an incision into the tissue surrounding the bone and the drilling of a hole through the bone parts to be joined. Due to the significant variation in bone size, configuration, and load requirements, a wide variety of bone fixation devices have been developed in the prior art. In general, the current standard of care relies upon a variety of metal wires, screws, and clamps to stabilize the bone fragments during the healing process. Following a sufficient bone healing period of time, the percutaneous access site or other site may require re-opening to permit removal of the bone fixation device.  
      Long bone fractures are among the most common encountered in the human skeleton. Many of these fractures and those of small bones and small bone fragments must be treated by internal and external fixation methods in order to achieve good anatomical position, early mobilization, and early and complete rehabilitation of the injured patient.  
      The internal fixation techniques commonly followed today frequently rely upon the use of Kirschner wires (K-wires), intramedullary pins, wiring, plates, screws, and combinations of the foregoing. The particular device or combination of devices is selected to achieve the best anatomic and functional condition of the traumatized bone with the simplest operative procedure and with a minimal use of foreign-implanted stabilizing material. A variety of alternate bone fixation devices are also known in the art, such as, for example, those disclosed in U.S. Pat. No. 4,688,561 to Reese, U.S. Pat. No. 4,790,304 to Rosenberg, and U.S. Pat. No. 5,370,646 to Reese, et al.  
      Notwithstanding the common use of the K-wire to achieve shear-force stabilization of bone fractures, K-wire fixation is attended by certain known risks. For example, a second surgical procedure is required to remove the device after healing is complete. Removal is recommended, because otherwise the bone adjacent to an implant becomes vulnerable to stress shielding as a result of the differences in the modulus of elasticity and density between metal and the bone.  
      In addition, an implanted K-wire may provide a site for a variety of complications ranging from pin-tract infections to abscesses, resistant osteomyelitis, septic arthritis, and infected nonunion.  
      Another potential complication involving the use of K-wires is in vivo migration. Axial migration of K-wires has been reported to range from 0 mm to 20 mm, which can both increase the difficulty of pin removal as well as inflict trauma to adjacent tissue.  
      As conventionally utilized for bone injuries of the hand and foot, K-wires project through the skin. In addition to the undesirable appearance, percutaneously extending K-wires can be disrupted or cause damage to adjacent structures such as tendons if the K-wire comes into contact with external objects.  
      Notwithstanding the variety of bone fasteners that have been developed in the prior art, there remains a need for a bone fastener of the type that can accomplish shear-force stabilization with minimal trauma to the surrounding tissue both during installation and following bone healing.  
      In addition, there remains a need for a simple, adjustable bone fixation device which may be utilized to secure soft tissue or tendon to bone.  
     SUMMARY OF THE INVENTION  
      There is provided in accordance with one aspect of the present invention, a fixation device for securing a first bone fragment to a second bone fragment. The fixation device comprises an elongate pin, having a proximal end and a distal end. At least one radially advanceable anchor is carried by the pin. An actuator, which is axially moveable with respect to the pin is also provided. Axial proximal movement of the pin with respect to the actuator causes at least a portion of the anchor to advance along a path which is inclined radially outwardly from the pin in the proximal direction. The device also includes a retention member with at least one retention structure in between the pin and the retention member, for permitting proximal movement of the pin with respect to the retention member but resisting distal movement of the pin with respect to the retention member.  
      The actuator may comprise a tubular body axially slidably carried on the pin. The anchor comprises at least one axially extending strip, having a free proximal end and a distal end, carried by the pin. The strip is moveable from an axial orientation to an inclined orientation in response to axial proximal retraction of the pin. In certain embodiments, at least two or four or more axially extending strips are provided.  
      The device may also include a first retention structure on the retention member for cooperating with a second retention structure on the pin to retain the device under compression. The retention member and the pin may comprise a bioabsorbable material, such as poly (L-lactide-co-D, L-lactide).  
      The distal end of the actuator may have a tapered surface, so that proximal retraction of the pin with respect to the actuator causes the anchor to incline outwardly as it slides along the tapered surface. The proximal end of the anchor may have a complementary tapered surface to slide along the tapered surface on the actuator. The pin may also have a relatively larger diameter near the distal end and a relatively smaller diameter proximally of the distal end.  
      In accordance with another aspect of the present invention, there is provided a bone fixation system for fixing two or more bone fragments. The fixation system comprises a first elongate tubular body, having a proximal end, a distal end and a longitudinal axis. A distal anchor is on the fixation device, moveable from a low profile orientation for distal insertion through a bore in the bone to an inclined orientation to resist axial proximal movement through the bore. An elongate pin is axially moveable within the tubular body and associated with the anchor, such that proximal retraction of the pin with respect to the tubular body advances the distal anchor from the axial orientation to the inclined orientation. The device also includes a second elongate tubular body, having a proximal end, a distal end and a longitudinal axis. At least one retention structure lies in between the second elongate tubular body and the elongate pin. The retention structure permits proximal movement of the elongate pin with respect to the second elongate tubular body but resists distal movement of the pin with respect to the second elongate tubular body. The first tubular body may be used to deploy the distal anchor, and may then be removed and replaced by the second tubular body. The second tubular body cooperates with the pin to apply compression to the bone.  
      The bone fixation device may also comprise at least one retention structure for retaining the compression across a fracture. The retention structure may comprise at least one annular ridge. A first retention structure may be on the second tubular body, and a second, complimentary retention structure may be provided on the pin.  
      The device may also comprise a proximal anchor, which is positioned on the second tubular body. The distal anchor comprises at least two axially extending strips spaced circumferentially apart around the pin.  
      The first tubular body may comprise a first tapered surface and the pin may comprise a second tapered surface such that proximal retraction of the pin with respect to the tubular body causes a radial enlargement of at least a portion of the tubular body.  
      Further features and advantages of the present invention will become apparent to those of skill in the art in view of the detailed description of preferred embodiments which follows, when considered together with the attached claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional schematic view of a bone fixation device of the present invention positioned within a fractured bone.  
       FIG. 2  is a longitudinal cross-sectional view through the pin body of the present invention.  
       FIG. 3  is a distal end elevational view of the pin body of  FIG. 2 .  
       FIG. 4  is a longitudinal cross-sectional view of the proximal anchor of the bone fixation device.  
       FIG. 5  is a proximal end elevational view of the proximal anchor of the bone fixation device.  
       FIG. 6  is a side elevational view of an alternate embodiment of the bone fixation device of the present invention.  
       FIG. 7  is a side elevational view of an alternate embodiment of the pin body in accordance with the present invention.  
       FIG. 8  is a longitudinal cross-sectional view through the pin body of  FIG. 7 .  
       FIG. 9  is a distal end elevational view of the pin body of  FIG. 7 .  
       FIG. 10  is an enlarged detail view of the distal end of the device shown in  FIG. 8 .  
       FIG. 11  is a cross-sectional view through a proximal anchor for use with the pin body of  FIG. 7 .  
       FIG. 12  is a proximal end elevational view of the proximal anchor end of  FIG. 11 .  
       FIG. 13  is a side elevational view of a guide wire that may be used with the pin body of  FIG. 7 .  
       FIG. 14  is a longitudinal cross-sectional view of the guide wire of  FIG. 13  and the pin body of  FIG. 7 .  
       FIG. 15  is a side elevational view of an alternate fixation device in accordance with the present invention, in the low profile configuration.  
       FIG. 16  is a side elevational view as in  FIG. 15 , with the fixation device in the implanted (radially enlarged) configuration.  
       FIG. 16A  is a side elevational cross section through an alternate distal anchor, in the implanted configuration.  
       FIG. 16B  is a side elevational fragmentary view of an anchor positioned along the length of the fixation device, shown in the implanted configuration.  
       FIG. 17  is a side elevational view of the pin illustrated in  FIG. 15 .  
       FIG. 18  is a side elevational detail view of the distal end of the pin illustrated in  FIG. 17 .  
       FIG. 19  is a side elevational detailed view of the retention structures on the pin illustrated in  FIG. 17 .  
       FIG. 20  is a side elevational view of a distal anchor and hub assembly of the fixation system illustrated in  FIG. 15 .  
       FIG. 21  is an end view of the anchor assembly illustrated in  FIG. 20 .  
       FIG. 22  is a side elevational view of the actuator of the device illustrated in  FIG. 15 .  
       FIG. 23  is a cross sectional view taken along the line  23 - 23  of the actuator illustrated in  FIG. 22 .  
       FIG. 24  is an end elevational view of the actuator illustrated in  FIG. 22 .  
       FIG. 25  is a detail view of a portion of the actuator illustrated in  FIG. 23 .  
       FIG. 26  is an anterior view of the distal tibia and fibula, with fixation devices across medial and lateral malleolar fractures.  
       FIG. 27A  is a side elevational view of a deployment actuator.  
       FIG. 27B  is cross-sectional view of the deployment actuator of  FIG. 27A  and a side elevational view of a fixation device as in  FIG. 15 .  
       FIG. 28A  is a side elevational view of an implantable sleeve, which may be used with the deployment actuator and fixation device of  FIG. 27B .  
       FIG. 28B  is a cross-sectional view taken along the line  28 B- 28 B of the retention member illustrated in  FIG. 28A .  
