Patent Publication Number: US-2006015105-A1

Title: Proximal anchors for bone fixation system

Description:
PRIORITY INFORMATION  
      This application claims the priority benefit under 35 U.S.C. § 119(e) of Provisional Application 60/468,377 filed May 6, 2003 and Provisional Application 60/471,973 filed May 20, 2003, the entire contents of these Provisional Applications are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to bone fixation devices, and, more particularly, to a bone fixation device with a proximal anchor.  
      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.  
      A variety of elongated implants (nail, screw, pin, etc.) have been developed, which are adapted to be positioned along the longitudinal axis of the femoral neck with a leading (distal) end portion in the femoral head so as to stabilize a fracture of the femoral neck. The elongated implant may be implanted by itself or connected to another implant such as a side plate or intramedullary rod. The leading end portion of the implant typically includes means to positively grip the femoral head bone (external threads, expanding arms, etc.), but the inclusion of such gripping means can introduce several significant problems. First, implants with sharp edges on the leading end portion, such as the externally threaded implants, exhibit a tendency to migrate proximally towards the hip joint bearing surface after implantation. This can occur when the proximal cortical bone has insufficient integrity to resist distal movement of the screw head. Such proximal migration under physiological loading, which is also referred to as femoral head cut-out, can lead to significant damage to the adjacent hip joint. Also, the externally threaded implants can generate large stress concentrations in the bone during implantation which can lead to stripping of the threads formed in the bone and thus a weakened grip. The movable arms of known expanding arm devices are usually free at one end and attached at the other end to the main body of the leading end portion of the implant. As a result, all fatigue loading is concentrated at the attached ends of the arms and undesirably large bending moments are realized at the points of attachment. In addition, conventional threaded implants generally exhibit insufficient holding power under tension, such that the threads can be stripped out of the femoral head either by overtightening during the implantation procedure or during post operative loading by the patient&#39;s weight.  
      Bone fasteners may also be used for the stabilization of fractures and/or fusion of various portions of the spine. Such fasteners are often inserted through the pedicles of the vertebra and may be used in combination with a variety of longitudinal elements such as rods or plates which span two or more vertebra. These systems may be affixed to either the posterior or the anterior side of the spine.  
      Notwithstanding the variety of bone fasteners that have been developed in the prior art, there remains a need for a simple, adjustable bone fixation device which may be utilized to secure a fracture, secure soft tissue or tendon to the bone and/or provide stability between bones (e.g., vertebrae).  
     SUMMARY OF THE INVENTION  
      There is provided in accordance with one embodiment of the present invention, a fixation device for securing a first bone fragment to a second bone fragment or a first bone to a second bone. Alternatively, the fixation device may be used to secure soft tissue to a bone. The fixation device comprises an elongate pin, having a proximal end and a distal end. At least one axially advanceable anchor is carried by the pin.  
      In another embodiment, an orthopedic fixation device comprises an elongate body, having a proximal end and a distal end. A distal anchor is on the distal end of the body. A retention structure is positioned on the elongate body, proximal to the anchor. A proximal anchor is moveably carried by the elongate body. The proximal anchor comprises a tubular sleeve with a radially outward extending head. At least one complementary retention structure is on the proximal anchor and is configured for permitting proximal movement of the elongate body with respect to the proximal anchor but resisting distal movement of the elongate body with respect the proximal anchor. A washer is angularly moveable with respect to the longitudinal axis of the tubular sleeve. The washer has an aperture that is elongated with respect to a first axis such that the washer permits greater angular movement in a plane containing the first axis.  
      In another embodiment, an orthopedic fixation device comprises an elongate pin, having a proximal end, a distal end and a first retention structure. At least one distal anchor carried by the elongate pin. A proximal anchor is axially moveable with respect to the elongate pin and comprises a split ring positioned within an annular recess formed within the proximal anchor. The split ring has at least one gap formed between two ends. The split ring is moveable between a first position and a second position. The second position is located closer to the longitudinal axis of the elongate pin as compared to the first portion so as to engage the first retention structure and prevent proximal movement of the proximal anchor with respect to the elongated pin while the first position allows distal movement of the proximal anchor with respect to the pin. An anti-rotational structure prevents rotation of the split ring with the recess about the longitudinal axis of the elongate pin.  
      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 positioned within a fractured bone.  
       FIG. 2  is a side elevational view of a pin body of the bone fixation device of  FIG. 1 .  
       FIG. 3  is a distal end elevational view of the pin body of  FIG. 2 .  
       FIG. 4  is a longitudinal cross-sectional view through the pin body of  FIG. 2 .  
       FIG. 5  is an enlarged detail view of the distal end of the device shown in  FIG. 2 .  
       FIG. 6  is a cross-sectional view of a proximal anchor of the bone fixation device of  FIG. 1 .  
       FIG. 7  is a proximal end view of the proximal anchor of  FIG. 6 .  
       FIG. 8  is a side view of a expansion guide wire.  
       FIG. 9  is a longitudinal cross-sectional view of the locking guide wire of  FIG. 8  and the pin body of  FIG. 8 .  
      FIGS.  10 A-E are perspective, side, top and bottom views of a modified embodiment of a pin body.  
      FIGS.  11 A-E perspective, side, top and bottom views of a modified embodiment of a locking guidewire.  
      FIGS.  12 A-F are perspective, side, top and bottom views of a portion of a modified embodiment of a proximal anchor.  
      FIGS.  13 A-D are perspective, side, and bottom views of the locking guidewire and pin body of  FIGS. 10-11E .  
       FIG. 14  is a posterior elevational posterior cross section through the proximal portion of the femur, having another embodiment of a bone fixation device positioned therein.  
       FIG. 15  is a side elevational cross section of a fixation device similar to that of  FIG. 14 .  
       FIG. 16  is a cross sectional view through an angularly adjustable proximal anchor plate.  
       FIG. 17  is a front perspective view of the anchor plate of  FIG. 16 .  
       FIG. 17A  is a perspective view of a flange and a fixation device.  
       FIG. 17B  is a partial cross-sectional side view of the flange of  FIG. 17A  and a housing of a proximal anchor.  
       FIG. 17C  is a bottom view of the flange of  FIG. 17A .  
       FIG. 18  is a side elevational view of a double helix distal anchor.  
       FIG. 19  is an anterior view of the distal tibia and fibula, with fixation devices across lateral and medial malleolar fractures.  
       FIG. 20  is a perspective view of another embodiment of a proximal anchor.  
       FIG. 21  is a side elevational view of the proximal anchor of  FIG. 20 .  
       FIG. 22  is a longitudinal cross-sectional view of the proximal anchor of  FIG. 20 .  
       FIG. 23  is an enlarged detail view of a portion of the proximal anchor shown in  FIG. 22 .  
       FIG. 23A  is an enlarged detail view of a portion of a modified embodiment of the proximal anchor shown in  FIG. 22 .  
       FIG. 23A  is a perspective view of another yet embodiment of a proximal anchor.  
       FIG. 24  is a perspective view of another yet embodiment of a proximal anchor.  
       FIG. 25  is a side elevational view of the proximal anchor of  FIG. 24 .  
       FIG. 26  is a longitudinal cross-sectional view of the proximal anchor of  FIG. 24 .  
       FIG. 27A  is an enlarged detail view of a portion of the proximal anchor of  FIG. 26  shown in a first position.  
       FIG. 27B  is an enlarged detail view of a portion of the proximal anchor of  FIG. 26  shown in a second position.  
       FIG. 27C  is side perspective view of a portion of a modified embodiment of the proximal anchor shown in  FIG. 26 .  
       FIG. 28  is an enlarged longitudinal cross-sectional view of a modified embodiment of a proximal anchor with the portion illustrated in  FIG. 27C .  
      FIGS.  29 A-C are perspective, side, and longitudinal cross-sectional views of a modified embodiment of a fixation device.  
       FIG. 29D  is an enlarged portion of  FIG. 29C .  
      FIGS.  30 A-F are perspective, side, top, bottom and cross-sectional views of a modified embodiment of a proximal anchor.  
       FIG. 31A  is a perspective view of another embodiment of a proximal anchor.  
       FIGS. 31B and 31C  are enlarged views of a portion of one embodiment of a proximal anchor.  
       FIG. 31D  is a front view of the proximal anchor of  FIG. 31A . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Although the application of the present invention will be initially 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 metaphysical 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 bone fixation devices described herein may also be used in a variety of techniques to stabilize the spine. For example, the bone fixation devices may be used as pedicle or facet screws that may be unilaterally or bilaterally symmetrically mounted on adjacent or non-adjacent vertebrae and used in combination one or more linkage rods or plates to facilitate fusion of one or more vertebrae. The bone fixation devices disclosed herein may also be used as a fixation screw to secure two adjacent vertebra to each other in a trans-laminar, trans-facet or facet-pedicle (e.g., the Boucher technique) applications. One of skill of the art will also recognize that the bone fixation devices disclosed herein may be used for posterior stability after laminectomy, artificial disc replacement, repairing odontoid fractures and other fractures of the spine, and other applications for providing temporary or permanent stability in the spinal column.  