       FIG. 29  is a cross-sectional schematic view of the deployment actuator and bone fixation device of  FIG. 27A  within a fractured bone.  
       FIG. 30  is a cross-sectional schematic view of the deployment actuator and bone fixation device within a fractured bone as in  FIG. 28 , with the fixation device in the implanted (radially enlarged) configuration.  
       FIG. 31  is a cross-sectional schematic view of the bone fixation device within a fractured bone as in  FIG. 28 , with the deployment actuator removed.  
       FIG. 32  is a cross-sectional schematic view of the bone fixation device within a fractured bone as in  FIG. 28 , with the implantable sleeve of  FIG. 28A  positioned around the bone fixation device.  
       FIG. 33  is a cross-sectional schematic view of the bone fixation device and the retention member within a fractured bone. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Although the application of the present invention will be disclosed in connection with the simplified bone fracture of  FIG. 1 , the methods and structures disclosed herein are intended for application in any of a wide variety of bones and fractures, as will be apparent to those of skill in the art in view of the disclosure herein. For example, the bone fixation device of the present invention is applicable in a wide variety of fractures and osteotomies in the hand, such as interphalangeal and metacarpophalangeal arthrodesis, transverse phalangeal and metacarpal fracture fixation, spiral phalangeal and metacarpal fracture fixation, oblique phalangeal and metacarpal fracture fixation, intercondylar phalangeal and metacarpal fracture fixation, phalangeal and metacarpal osteotomy fixation as well as others known in the art. A wide variety of phalangeal and metatarsal osteotomies and fractures of the foot may also be stabilized using the bone fixation device of the present invention. These include, among others, distal metaphyseal osteotomies such as those described by Austin and Reverdin-Laird, base wedge osteotomies, oblique diaphyseal, digital arthrodesis as well as a wide variety of others that will be known to those of skill in the art. Fractures and osteotomies and arthrodesis of the tarsal bones such as the calcaneus and talus may also be treated. Spiked washers can be used, attached to the collar or freely movable beneath the collar. The bone fixation device may be used with or without plate(s) or washer(s), all of which can be either permanent, absorbable or comprising both.  
      Fractures of the fibular and tibial malleoli, pilon fractures and other fractures of the bones of the leg may be fixated and stabilized with the present invention with or without the use of plates, both absorbable or non-absorbing types, and with alternate embodiments of the current invention. One example is the fixation of the medial malleolar avulsion fragment fixation with the radially and axially expanding compression device. Each of the foregoing may be treated in accordance with the present invention, by advancing one of the fixation devices disclosed herein through a first bone component, across the fracture, and into the second bone component to fix the fracture.  
      The fixation device of the present invention may also be used to attach tissue or structure to the bone, such as in ligament reattachment and other soft tissue attachment procedures. Plates and other implants may also be attached to bone, using either resorbable or nonreabsorbable fixation devices disclosed herein depending upon the implant and procedure. The fixation device may also be used to attach sutures to the bone, such as in any of a variety of tissue suspension procedures.  
      For example, peripheral applications for the fixation devices include utilization of the device for fastening soft tissue such as capsule, tendon or ligament to bone. It may also be used to attach a synthetic material such as marlex mesh, to bone or allograft material such as tensor fascia lata, to bone. In the process of doing so, retention of the material to bone may be accomplished with the collar as shown, with an enlarged collar to increase contact surface area, with a collar having a plurality of spikes to enhance the grip on adjacent tissue, or the pin and or collar may be modified to accept a suture or other material for facilitation of this attachment.  
      Specific examples include attachment of the posterior tibial tendon to the navicular bone in the Kidner operation. Navicular-cuneiform arthrodesis may be performed utilizing the device and concurrent attachment of the tendon may be accomplished. Attachment of the tendon may be accomplished in the absence of arthrodesis by altering the placement of the implant in the adjacent bone.  
      Ligament or capsule reattachment after rupture, avulsion of detachment, such as in the ankle, shoulder or knee can also be accomplished using the devices disclosed herein.  
      The fixation devices may be used in combination with semi tubular, one-third tubular and dynamic compression plates, both of metallic and absorbable composition, preferably by modifying the collar to match the opening on the plate.  
      The cannulated design disclosed below can be fashioned to accept an antibiotic impregnated rod for the slow release of medication and/or bone growth or healing agents locally. This may be beneficial for prophylaxis, especially in open wounds, or when osteomyelitis is present and stabilization of fracture fragments is indicated. The central lumen can also be used to accept a titanium or other conductive wire or probe to deliver an electric current or electromagnetic energy to facilitate bone healing.  
      A kit may be assembled for field use by military or sport medical or paramedical personnel. This kit contains an implanting tool, and a variety of implant device size and types, a skin stapler, bandages, gloves, and basic tools for emergent wound and fracture treatment. Antibiotic rods would be included for wound prophylaxis during transport.  
      Referring to  FIG. 1 , there is illustrated generally a bone  10 , shown in cross-section to reveal an outer cortical bone component  12  and an inner cancellous bone component  14 . A fracture  16  is schematically illustrated as running through the bone  10  to at least partially divide the bone into what will for present purposes be considered a proximal component  19  and distal component  21 . The fracture  16  is simplified for the purpose of illustrating the application of the present invention. However, as will be understood by those of skill in the art, the fracture  16  may extend through the bone at any of a wide variety of angles and depths. The bone fixation device of the present invention may be useful to stabilize two or more adjacent components of bone as long as each component may be at least partially traversed by the bone fixation device and anchored at opposing sides of the fracture to provide a sufficient degree of stabilization.  
      A proximal aperture  18  is provided in the proximal component  19  of the bone  10 , such as by drilling, as will be discussed. A distal aperture  20  is provided in an opposing portion of bone such as in distal bone component  21  and is connected to the proximal aperture  18  by way of a through hole  22 , as is known in the art, in a through hole application. The fixation device may also be useful in certain applications where the distal end of the device resides within the bone.  
      The bone fixation device  24  is illustrated in  FIG. 1  in its installed position within the through hole  22 . The bone fixation device  24  generally comprises an elongate pin  26  having a proximal end  28 , a distal end  30 , and an elongate pin body  32  extending therebetween.  
      The distal end  30  of pin  26  is provided with a distal anchor  34 , as will be discussed. A proximal anchor  36  is also provided, such as a radially outwardly extending collar  38  connected to a tubular housing  40  adapted to coaxially receive the pin body  32  therethrough.  
      The radially interior surface of the tubular housing  40 , in the illustrated embodiment, is provided with a plurality of retention structures  42 . Retention structures  42  cooperate with corresponding retention structures  44  on the surface of pin body  32  to permit advancement of the proximal anchor  36  in the direction of the distal anchor  34  for properly sizing and tensioning the bone fixation device  24 . Retention structures  42  then cooperate with retention structures  44  to provide a resistance to movement of the proximal anchor  36  in the proximal direction relative to pin body  32 .  
      In use, the proximal projection of pin  26  which extends beyond the proximal anchor  36  after tensioning is preferably removed, such as by cutting, to minimize the projection of the bone fixation device  24  from the surface of the bone.  
      One embodiment of the pin  26 , adapted for fixing oblique fractures of the fibula or metatarsal bone(s) is illustrated in  FIG. 2 . The bone fixation device  24  of this embodiment uses a generally cylindrical pin body  32 . Although the present invention is disclosed as embodied in a pin body  32  having a generally circular cross section, cross sections such as oval, rectangular, square or tapered to cause radial along with axial bone compression or other configurations may also be used as desired for a particular application.  
      Pin body  32  generally has an axial length of within the range of from about 5 mm or about 10 mm to about 70 mm in the as-manufactured condition. In one embodiment intended for small bones in the foot, the pin body  32  has an axial length of about 19 mm. The illustrated embodiment shows a solid pin body  32 . However, a cannulation may be provided along the longitudinal axis of the body to allow introduction of the pin over a wire as is understood in the art. Hollow tubular structures may also be used.  
      The retention structures  44  on the surface of pin body  32  in the illustrated embodiment comprise a plurality of annular ramp or ratchet-type structures which permit the proximal anchor  36  to be advanced in a distal direction with respect to pin body  32 , but which resist proximal motion of proximal anchor  36  with respect to pin body  32 . Any of a variety of ratchet-type structures can be utilized in the present invention. The annular ramped rings illustrated in  FIG. 2  provide, among other advantages, the ability of the ratchet to function regardless of the rotational orientation of the proximal anchor  36  with respect to the pin body  32 . In an embodiment having a noncircular cross section, or having a rotational link such as an axially-extending spline on the pin body  32  for cooperating with a complementary keyway on proximal anchor  36 , the retention structures  42  can be provided on less than the entire circumference of the pin body as will be appreciated by those of skill in the art. Thus, ratchet structures can be aligned in an axial strip such as at the bottom of an axially extending channel in the surface of the pin body.  