      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 (i.e., a blind hole application).  
      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 illustrated bone fixation  24  device and modified embodiments of the bone fixation device  24  are disclosed in U.S. Pat. No. 6,648,890, issued on Nov. 18, 2003, which is hereby incorporated by reference herein.  
      The distal end  30  of pin  26  is provided with a distal anchor  34 , as will be discussed below. A proximal anchor  36  is also provided.  
      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 the embodiment illustrated in  FIGS. 6 and 7 , the proximal anchor  36  comprise a collar  38  for contacting the proximal bone component  19 . Collar  38  may comprises a radially-outwardly extending annular ramp or 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 collar  38  is connected to a tubular housing  40  adapted to coaxially receive the pin body  32  therethrough.  
      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 cannulated pin body  32 , which defines a central lumen  11  to allow introduction of the pin over a wire as is understood in the art. Hollow tubular structures may also be used. However, in other embodiments, a solid pin body may be provided. Such an embodiment is disclosed in co-pending U.S. Pat. No. 6,648,890, filed Apr. 10, 2001, which was incorporated by reference above.  
      In the illustrated embodiment, the retention structures  44  of the pin  26  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, the collar  38  (see  FIGS. 6 and 7 ) 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. 7 .  
      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. 4 , 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. In still other embodiments, the proximal end  28  of the central lumen  11  may be configured with an non-round cross-section for rotational engagement with an installation or removal tool.  
      The retention structures  44  can also 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. Such a ramp or ratchet-type structure 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.  
      In a similar manner, 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.  
      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 above. 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 .  
      With reference to  FIGS. 2-5 , the distal anchor  34  in the illustrated embodiment comprises a plurality of ramped extensions or barbs  50  for engaging the distal cortical bone, the interior cancellous bone or other surfaces. As will be explained below, the extensions or barbs  50  are positioned or compressible radially inward for the purpose of advancing the pin  32  into, and, in some applications, through the hole  22 . Barbs  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 barbs  50  to seat firmly against the outside surface of distal bone component  21 , as illustrated in  FIG. 1 .  
      The illustrated embodiment includes four barbs  50  ( FIG. 3 ), oriented at 90° with respect to each other. However, 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. 2 . 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.  
      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 , and thereafter resist proximal withdrawal of the pin body  32 . 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  20  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.  
      Additional description of the distal anchor and alternate distal anchor designs are described in co-pending U.S. Pat. No. 6,648,890, which is hereby incorporated by reference herein.  
      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.  
      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 .  
      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 .  
      Following appropriate positioning 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 pin cutters 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.  
      As mentioned above, in some embodiments, the retention structures  44  on the surface of the pin body comprise a plurality of ratchet-type structures. In such embodiments, proximal traction is preferably 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.  
      For any of the ratchet-type 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.  
      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.  
      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. 2 , 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. 6 , 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.  
       FIG. 8  illustrates a expansion 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&#39;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. 9 , 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.  
      For example,  FIGS. 10A-13C  illustrate an exemplary embodiment of a locking guidewire  150 ′ and a pin  26 ′ with such radially inwardly and radially outwardly directed surface structures. In this exemplary embodiment, the locking guidewire  150 ′ includes a radially outwardly directed flange  151 ′, which is formed by the locking portion  156 ′ of the guidewire  150 ′. The flange  151 ′ is configured to allow a portion of the locking portion  156 ′ to be retracted into the first portion  160 ′ of the lumen  11 ′ (see FIGS.  10 A-E) while preventing the distal portion  156 ′ from passing proximally through the pin  26 ′ or resisting such movement. To resist distal migration of the locking guidewire  150 ′, the illustrated embodiment includes a radially outwardly directed flange  153 ′, which cooperates with a corresponding cylindrical groove  155 ′ (see  FIG. 10A ) provided on the inside surface of the lumen  11 ′. The complementary surface structures  153 ′,  155 ′ may be toleranced such that the locking guidewire  150 ′ 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 addition, the complementary surface structures  153 ′,  155 ′ may be toleranced such that proximally withdrawing the locking guidewire  150 ′ into the lumen  11 ′ produces audible or tactile feedback when the complementary surface structures  153 ′,  155 ′ are properly engaged. Such feedback can be used by the surgeon to indicate that the locking guidewire  150 ′ is properly positioned within the pin  26 ′. The proximal end of the locking guidewire  150 ′ may also include visual indicia (e.g, color bands, grooves, etc.), which may be referenced with respect the proximal end of the pin to confirm that locking guidewire has been properly retracted. The locking portion  156  on the exemplary pull pin of FIGS.  11 A-C also includes a conical or tapered portion  157 ′, which may configured to interact with the lumen of the pin so as to gradually expand the distal end  30 ′ of the pin  26 ′.  
      FIGS.  12 A-F illustrate a portion of another exemplary embodiment of a proximal anchor. In this embodiment, the proximal anchor  600  includes a tubular housing  602 , that may be attached to, coupled to, or integrally formed, partially or wholly, with a flange  601  (see FIGS.  13 A-C), which in the illustrated embodiment includes a bone contacting surface  603  and has one or more anti-rotational features  605  (e.g., flat sides). The tubular housing  602  includes an inner surface with one or more teeth or flanges  610 , which are configured to engage the grooves or ridges on the body  405 . One or more slots or openings  604  are formed in the tubular housing to form one or more bridges  606 , which carry the grooves or ridges  605 . The anchor  600  can be pushed towards the distal end of the body and the teeth can slide along and be lifted over the retention structures  406  of the body as the bridge is flexed away from the body. The number and shape of the openings and bridges may be varied depending of the desired flexing of the bridges when the proximal anchor is moved distally over the body and the desired retention force of the distal anchor when appropriately tensioned. In one embodiment, the teeth on the proximal anchor and the grooves on the body  405  ( FIG. 10A ) may be configured such that the proximal anchor can be rotated or threaded onto the pin in the proximal direct and/or so that the proximal anchor can be removed by rotation.  
      In use, after the clinician assesses the bone, selects a bone drill and drills a through hole  22  ( FIG. 1 ), 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 pin cutters which are routinely available in the clinical setting, or snapped off using designed break points as has been discussed. In certain embodiments, the proximal end  28  of the pin body and/or the proximal end  154  of the guidewire  150  may be removed by cauterizing. Cauterizing may fuse the proximal anchor  36  to the body  32  thereby adding to the retention force between the proximal anchor  36  and the body  28 . Such fusion between the proximal anchor and the body may be particularly advantageous if the pin and the proximal anchor are made from a bioabsorbable and/or biodegradable material. In this manner, as the material of the proximal anchor and/or the pin is absorbed or degrades, the fusion caused by the cauterizing continues to provide retention force between the proximal anchor and the pin.  
      Referring to  FIG. 14 , there is illustrated a posterior side elevational view of the proximal portion of a femur  210 , having another embodiment of a fixation device  212  positioned therein. Detailed descriptions of this and alternative fixation devices can be found in co-pending U.S. Pat. No. 6,511,481 issued on Jan. 28, 2003 entitled METHOD AND APPARATUS FOR FIXATION OF PROXIMAL FEMORAL FRACTURE, U.S. patent application Ser. No. 10/012,687 filed on Nov. 13, 2001 entitled DISTAL BONE ANCHORS FOR BONE FIXATION WITH SECONDARY COMPRESSION and U.S. patent application Ser. No. 09/991,367 filed on Nov. 13, 2001 entitled METHOD AND APPARATUS FOR BONE FIXATION WITH SECONDARY COMPRESSION, which are hereby incorporated by reference herein. Although this embodiment of a fixation device is disclosed in the context of fractures of the proximal femur, as with the embodiments described above, 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.  
      The proximal end of the femur  210  comprises a head  214  connected by way of a neck  216  to the long body or shaft  217  of the femur  210 . As illustrated in  FIG. 10 , the neck  216  is smaller in diameter than the head  214 . The neck  216  and head  214  also lie on an axis which, on average in humans, crosses the longitudinal axis of the body  217  of the femur  210  at an angle of about 126°. The risk of fracture at the neck  216  is thus elevated, among other things, by the angular departure of the neck  216  from the longitudinal axis of the body  217  of femur  210  and also the reduced diameter of the neck  216  with respect to the head  214 .  
      The greater trochanter  218  extends outwardly above the junction of the neck  216  and the body  217  of the femur  210 . On the medial side of the greater trochanter  218  is the trochanteric fossa  220 . This depression accommodates the insertion of the obturator externus muscle. The lesser trochanter  221  is located posteromedially at the junction of the neck  216  and the body  217  of the femur  210 . Both the greater trochanter  218  and the lesser trochanter  221  serve for the attachment of muscles. On the posterior surface of the femur  210  at about the same axial level as the lesser trochanter  221  is the gluteal tuberosity  222 , for the insertion of the gluteus maximus muscle. Additional details of the femur are well understood in the art and not discussed in further detail herein.  