      A single embodiment of the bone fixation device can be used for fixing fractures in bones having any of a variety of diameters. This is accomplished by providing the retention structures  44  over a predetermined axial working length of the pin body  32 . For example, in the illustrated embodiment, the retention structures  44  commence at a proximal limit  46  and extend axially until a distal limit  48 . Axially extending the retention zone between limits  46  and  48  will extend the effective range of bone thicknesses which the pin  32  can accommodate. Although the retention structures  44  may alternatively be provided throughout the entire length of the pin body  32 , retention structures  44  may not be necessary in the most distal portions of pin body  32  in view of the minimum diameter of bones likely to be fixed.  
      In one embodiment of the invention, the distal limit  48  of retention structures  44  is spaced apart from the distal end  30  of pin body  32  by a distance within the range of from about 4 mm to about 20 mm, and, in embodiments for small bones in the foot, from about 4 mm to about 8 mm. The axial length of the portion of the pin body  32  having retention structures  44  thereon, from proximal limit  46  to distal limit  48 , is generally within the range of from about 4 mm to about 8 mm, and was approximately 6 mm in an embodiment having a pin body length of about 19 mm. Depending upon the anchor design, the zone between proximal limit  46  and distal limit  48  may extend at least about 50%, and in some embodiments in excess of about 75% or even in excess of 90% of the length of the pin body.  
      In general, the minimum diameter of the pin body  32  is a function of the construction material of the pin and the desired tensile strength for a given application. The maximum diameter is established generally by the desire to minimize the diameter of the through hole  22  while still preserving a sufficient structural integrity of the fixation device  24  for the intended application.  
      The diameter of pin body  32  will generally be in the range of from about 1.5 mm or 1.8 mm for small bones of the foot and hand to as large as 7.0 mm or larger for bones such as the tibia. In one absorbable embodiment of the invention intended for use in the first metatarsal, the pin  24  comprises poly (L, co-D,L-lactide) and has a diameter of about 1.8 mm. Any of a variety of other materials may also be used, as discussed infra.  
      The distal anchor  34  in the illustrated embodiment comprises a plurality of ramped extensions  50  which incline radially outwardly in the proximal direction. Extensions  50  are positioned or compressible radially inwardly for the purpose of advancing the pin  32  into, and, in some applications, through the through hole  22 . Extensions  50  preferably exert a radially outwardly directed bias so that they tend to extend radially outwardly from the pin body  32  once the distal anchor  34  has advanced out through the distal aperture  20  in bone  10 . Proximal traction on the proximal end  28  of pin body  32  will thereafter tend to cause extensions  50  to seat firmly against the outside surface of distal bone component  21 , as illustrated in  FIG. 1 . In accordance with an optional feature which can be included in any of the embodiments herein, the pin body  32  is provided with a central lumen extending axially therethrough (cannulated) for introduction over a guide pin as will be understood by those of skill in the art.  
      Although any of a variety of alternate designs for distal anchor  34  may be utilized in the context of the present invention, any such distal anchors  34  preferably permit axial distal motion of pin body  32  through the through hole  22 , and thereafter resist proximal withdrawal of the pin body  32  from through hole  22 . As will be appreciated by those of skill in the art, this feature allows the bone fixation device  24  to be set within a bone through a single proximal percutaneous puncture or incision, without the need to expose the distal component  21  or “backside” of the bone. This can be accomplished by biased anchors which are formed integrally with the pin, or which are attached during manufacturing. Distal anchors may also be hinged to the pin body, and may be deployed by a push or pull wire extending through the pin body if the desired construction material does not permit adequate spring bias.  
      For a through hole having a diameter of about 2.3 mm, pin bodies  32  having an outside diameter of about 1.8 mm in the areas other than retention structures  44 , and a maximum outside diameter of about 2.24 mm in the area of retention structures  44  have been found to be useful. In this embodiment, the maximum outside diameter of the distal anchor  34  was approximately 2.92 mm in the relaxed state. The axial length from the distal tip of distal end  30  to the proximal extent of extensions  50  was about 1.21 mm.  
      The pin body  32 , together with the distal anchor  34  and other components of the present invention can be manufactured in accordance with any of a variety of techniques which are well known in the art, using any of a variety of medical-grade construction materials. For example, the pin body  32  and other components of the present invention can be injection-molded from a variety of medical-grade polymers including high or other density polyethylene, nylon and polypropylene. Distal anchor  34  can be separately formed from the pin body  32  and secured thereto in a post-molding operation, using any of a variety of securing techniques such as solvent bonding, thermal bonding, adhesives, interference fits, pivotable pin and aperture relationships, and others known in the art. Preferably, however, the distal anchor  34  is integrally molded with the pin body  32 , if the desired material has appropriate physical properties.  
      Retention structures  44  can also be integrally molded with the pin body  32 . Alternatively, retention structures  44  can be machined or pressed into the pin body  32  in a post-molding operation, or secured using other techniques depending upon the particular design.  
      A variety of polymers which may be useful for the anchor components of the present invention are identified below. Many of these polymers have been reported to be biodegradable into water-soluble, non-toxic materials which can be eliminated by the body: 
          Polycaprolactone     Poly (L-lactide)     Poly (DL-lactide)     Polyglycolide     Poly (L-Lactide-co-D, L-Lactide)     70:30 Poly (l-Lactide-co-D, L-Lactide)     95:5 Poly (DL-lactide-co-glycolide)     90:10 Poly (DL-lactide-co-glycolide)     85:15 Poly (DL-lactide-co-glycolide)     75:25 Poly (DL-lactide-co-glycolide)     50:50 Poly (DL-lactide-co-glycolide)     90:10 Poly (DL-lactide-co-caprolactone)     75:25 Poly (DL-lactide-co-caprolactone)     50:50 Poly (DL-lactide-co-caprolactone)     Polydioxanone     Polyesteramides     Copolyoxalates     Polycarbonates     Poly (glutamic-co-leucine)        

      The desirability of any one or a blend of these or other polymers can be determined through routine experimentation by one of skill in the art, taking into account the mechanical requirements, preferred manufacturing techniques, and desired reabsorption time. Optimization can be accomplished through routine experimentation in view of the disclosure herein.  
      Alternatively, the anchor components can be molded, formed or machined from biocompatible metals such as Nitinol, stainless steel, titanium, and others known in the art. In one embodiment, the components of the bone fixation device  24  are injection-molded from a bioabsorbable material, to eliminate the need for a post-healing removal step. One suitable bioabsorbable material which appears to exhibit sufficient structural integrity for the purpose of the present invention is poly-p-dioxanone, such as that available from the Ethicon Division of Johnson &amp; Johnson. Poly (L-lactide, or co-DL-lactide) or blends of the two may alternatively be used. As used herein, terms such as bioabsorbable, bioresorbable and biodegradable interchangeably refer to materials which will dissipate in situ, following a sufficient bone healing period of time, leaving acceptable byproducts. All or portions of any of the devices herein, as may be appropriate for the particular design, may be made from allograft material, or synthetic bone material as discussed elsewhere herein.  
      The bioabsorbable implants of this invention can be manufactured in accordance with any of a variety of techniques known in the art, depending upon the particular polymers used, as well as acceptable manufacturing cost and dimensional tolerances as will be appreciated by those of skill in the art in view of the disclosure herein. For example, any of a variety of bioabsorbable polymers, copolymers or polymer mixtures can be molded in a single compression molding cycle, or the surface structures can be machined on the surface of the pin or sleeve after the molding cycle. It is also possible to use the techniques of U.S. Pat. No. 4,743,257, the entire disclosure of which is incorporated herein by reference, to mold absorbable fibers and binding polymers together, to create a fiber-reinforced absorbable anchor.  
      An oriented or self-reinforced structure for the anchor can also be created during extrusion or injection molding of absorbable polymeric melts through a suitable die or into a suitable mold at high speed and pressure. When cooling occurs, the flow orientation of the melt remains in the solid material as an oriented or self-reinforcing structure. The mold can have the form of the finished anchor component, but it is also possible to manufacture the anchor components of the invention by machining injection-molded or extruded semifinished products. It may be advantageous to make the anchors from melt-molded, solid state drawn or compressed, bioabsorbable polymeric materials, which are described, e.g., in U.S. Pat. Nos. 4,968,317 and 4,898,186, the entire disclosures of which are incorporated herein by way of this reference.  
      Reinforcing fibers suitable for use in the anchor components of the present invention include ceramic fibers, like bioabsorbable hydroxyapatite or bioactive glass fibers. Such bioabsorbable, ceramic fiber reinforced materials are described, e.g., in published European Patent Application No. 0146398 and in WO/96/21628, the entire disclosures of which are incorporated herein by way of this reference.  
      As a general feature of the orientation, fiber-reinforcement or self-reinforcement of the anchor components, many of the reinforcing elements are oriented in such a way that they can carry effectively the different external loads (such as tensile, bending and shear loads) that are directed to the anchor as used.  
      The oriented and/or reinforced anchor materials for many applications have tensile strengths in the range of about 100-2000 MPa, bending strengths in the range of about 100-600 MPa and shear strengths in the range of about 80-400 MPa, optimized for any particular design and application. Additionally, they are relatively stiff and tough. These mechanical properties may be superior to those of non-reinforced or non-oriented absorbable polymers, which often show strengths between about 40 and 100 MPa and are additionally may be flexible or brittle. See, e.g., S. Vainionpaa, P. Rokkanen and P. Tormnld, “Surgical Applications of Biodegradable Polymers in Human Tissues”, Progr. Polym. Sci., Vol. 14, (1989) at 679-716, the full disclosure of which is incorporated herein by way of this reference.  
      The anchor components of the invention (or a bioabsorbable polymeric coating layer on part or all of the anchor surface), may contain one or more bioactive substances, such as antibiotics, chemotherapeutic substances, angiogenic growth factors, substances for accelerating the healing of the wound, growth hormones, antithrombogenic agents, bone growth accelerators or agents, and the like. Such bioactive implants may be desirable because they contribute to the healing of the injury in addition to providing mechanical support.  
      In addition, the anchor components may be provided with any of a variety of structural modifications to accomplish various objectives, such as osteoincorporation, or more rapid or uniform absorption into the body. For example, osteoincorporation may be enhanced by providing a micropitted or otherwise textured surface on the anchor components. Alternatively, capillary pathways may be provided throughout the pin and collar, such as by manufacturing the anchor components from an open cell foam material, which produces tortuous pathways through the device. This construction increases the surface area of the device which is exposed to body fluids, thereby generally increasing the absorption rate. Capillary pathways may alternatively be provided by laser drilling or other technique, which will be understood by those of skill in the art in view of the disclosure herein. In general, the extent to which the anchor can be permeated by capillary pathways or open cell foam passageways may be determined by balancing the desired structural integrity of the device with the desired reabsorption time, taking into account the particular strength and absorption characteristics of the desired polymer.  
      One open cell bioabsorbable material is described in U.S. Pat. No. 6,005,161 as a poly(hydroxy) acid in the form of an interconnecting, open-cell meshwork which duplicates the architecture of human cancellous bone from the iliac crest and possesses physical property (strength) values in excess of those demonstrated by human (mammalian) iliac crest cancellous bone. The gross structure is said to maintain physical property values at least equal to those of human, iliac crest, cancellous bone for a minimum of 90 days following implantation. The disclosure of U.S. Pat. No. 6,005,161 is incorporated by reference in its entirety herein.  
      The anchors of the present invention may be sterilized by any of the well known sterilization techniques, depending on the type of material. Suitable sterilization techniques include heat sterilization, radiation sterilization, such as cobalt  60  irradiation or electron beams, ethylene oxide sterilization, and the like.  
      In the embodiment illustrated in  FIG. 4 , the proximal anchor  36  comprises a collar  38  for contacting the proximal bone component  19 . Collar  38  preferably comprises a radially-outwardly extending annular flange to optimize contact with the proximal bone component  19 . Alternatively, proximal collar  38  may comprise one or more radially-outwardly extending stops, a frusto-conical plug, or other structures which stop the distal progress of proximal anchor  36  with respect to the through hole  22  or blind hole, depending upon the application.  
      The pin body  32  cooperates with a proximal anchor  36  to accomplish the fixation function of the present invention. Proximal anchor  36  is preferably axially movably carried by the pin body  32  throughout a sufficient axial range of motion to accommodate a variety of bone diameters.  
      Collar  38  is axially movably disposed with respect to pin body  32  such as by connection to a tubular housing  40 . Tubular housing  40  is concentrically positioned on pin body  32 , and is provided on its interior surface with at least one, and preferably a plurality, of retention structures  42 . Retention structures  42  are configured to cooperate with the complementary retention structures  44  on the pin body  32  to permit axial distal advancement of collar  38  with respect to pin body  32 , but resist proximal motion of collar  38  with respect to pin body  32 , as has been discussed.  
      In one embodiment of the present invention, the minimum interior diameter of the tubular housing  40  is about 2.00 mm. The maximum interior diameter of the tubular housing  40 , at the radial outwardmost bottom of the annular recesses adapted to cooperate with annular ridges  44  on pin body  32 , is about 2.17 mm. The outside diameter of the collar  38  is about 2.70 mm, and the thickness in the axial direction of annular collar  38  is about 0.20 mm.  
      The retention structures  42  may comprise any of a variety of complementary surface structures for cooperating with the corresponding structures  44  on the pin  32 , as is discussed elsewhere herein. In the illustrated embodiment, the retention structures are in the form of a plurality of annular rings or helical threads, which extend axially throughout the length of the tubular housing  40 . The retention structure  42  may alternatively comprise a single thread, ridge or groove or a plurality of structures which extend only part way (e.g., at least about 10% or 25% or more) along the length of the tubular housing  40 . Retention force may be optimized by providing threads or other structures along a substantial portion, e.g., throughout at least 75% or 80% of the axial length of the tubular housing  40 .  
      The overall length of the tubular housing  40  may be maximized with respect to the depth of the target borehole for a particular application. For example, in a device intended to fix bones having a diameter within the range of from about 15-20 mm, the axial length of the tubular body  40  is preferably at least about 8 mm or 10 mm, and, more preferably, at least about 12 mm or 14 mm. In this manner, the axial length of the zone of retention structures  42  is maximized, thereby increasing the tensile strength of the implanted device. The proximal anchor  36  can be readily constructed using other dimensions and configurations while still accomplishing the desired function, as will be apparent to those of skill in the art in view of the disclosure herein.  
      In use, a bone is first identified having a fracture which is fixable by a pin-type fixation device. The clinician assesses the bone, selects a bone drill and drills a through hole  22  in accordance with conventional techniques.  
      A bone fixation device  24  having an axial length and outside diameter suitable for the through hole  22  is selected. The distal end  30  of the bone fixation device  24  is percutaneously or otherwise advanced towards the bone, and subsequently advanced through the through hole  22  until distal anchor  34  exits the distal aperture  20 . The proximal anchor  36  may be positioned on the bone fixation device  24  prior to positioning of the pin body  32  in the through hole  22 , or following placement of the pin body  32  within through hole  22 .  
      Proximal traction is applied to the proximal end  28  of pin body  32 , to seat the distal anchor  34 . While proximal traction is applied to the proximal end  28  of pin body  32 , such as by conventional hemostats or a calibrated loading device, the proximal anchor  36  is advanced distally until the anchor  36  fits snugly against the proximal component  19  of the bone. Appropriate tensioning of the bone fixation device  24  is accomplished by tactile feedback or through the use of a calibration device for applying a predetermined load on implantation.  
      Following appropriate tensioning of the proximal anchor  36 , the proximal end  28  of the pin body  32  may be cut off and removed. Pin body  32  may be cut using conventional bone forceps which are routinely available in the clinical setting. Alternatively, a pin may be selected such that it is sized to fit the treatment site such that following tension no proximal extension remains.  
      Following trimming the proximal end  28  of pin  26 , the access site may be closed and dressed in accordance with conventional wound closure techniques.  
      Preferably, the clinician will have access to an array of bone fixation devices  24 , having different diameters and axial lengths. These may be packaged one or more per package in sterile envelopes or peelable pouches, or in dispensing cartridges which may each hold a plurality of devices  24 . Upon encountering a bone for which the use of a fixation device is deemed appropriate, the clinician will assess the dimensions and load requirements of the bone, and select a bone fixation device from the array which meets the desired specifications.  
      Referring to  FIG. 6 , there is disclosed an alternate embodiment of the fixation pin. The fixation pin  26  illustrated in  FIG. 6  may be identical to the embodiments previously discussed, except with respect to the proximal anchor  52 . Proximal anchor  52  comprises a radially outwardly extending annular collar  54  or other structure for resisting motion of the proximal anchor  52  in a distal direction through the aperture in the bone. Collar  54  is connected to a proximal portion of the tubular housing  56 , analogous to housing  40  previously discussed. Tubular housing  56  is adapted to receive the pin body  32  therethrough.  
      The radially inwardly facing surface of tubular housing  56  is provided with a plurality of retention structures  58 . In this embodiment, retention structures  58  comprise a plurality of recesses or grooves which extend radially outwardly into the tubular housing  56 . Retention structures  58  are adapted to cooperate with corresponding retention structure  60  secured to or integral with the pin  32 . Retention structure  60  in this embodiment comprise a plurality of radially outwardly extending annular rings or threads, which are adapted to be received within the corresponding retention structures  58 . In this embodiment, the proximal anchor  52  is unable to move in an axial direction with respect to pin  32  unless sufficient axial force is applied to plastically-deform the retention structures  58  and/or retention structures  60  so that the tubular housing  56  snaps, ridge by ridge, in the direction of the axial force. The precise amount of axial force necessary to overcome the resistance to motion of proximal anchor  52  with respect to pin  32  can be optimized through appropriate tolerancing of the corresponding retention structures, together with the selection of materials for the proximal anchor  52  and/or pin  32 . Preferably, the tolerances and construction details of the corresponding retention structures  58  and  60  are optimized so that the proximal anchor  52  may be advanced distally over the pin  32  using manual force or an installation tool, and the proximal anchor  52  will have a sufficient retention force to resist movement of the bone fragments under anticipated use conditions.  
      Referring to  FIGS. 7-14 , there is illustrated an alternate embodiment of the fixation device of the present invention. This embodiment is optimized for construction from a metal, such as titanium or titanium alloy, although other materials including those disclosed elsewhere herein may be utilized for the present embodiment. Referring to  FIGS. 7 and 8 , the fixation device includes a body  32  which is in the form of a pin  26  extending between a proximal end  28  and a distal end  30 . The distal end  30  includes a plurality of friction enhancing or interference fit structures such as ramped extensions or barbs  50 , for engaging the distal cortical bone or other surface or interior cancellous bone as has been described.  
      Although the illustrated embodiment includes four barbs  50 , oriented at 90° with respect to each other, anywhere from one to about twelve or more barbs  50  may be utilized as will be apparent to those of skill in the art in view of the disclosure herein. The barbs  50  may be radially symmetrically distributed about the longitudinal axis of the pin  26 . Each barb  50  is provided with a transverse engagement surface  21 , for contacting the distal surface of the cortical bone or other structure or surface against which the barb  50  is to anchor. Transverse engagement surfaces  21  may lie on a plane which is transverse to the longitudinal axis of the pin  26 , or may be inclined with respect to the longitudinal axis of the pin  26 .  
      Each of the transverse engagement surfaces  21  in the illustrated embodiment lies on a common plane which is transverse to the longitudinal axis of the pin  26 . Two or more planes containing engagement surfaces  21  may alternatively be provided. The transverse engagement surfaces  21  may also lie on one or more planes which are non-normal to the longitudinal axis of pin  26 . For example, the plane of a plurality of transverse engagement surfaces  21  may be inclined at an angle within the range of from about 35° or 45° to about 90° with respect to the longitudinal axis of the pin  26 . The plane of the transverse engagement surface may thus be selected to take into account the angle of the distal surface of the bone through which the pin may be positioned, as may be desired in certain clinical applications.  
      In order to facilitate the radially inward compression of the barbs  50  during the implantation process, followed by radially outward movement of the barbs  50  to engage the distal bone surface, each barb  50  in the illustrated embodiment is carried by a flexible or hinged lever arm  23 . Lever arms  23  may be formed by creating a plurality of axial slots  15  in the sidewall of the pin  26 . The axial slots  15  cooperate with a central lumen  11  to isolate each barb  50  on a unique lever arm  23 . The axial length of the axial slots  15  may be varied, depending upon the desired length over which flexing is desirably distributed, the desired range of lateral motion, and may vary depending upon the desired construction material. For a relatively rigid material such as titanium, axial lengths of the axial slot  15  in excess of about 0.1 inches and preferably in excess of about 0.2 inches are utilized on a pin  26  having an outside diameter of about 0.1 inches and a length of about 1.25 inches. Axial slots  15  will generally extend within a range of from about 5% to about 90%, and often within about 10% to about 30% of the overall length of the pin  26 .  
      The circumferential width of the slots  15  at the distal end  30  is selected to cooperate with the dimensions of the barbs  50  to permit radial inward deflection of each of the barbs  50  so that the pin  26  may be press fit through a predrilled hole having an inside diameter approximately equal to the outside diameter of the pin  26  just proximal to the transverse engagement surfaces  21 . For this purpose, each of the slots  15  tapers in circumferential direction width from a relatively larger dimension at the distal end  30  to a relatively smaller dimension at the proximal limit of the axial slot  15 . See  FIG. 7 . In the illustrated embodiment, each slot  15  has a width of about 0.20 inches at the proximal end and a width of about 0.035 inches at the distal end in the unstressed orientation. The width of the slot  15  may taper continuously throughout its length, or, as in the illustrated embodiment, is substantially constant for a proximal section and tapered over a distal section of the slot  15 . The wall thickness of the lever arm  23  may also be tapered to increase the diameter of the central lumen  11  in the distal direction. This will allow a lower compressed crossing profile before the inside surfaces of the lever arms bottom out against each other.  
      The pin  26  is additionally provided with a plurality of retention structures  44  as has been discussed. Retention structures  44  are spaced apart axially along the pin  26  between a proximal limit  46  and a distal limit  48 . The axial distance between proximal limit  46  and distal limit  48  is related to the desired axial travel of the proximal anchor  36 , and thus the range of functional sizes of the pin. In one embodiment of the pin  26 , the retention structures  44  comprise a plurality of threads, adapted to cooperate with the complimentary retention structures  42  on the proximal anchor  36 , which may be a complimentary plurality of threads. In this embodiment, the proximal anchor  36  may be distally advanced along the pin  26  by rotation of the proximal anchor  36  with respect to the pin  26 . Proximal anchor  36  may advantageously be removed from the pin  26  by reverse rotation, such as to permit removal of the pin  26  from the patient. For this purpose, collar  38  is preferably provided with a gripping configuration or structure to permit a removal tool to rotate collar  38  with respect to the pin  26 . Any of a variety of gripping surfaces may be provided, such as one or more slots, flats, bores, or the like. In the illustrated embodiment, the collar  38  is provided with a polygonal, and in particular, a hexagonal circumference, as seen in  FIG. 12 .  
      The proximal end  28  of the pin  26  is similarly provided with a structure  29  for permitting rotational engagement with an installation or a removal tool. Rotational engagement may be accomplished using any of a variety of shapes or configurations, as will be apparent to those of skill in the art. One convenient structure is to provide the proximal end  26  with one or more flat side walls, for rotationally engaging a complimentary structure on the corresponding tool. As illustrated in  FIG. 9 , the proximal end  26  may be provided with a structure  29  having a square cross-section. Alternatively, the exterior cross-section through proximal end  28  may be any of a variety of configurations to permit rotational coupling, such as triangular, hexagonal, or other polygons, or one or more axially extending flat sides or channels on an otherwise round body.  
      The foregoing structures enable the use of an installation and/or deployment tool having a concentric core within a sleeve configuration in which a first component (e.g. a sleeve) engages the proximal anchor  36  and a second component (e.g. a core) engages the proximal rotational engagement structure  29  of pin  26 . The first component may be rotated with respect to the second component, so that the proximal anchor  36  may be rotated onto or off of the retention structures  44  on pin  26 . In a modified arrangement, a first tool (e.g., a pair of pliers or a wrench) may be used to engage the proximal anchor  36  and a second tool (e.g., a pair of pliers or a wrench) may be used to engage the proximal rotational engagement structure  29  of pin  26 . In such an arrangement, the first tool may be rotated with respect to the second tool (or vice versa), so that the proximal anchor  36  may be rotated onto or off the retention structures  44  on the pin  26 .  
      Alternatively, the retention structures  42  on the proximal anchor  36  may be toleranced to permit distal axial advancement onto the pin  26 , such as by elastic deformation, but require rotation with respect to the pin  26  in order to remove the proximal anchor  36  from the pin  26 .  
      Any of a variety of alternative retention structures may be configured, to permit removal of the proximal anchor  36  such as following implantation and a bone healing period of time. For example, the retention structures  44  such as threads on the pin  26  may be provided with a plurality of axially extending flats or interruptions, which correspond with a plurality of axial flats on the retention structures  42  of proximal anchor  36 . This configuration enables a partial rotation (e.g. 90°) of the proximal anchor  36  with respect to the pin  26 , to disengage the corresponding retention structures and permit axial withdrawal of the proximal anchor  36  from the pin  26 . One or both of the retention structures  44  and  42  may comprise a helical thread or one or more circumferentially extending ridges or grooves. In a threaded embodiment, the thread may have either a fine pitch or a course pitch. A fine pitch may be selected where a number of rotations of proximal anchor  36  is desired to produce a relatively small axial travel of the anchor  36  with respect to the pin  26 . In this configuration, relatively high compressive force may be achieved between the proximal anchor  36  and the distal anchor  34 . This configuration will also enable a relatively high resistance to inadvertent reverse rotation of the proximal anchor  36 . Alternatively, a relatively course pitch thread such as might be found on a luer connector may be desired for a quick twist connection. In this configuration, a relatively low number of rotations or partial rotation of the proximal anchor  36  will provide a significant axial travel with respect to the pin  26 . This configuration may enhance the tactile feedback with respect to the degree of compression placed upon the bone. The thread pitch or other characteristics of the corresponding retention structures can be optimized through routine experimentation by those of skill in art in view of the disclosure herein, taking into account the desired clinical performance.  
      Referring to  FIG. 7 , at least a first break point  31  may be provided to facilitate breaking the proximal portion of the pin  26  which projects proximally of the collar  38  following tensioning of the fixation system. Break point  31  in the illustrated embodiment comprises an annular recess or groove, which provides a designed failure point if lateral force is applied to the proximal end  28  while the remainder of the attachment system is relatively securely fixed. At least a second break point  33  may also be provided, depending upon the axial range of travel of the proximal anchor  36  with respect to the pin  26 .  
      In one embodiment having two or more break points  31 ,  33 , the distal break point  31  is provided with one or more perforations or a deeper recess than the proximal break point  33 . In this manner, the distal break point  31  will preferentially fail before the proximal break point  33  in response to lateral pressure on the proximal end  28 . This will ensure the minimum projection of the pin  26  beyond the collar  38  following deployment and severing of the proximal end  28  as will be appreciated in view of the disclosure herein.  
      Proximal projection of the proximal end  28  from the proximal anchor  36  following implantation and breaking at a breakpoint  31  may additionally be minimized or eliminated by allowing the breakpoint  31  or  33  to break off within the proximal anchor  36 . Referring to  FIG. 11 , the retention structure  42  may terminate at a point  61  distal to a proximal surface  63  on the anchor  36 . An inclined or tapered annular surface  65  increases the inside diameter of the central aperture through proximal anchor  36 , in the proximal direction. After the proximal anchor  36  has been distally advanced over a pin  26 , such that a breakpoint  31  is positioned between the proximal limit  61  and the proximal surface  63 , lateral pressure on the proximal end  28  of pin  26  will allow the breakpoint  31  to break within the area of the inclined surface  65 . In this manner, the proximal end of the pin  26  following breaking resides at or distally of the proximal surface  63 , thus minimizing the profile of the device and potential tissue irritation.  
      For any of the (axially deployable) embodiments disclosed above, installation can be simplified through the use of an installation tool. The installation tool may comprise a pistol grip or plier-type grip so that the clinician can position the tool at the proximal extension of pin  32  and through one or more contractions with the hand, the proximal anchor  36 ,  52  and distal anchor  34  can be drawn together to appropriately tension against the bone fragments. The use of a precalibrated tool can permit the application of a predetermined tension in a uniform manner from pin to pin.  
      Calibration of the installation device to set a predetermined load on the pin can be accomplished through any of a variety of means which will be understood to those of skill in the art. For example, the pin  32  may be provided with one or more score lines or transverse bores or other modifications which limit the tensile strength of the part at one or more predetermined locations. In this manner, axial tension applied to the proximal end  28  with respect to the collar  54  will apply a predetermined load to the bone before the pin  32  will separate at the score line. Alternatively, internal structures within the installation tool can be provided to apply tension up to a predetermined limit and then release tension from the distal end of the tool.  
      FIGS.  13  illustrates a locking guide wire  150  that may be used with the fixation device described above. The guide wire has a distal end  152  and a proximal end  154 . The illustrated guide wire  150  comprises a locking portion  156  that is located at the distal end  152  of the guide wire  150  and an elongated portion  158  that preferably extends from the distal portion  156  to the proximal end  154  of the guide wire  150 . The diameter D 1  of the elongated portion  158  is generally smaller than the diameter D 2  of the distal portion  154 . The guide wire  150  can be made from stainless steel, titanium, or any other suitable material. Preferably, in all metal systems, the guidewire  150  and locking portion  156  are made from the same material as the remainder of the fixation device to prevent cathodic reactions.  
      The locking portion  156  on guidewire  150  can take any of a variety of forms, and accomplish the intended function as will be apparent to those of skill in the art in view of the disclosure herein. For example, a generally cylindrical locking structure, as illustrated, may be used. Alternatively, any of a variety of other configurations in which the cross section is greater than the cross section of the proximal portion  158  may be used. Conical, spherical, or other shapes may be utilized, depending upon the degree of compression desired and the manner in which the locking portion  156  is designed to interfit with the distal end  30  of the pin.  
      The guide wire  150  is configured such that its proximal end can be threaded through the lumen  11  of the pin  26 . With reference to  FIG. 8 , the lumen  11  preferably comprises a first portion  160  and a second portion  162 . The first portion  160  is generally located at the distal end  30  within the region of the lever arms of the pin  26 . The second portion  162  preferably extends from the first portion  160  to the proximal end  28  of the pin  26 . The inside diameter of the first portion  160  is generally larger than the diameter of the second portion  162 . As such, the junction between the first portion  160  and the second portion  162  forms a transverse annular engagement surface  164 , which lies transverse to the longitudinal axis of the pin  26 .  
      As mentioned above, the guide wire  150  is configured such that its proximal end can be threaded through the lumen  11  of the pin  26 . As such, the diameter D 1  of the elongated portion  158  is less than the diameter of the second portion  162  of the lumen  11 . In contrast, the diameter D 2  of distal portion  156  preferably is slightly smaller than equal to or larger than the diameter of the first portion  160  and larger than the diameter of the second portion  162 . This arrangement allows the distal portion  156  to be retracted proximally into the first portion  160  but prevents the distal portion  156  from passing proximally through the pin  26 .  
      In addition, any of a variety of friction enhancing surfaces or surface structures may be provided, to resist distal migration of the locking guidewire  150 , post deployment. For example, any of a variety of radially inwardly or radially outwardly directed surface structures may be provided along the length of the locking guidewire  150 , to cooperate with a corresponding surface structure on the inside surface of the lumen  11 , to removably retain the locking guidewire  150  therein. In one embodiment, a cylindrical groove is provided on the inside surface of the lumen  11  to cooperate with a radially outwardly extending annular flange or ridge on the outside diameter of the locking guidewire  150 . The complementary surface structures may be toleranced such that the locking guidewire or guide pin may be proximally retracted into the lumen  11  to engage the locking structure, but the locking structure provides a sufficient resistance to distal migration of the locking guidewire  150  such that it is unlikely or impossible to become disengaged under normal use.  
      In use, after the clinician assesses the bone, selects a bone drill and drills a through hole  22 , the distal end  152  of the guide wire  150  and the distal end  30  of the pin  26  are advanced through the through hole until the distal portion  156  and the barbs  50  exit the distal aperture  20 . The proximal anchor  36  may be positioned on the bone fixation device  24  prior to positioning of the pin body  32  in the through hole  22 , or following placement of the pin body  32  within through hole  22 .  
      The guide wire  150  is preferably thereafter retracted until the distal portion  156  enters, at least partially, the first portion  160  of the pin  26  (see  FIG. 14 ). The proximal anchor  36  can then be rotated or otherwise distally advanced with respect to the pin body  26  so as to seat the distal anchor  34  snugly against the distal component  21  of the bone. As such, at least a part of the distal portion  156  of the guide wire  150  becomes locked within the first portion  150  of the pin  26 . This prevents the barbs  50  and lever arms  24  from being compressed radially inward and ensures that the barbs  50  remain seated snugly against the distal component  21  of the bone.  
      Following appropriate tensioning of the proximal anchor  36 , the proximal end  28  of the pin body  32  and the proximal end  154  of the guide wire  150  are preferably cut off or otherwise removed. These components may be cut using conventional bone forceps which are routinely available in the clinical setting, or snapped off using designed break points as has been discussed.  
       FIG. 15  shows a bone fixation device  200 , which may be used either in a through hole application such as that illustrated in  FIG. 1 , or in a blind hole as in  FIG. 26  in which the distal anchor is deployed within cancellous bone. The fixation device  200  has a distal portion  215  and a proximal portion  220 . In general, as a component of the proximal portion  220  of the device is axially moved, an anchor component on the distal portion  215  of the device advances away from the longitudinal axis of the device to engage cancellous bone.  
      As with other embodiment disclosed herein, the bone fixation device  200  may be used alone, in multiples such as two or three or four or more per fixation, and/or together with plates, intramedullary nails, or other support structures. The bone fixation device  200  may also be used in any of a variety of locations on the body, as has been discussed previously. These include, for example, femur neck fractures, medial and lateral malleolar fractures, condylar fractures, epicondylar fractures, and colles fractures (distal radius and ulnar).  
      The bone fixation device generally comprises an elongate pin  205  having a proximal end  222 , a distal end  224  and an actuator  210 . As the actuator  210  is advanced distally with respect to the pin  205 , the distal portion of the pin  205  expands, engaging the bone.  FIG. 16  shows the device  200  in a deployed mode, such that the distal portion of the pin  205  is in an expanded state.  
      The pin  205  can have any of a variety of dimensions, depending upon the intended use environment. In one embodiment, useful, for example, in a malleolar fracture, the pin has an overall length of about 2.5 inches and a diameter of about 0.136 inches between the retention structures  240  and the distal end  224 . See  FIG. 17 . The outside diameter of the pin  205  proximally of the retention structures  240  may be somewhat smaller, and, in the illustrated embodiment, the outside diameter is about 0.130 inches. The axial length of the retention zone which includes retention structures  240  can also be varied widely, depending upon the range of travel desired for the proximal anchor as has been discussed in connection with previous embodiments. In the illustrated embodiment, the axial length of the retention structure  240  zone is about 0.240 inches.  
      The distal end  224  of pin  205  comprises a transverse surface  225  such as an annular flange formed by a radially enlarged head  227 . See  FIG. 18 . The head  227  is provided with a frusto conical tapered surface  229 , to facilitate introduction of the device into and advancement through a bore in a bone. The transverse surface  225  is provided to retain a hub  235 , as will be discussed below. In one embodiment, the distal end of the pin  224  immediately proximal to the transverse surface  225  has an outside diameter of about 0.144 inches, and the adjacent portion of the head  227  has an outside diameter of about 0.172 inches to provide a transverse surface  225  having a radial dimension of about 0.014 inches. The pin  205  may be cannulated as has been previously discussed.  
      In the illustrated embodiment, the distal tapered surface  229  is substantially smooth, to permit insertion into a predrilled borehole. Alternatively, the distal surface  229  may include a drill tip, such as one or more sharpened edges to enable introduction of the fixation device  200  into a bone without the requirement of predrilling a borehole. In a self drilling embodiment of the bone fixation device  200 , the proximal end  222  of the pin  205  may be attached directly to a drill using a conventional chuck connection, or may be provided with a slot, or a hexagonal cross section or other rotational interlock structure for coupling to a rotational driving device.  
       FIG. 19  shows a detailed view of the retention structure  240  on the pin for restricting movement between the actuator  210  and the pin  205 . The retention structure may comprise a plurality of recesses, grooves, or serrations, including helical threads, which extend radially inwardly or outwardly often in an annular configuration. The retention structure  240  may include one or more ramped surfaces that incline radially inwardly in the proximal direction. These structures, and the complementary structures which may be used on the actuator  210  have been disclosed elsewhere herein. In the illustrated embodiment, the retention structure  240  comprises a plurality of annular ramped rings, each ramp having a length in the axial direction of about 0.016 inches. The ramped surfaces incline radially outwardly in the distal direction, to facilitate distal advancement of the proximal anchor with respect to the pin  205 , and resist proximal motion of the proximal anchor with respect to the pin  205 , as is discussed elsewhere herein.  
      A radially advanceable anchor  230  ( FIGS. 20 and 21 ) is provided at the distal end of the pin. The anchor  230  is shown as having four axially extending strips or tines  231 ,  232 ,  233 ,  234  carried by the pin; however, the anchor  230  may have one or two or a plurality of axially extending strips. The strips  231 - 234  are moveable from an axial orientation (for insertion) to an inclined orientation in response to an axial proximal retraction of the pin relative to the actuator  210 . The proximal end of the strips  231 - 234  are free, to permit radial enlargement. The distal end of the strips  231 - 234  are attached to the distal end of the pin  205  either directly (e.g.  FIG. 16A ), or indirectly such as in  FIGS. 15-16 . The collapsed anchor  230  may be provided with an outside diameter that is less than the outside diameter of the actuator tube  210  and the head  227  of the pin  205 , to facilitate insertion into the hole without placing stress on the anchor  230 .  
      In the illustrated embodiment, the anchor  230  is formed as a separate component of the fixation system. This enables the pin  205  and the actuator  210  to be conveniently manufactured from a bioabsorbable material, while the anchor assembly  230  may be made from any of a variety of biocompatible metals such as stainless steel, titanium or nickel titanium alloys such as nitinol. This variety of a hybrid absorbable-nonabsorbable fixation device takes advantage of the strength and flexibility of nitinol or other metal in the area of the strips  231 - 234 , yet leaves only a minimal amount of metal within the bone following dissolution of the bioabsorbable component. In addition, the long term indwelling component (the metal anchor) does not span the fracture.  
      Although sometimes referred to herein as “strips” the moveable anchor components  231 - 234  may take any of a variety of shapes, depending upon the desired construction materials, manufacturing technique and performance. In the illustrated embodiment, the anchor  230  may formed from a piece of tubing stock, such as nitinol tubing, by laser etching or other cutting technique. The anchor  230  has an outside diameter of about 0.172 inches, and an axial length of about 0.394 inches. Each of the strips  231 - 234  has a width in the circumferential direction of approximately 0.08 inches and a radial direction wall thickness of no more than about 0.014 inches. However, any of a variety of dimensions may be utilized, as will be apparent to those of skill in the art in view of the disclosure herein. In addition, more or fewer than four axially extending tines  231 - 234  may be readily provided.  
      The strips  231 - 234  may alternatively be formed from a round cross section material such as wire, or other separate component which is assembled or fabricated into a finished multi strip anchor  230 .  
      In the illustrated anchor  230 , the proximal end  237  of each strip is provided with a ramped surface  239 . The ramped surface causes the radial thickness of the strip to decrease in the proximal direction. This ramped surface  239  cooperates with a complementary ramped surface on the actuator, discussed elsewhere herein, to facilitate radial outward advancement of the anchor in response to proximal retraction of the pin  205  with respect to the actuator  210 .  
      The ramped surface  239  on each tine or strip also acts as a leading cutting edge to permit each tine to cut into cancellous bone as it advances along a path which will normally be inclined radially outwardly in the proximal direction, in response to proximal retraction of the pin with respect to the actuator. Placing the ramped surface on the radially inwardly facing surface of the tine may allow the tine to seek a maximum angle with respect to the longitudinal axis of the pin, following deployment. This anchor construction thus enables each of the tines to create its own path through the bone such that the cross section of the tine substantially fills the cross section of the path which it creates. The length of the path along the axis of the tine is generally at least about two times, and in certain embodiments is at least as much as three times or five or more times the average cross section of the tine. The path may be substantially linear or curved, such as slightly concave outwardly from the axis of the pin.  
      The pin  205  may comprise two or more anchors  230  along its length, each anchor comprising one or two or more (e.g., 4) axial strips. The anchors are each moveable from an axial orientation for distal insertion through a bore in the bone to an inclined orientation to resist proximal axial movement through the bone. In certain embodiments, the anchor  230  and pin  205  are provided with a mechanical interlock such as a projection and slot or other complementary surface structures to prevent rotation of the anchor  230  with respect to the pin  205 .  
      Referring to  FIG. 16B , there is illustrated an in-line or intermediate anchor  230 , which may be used in combination with a distal anchor such as that illustrated in  FIG. 220 , to provide two cancellous bone anchors spaced axially apart along the fixation device. In one embodiment, each of the two cancellous bone anchors is provided with four axially extending anchor strips  231 - 234 . In the embodiment of  FIG. 16B , each of the anchor strips  232  and  234  may be provided with an inclined surface  239  as has been discussed, to cooperate with a complementary inclined surface on the actuator  210 . Actuator  210  may be provided with an opening  242  corresponding to each strip  232 , to permit functioning of the anchor as will be understood by reference to  FIG. 16B . In a hybrid absorbable-nonabsorbable fixation device, the intermediate anchor  230  may be formed from a structure similar to that illustrated in  FIG. 20 , which is molded into the pin or fit into a recess or against a transverse stop surface on the pin  205 . Alternatively, the pin  205  may be formed from a tubular metal stock, and each of the axial strips  232  is formed by cutting a channel  244  such as a U shaped channel using conventional laser cutting or other techniques to isolate the anchor strip. The anchor strips may then be biased radially outwardly by prebending them slightly in excess of the elastic limit, to facilitate each strip  232  entering the corresponding aperture  242 . Variations on the foregoing anchor structures may be readily envisioned by those of skill in the art in view of the disclosure herein.  
      The illustrated anchor assembly  230  ( FIG. 20 ) includes a hub  235  carried by the pin, such that the distal end of each axially extending strip is attached to the hub. The hub may comprise an annular ring, which is rigidly affixed to or slidably carried by the pin  205 . The axial strips may also be fixed directly to the pin  205 , such as illustrated in  FIG. 16A  in which the strips are integrally formed with the pin  205 . The shaft of pin  205  can be solid or cannulated to allow for insertion of guides such as k-wires.  
      The hub  235  or other structure which carries the anchor tines may be rotationally locked to the pin  205  and/or the actuator  210 . Any of a variety of key or spline type relationships between the hub  235  and the pin  205  may be used. For example, an axially extending recess or groove in the pin  205  can receive a radially inwardly directed projection or extension of the hub  235 . Alternatively, nonround complementary cross sectional configurations can be utilized for the pin  205  in the area of the hub  235 . As a further alternative, the hub  235  or anchor tines can be insert molded within a bioreasorbable or other polymeric pin  205 . Similar antirotation locks can be utilized for the proximal anchor or collar, as discussed elsewhere herein. Antirotation structures may be desirable in certain applications where rotation of the first and second bone fragments about the axis of the fixation pin may be clinically disadvantageous.  
      The actuator  210 , as shown in  FIG. 22 , comprises a tubular body  212  axially slidable on the pin.  FIGS. 23-25  show additional views of the actuator. The distal end of the actuator may be provided with a tapered surface  246 , such that proximal retraction of the pin with respect to the actuator causes the anchor to incline outwardly as it slides along the tapered surface. In one embodiment, the tapered surface  246  is provided on a metal leading ring  247 , on an otherwise polymeric (e.g., absorbable) tubular body.  
      The actuator  210  can take any of a variety of forms, in addition to the tubular structure illustrated in  FIGS. 22-25 . For example, the actuator  210  may extend axially moveably within an internal lumen inside of the pin  205 . Alternatively, the actuator  210  may comprise an axially extending pull wire or strip which extends along side of the pin  205 . In an embodiment of the type illustrated in  FIG. 22 , and dimensioned, for example, for use in a malleolar fracture, the tubular body  210  has an axial length of about 1.55 inches, and an outside diameter of about 0.172 inches. The inside diameter is approximately 0.138 inches, and the distal ramped surface  246  inclines at an angle of about 30° with respect to the longitudinal axis of the device. At least one axially extending stress release slot  242  extends through the retention structure  244 , discussed below. The stress release slot has an axial length of about 0.200 inches, and a width of about 0.018 inches. The proximal collar  238 , discussed below, has an outside diameter of about 0.275 inches.  
      The actuator  210  further comprises a collar  238 . Collar  238  is axially movably disposed with respect to pin  205  by connection to actuator  210 . The collar  238  seats against the proximal bone fragment to retain compression across the fracture. Collar  238  can be any of a variety of shapes or sizes, as has been discussed. The outer periphery of collar  238  can also have a radius in the axial direction or other adaptations to allow for countersinking or for cooperation with or to function as a fixation plate. The collar can act as a washer, with or without spikes for engaging tissue.  
      A retention structure  242  is preferably located on the actuator  210 , permitting proximal movement of the pin with respect to the actuator, but resisting distal movement of the pin with respect to the actuator as has been discussed. The retention structure  242  may comprise a plurality of inwardly or outwardly extending annular rings or threads. The retention structure  242  on the actuator cooperates with the retention structure  240  on the pin to retain the device under compression. The retention structure may also include a rotational link or axially extending spline for cooperating with a complementary keyway or structure on the pin  205  to prevent rotation of the pin with respect to the actuator  210 .  
      The actuator can be made from any of a variety of suitable materials or combination of materials. Preferably, the anchor is made from a metallic material, such as titanium or titanium alloy, although other materials including those disclosed elsewhere herein may be utilized for the present invention. The pin and actuator are preferably made of a bioabsorbable material, as previously discussed herein, such as Poly (L-lactide-co-D, L-lactide).  
      The proximal portion of the pin  205  can be sized to length or longer than required. The proximal portion of pin  205  which extends beyond the proximal end of actuator  210  after tensioning is preferably removed to minimize the projection of bone fixation device  200  from the surface of the bone. As previously discussed, at least a first break point may be provided on the pin  205  to facilitate breaking the proximal portion of the pin  205 . A break point, which may be an annular recess, groove, or notch, provides a designed failure point if lateral force is applied to the proximal end  220  while the remainder of the fixation device  200  is securely fixed. At least a second break point may also be provided. Alternative methods of sizing to length may also be utilized, as known to those of skill in the art.  
      Although the present invention is disclosed as embodied in a bone fixation device  200  having a generally circular cross section, cross sections such as oval, rectangular, or square may be used. Independently, the pin  205  may be tapered along its length to cause radial along with axial bone compression. Furthermore, the device may be used in combination with support features, such as plates or intramedullary nails.  
       FIG. 26  demonstrates the device  200  deployed for fracture fixation of the medial and lateral malleolar fractures. As the pin is proximally retracted with respect to the actuator, the anchor deploys radially outwardly from the pin in the proximal direction. The anchor  230  is typically embedded into the cancellous portion of the bone. The collar supports the proximal fragment of bone, provides compression as locking tension increases on the shaft, and initiates expansion of the umbrella.  
       FIGS. 27A and 27B  illustrate a removable deployment actuator  300 , which may be used with the bone fixation device  200  described above in either a through hole application such as that illustrated in  FIG. 1 , or a blind hole application such as that illustrated in  FIG. 26 . The deployment actuator  300  comprises an elongate body such as tubular body  302 , which is axially movable with respect to the pin  205 . In the illustrated embodiment, the tubular actuator is concentrically carried by, and axially movable along the pin  205 . As with the actuator  210  described above, the distal end  303  of the deployment actuator  304  may be provided with a tapered surface  304 , such that proximal retraction of the pin  205  with respect to the deployment actuator  300  causes the anchor  230  to incline outwardly as it slides along the tapered surface  304 .  
      The deployment actuator  300  preferably comprises a proximal engagement structure such as a collar  308 . The collar  308  is axially movably disposed with respect to pin  205  by connection to the deployment actuator  300 . The collar  308  may therefore be used to resist axial movement of the deployment actuator  300  with respect to the pin  205  as will be explained below. The collar  308  can be any of a variety of shapes or sizes, to facilitate manual grasping by the clinician. Alternatively, the proximal engagement structure may comprise any of a variety of ridges, grooves, threads or other locking structures to permit engagement by complementary locking structures on a deployment tool.  
      Unlike the actuator  210 , the deployment actuator  300  preferably does not include a retention structure for engaging the pin  205 . As such, proximal and distal movement of the pin  205  with respect to the deployment actuator  300  is permitted. The deployment actuator  300  can be made from any of a variety of suitable materials or combination of materials. Preferably, the actuator is made from a metallic material, such as stainless steel, titanium or titanium alloy, or a polymeric material, such as polyethylene, PEBAX, PEEK, nylon, or PTFE, although other materials including those disclosed elsewhere herein may be utilized. In one embodiment, the tapered surface  304  is provided on a metal leading ring  305  on an otherwise polymeric tubular body  302 .  
      The deployment actuator  300  is used in combination with a retention member  320 , which is illustrated in  FIGS. 28A and 28B . The retention member  320  may be substantially similar in construction to the actuator  210  of  FIGS. 22 and 23 . As such, like numbers are used to refer to parts similar to those of  FIGS. 22 and 23 . The retention member  320  generally comprises an elongate body such as tubular body  212 , which is axially movable with respect to the pin  205 . Retention member  320  is provided with a proximal anchor or engagement structure for engaging a proximal surface of the bone, a plate, soft tissue or other surface depending upon the intended application. In the illustrated embodiment, the proximal anchor comprises a collar  238 .  
      A retention structure  242  is configured to cooperate with the complementary retention structure  240  on the pin  205  to facilitate distal advancement of the retention member  320  with respect to the pin  205 , and resist proximal motion of the retention member  320  with respect to the pin  205  in a manner as is discussed elsewhere herein.  
      The distal end of the retention member  320  may be provided with a blunt distal surface  322 , which as will be explained below may secure the anchor  230  in an expanded or deployed position. In one embodiment, the blunt surface  322  is provided on a metal leading ring  324 , on an otherwise polymeric (e.g,. absorbable) tubular body.  
      Alternatively, the axial length of the retention member  320  may be selected such that it is shorter than the reasonably anticipated axial distance from the proximal surface of the proximal cortical bone or tissue and the deployed position of the anchor  230 . This allows a retention member  320  to be able to apply compression throughout a range of sizes, which may be desirable as a consequence of differing bone dimensions, or differing deployed positions of the distal anchor  230  with respect to the proximal bone or tissue surface.  
      In use, a deployment tool may be used to position the pin  205  within the bone  10 . In through hole applications, the pin  205  is advanced through a through hole until the anchor  230  exits the distal aperture (not shown). For blind hole applications, the anchor  230  is positioned in the cancellous portion  14  of the bone. See  FIG. 29 . In either application, the deployment actuator  300  may be positioned on the pin  205  before or after placement of the pin  205  and the anchor  234 .  
      As shown in  FIG. 30 , the deployment actuator  300  is used to deploy the anchor  230 . This may be accomplished in several ways. For example, the deployment tool may be configured to advance the deployment actuator  300  distally with respect to the pin  205 . Alternatively, the pin  205  can be proximally withdrawn with respect to the deployment actuator  300 . In yet another arrangement, the pin  205  can be proximally withdrawn while the deployment actuator  300  is simultaneously or sequentially distally advanced. In all of these arrangements, the distal end  303  of the deployment actuator  300  causes the anchor  234  to incline outwardly. The collar  308  may be used to proximally or distally move, or resist movement of, the deployment actuator  300 .  
      Following deployment of the anchor  230 , the deployment actuator  300  is proximally removed while the pin  205  remains anchored within the bone  10 . See  FIG. 31 . A deployment tool may then be used to insert the retention member  320  over or along the pin  205 . See  FIG. 32 . Alternatively, the surgeon can manually position the retention member  320  over the pin  205 . A deployment tool may then be used to appropriately compress the fracture  16  between the anchor  230  and the collar  238  of the implantable sleeve. See  FIG. 33 . As mentioned above, the retention structures  242  on the collar  238  permit distal movement of the collar  238  with respect to the pin  205 , but resist proximal movement of the collar  238  with respect to the pin  205 . The proximal portion of the pin  205  can be removed after the desired compression has been achieved, as described above.  
      The deployment actuator and the method for deploying a bone fixation device described above have several advantages. For example, because the deployment actuator  300  is removed from the patient&#39;s body, it can be formed from a different material compared to the retention member  320 , which is configured to remain in the patient&#39;s body. For example, the deployment actuator  300  may be formed from a less expensive and/or non-bioabsorbable material while the retention member  320  is formed from a relatively more expensive and/or bioabsorbable material. In addition, it may be advantageous to form the deployment actuator  300  from a more rugged material (e.g., a metal) as compared to the retention member  320 , because of the column strength desired to deploy the anchor  234 . In one application of the invention, the retention member  320  and pin  205  comprise a bioabsorbable material such as any of those disclosed previously herein. The distal anchor  230  may comprise a metal such as a titanium alloy. Following an absorption period of time, only the distal anchor  230  remains within the patient.  
      The specific dimensions of any of the bone fixation devices of the present invention can be readily varied depending upon the intended application, as will be apparent to those of skill in the art in view of the disclosure herein. Features from the various embodiments described above may also be incorporated into the others.  
      Although the present invention has been described in terms of certain preferred embodiments, other embodiments of the invention including variations in dimensions, configuration and materials will be apparent to those of skill in the art in view of the disclosure herein. In addition, all features discussed in connection with any one embodiment herein can be readily adapted for use in other embodiments herein. The use of different terms or reference numerals for similar features in different embodiments does not imply differences other than those which may be expressly set forth. Accordingly, the present invention is intended to be described solely by reference to the appended claims, and not limited to the preferred embodiments disclosed herein.