       FIG. 14  illustrates a fracture  224  which crosses the femur approximately in the area of the greater trochanter  218 . Fractures of the proximal portion of the femur  210  are generally classified as femoral neck fractures, intertrochanteric fractures and subtrochanteric fractures. All of these fractures will be deemed femoral neck fractures for the purpose of describing the present invention.  
      Referring to  FIGS. 14 and 15 , the fixation device  212  comprises a pin body  228  extending between a proximal end  230  and a distal end  232 . The length, diameter and construction materials of the body  228  can be varied, depending upon the intended clinical application. In an embodiment optimized for femoral neck fractures in an adult human population, the body  228  will generally be within the range of from about 45 mm to about 120 mm in length after sizing, and within the range of from about 3 mm to about 8 mm in maximum diameter. The major diameter of the helical anchor, discussed below, may be within the range of from about 6 mm to about 12 mm. In general, the appropriate dimensions of the body  228  will vary, depending upon the specific fracture. In rough terms, for a malleolar fracture, shaft diameters in the range of from about 3 mm to about 4.5 mm may be used, and lengths within the range of from about 25 mm to about 70 mm. For condylar fractures, shaft diameters within the range of from about 4 mm to about 6.5 mm may be used with lengths within the range of from about 25 mm to about 70 mm. For colles fractures (distal radius and ulna), diameters within the range of from about 2.5 mm to about 3.5 mm may be used with any of a variety of lengths within the range of from about 6 mm to about 120 mm.  
      In one embodiment, the body  228  comprises titanium. However, as will be described in more detail below, other metals or bioabsorbable or nonabsorbable polymeric materials may be utilized, depending upon the dimensions and desired structural integrity of the finished fixation device  212 .  
      The distal end  232  of the body  228  is provided with a cancellous bone anchor or distal anchor  234 . Additional details of the illustrated cancellous bone anchor and other embodiments are described below and in co-pending U.S. patent application Ser. No. 10/012,687 filed on Nov. 13, 2001 entitled DISTAL BONE ANCHORS FOR BONE FIXATION WITH SECONDARY COMPRESSION, which was incorporated by reference above. In general, the cancellous bone anchor  234  is adapted to be rotationally inserted into the cancellous bone within the head  214  of the femur  210 , to retain the fixation device  212  within the femoral head.  
      The proximal end  230  of the body  228  is provided with a proximal anchor  236 . As with the embodiments described with reference to  FIGS. 1-9 , the proximal anchor  236  is axially distally moveable along the body  228 , to permit compression of the fracture  224  as will be apparent from  FIG. 14 . Complimentary locking structures such as threads or ratchet like structures between the proximal anchor  236  and the body  228  resist proximal movement of the anchor  236  with respect to the body  228  under normal use conditions. The proximal anchor  36  can be axially advanced along the body  228  either with or without rotation, depending upon the complementary locking structures as will be apparent from the disclosure herein.  
      In the illustrated embodiment, proximal anchor  236  comprises a housing  238  such as a tubular body, for coaxial movement along the body  228 . The housing  238  is provided with one or more surface structures  240  such as radially inwardly projecting teeth or flanges, for cooperating with complementary surface structures  242  on the body  228 . The surface structures  240  and complementary surface structures  242  permit distal axial travel of the proximal anchor  236  with respect to the body  228 , but resist proximal travel of the proximal anchor  236  with respect to the body  228 . Any of a variety of complementary surface structures which permit one way ratchet like movement may be utilized, such as a plurality of annular rings or helical threads, ramped ratchet structures and the like for cooperating with an opposing ramped structure or pawl.  
      Retention structures  242  are spaced axially apart along the body  228 , between a proximal limit  254  and a distal limit  256 . The axial distance between proximal limit  254  and distal limit  256  is related to the desired axial range of travel of the proximal anchor  236 , and thus the range of functional sizes of the fixation device  212 . In one embodiment of the fixation device  212 , the retention structure  242  comprise a plurality of threads, adapted to cooperate with the retention structures  240  on the proximal anchor  236 , which may be a complementary plurality of threads. In this embodiment, the proximal anchor  236  may be distally advanced along the body  228  by rotation of the proximal anchor  236  with respect to the body  228 . Proximal anchor  236  may be advantageously removed from the body  28  by reverse rotation, such as to permit removal of the body  28  from the patient. In this embodiment, a washer or flange  244  is preferably provided with a gripping structure to permit a removal tool to rotate the flange  244  with respect to the body  228 . Any of a variety of gripping structures may be provided, such as one or more slots, flats, bores or the like. In one embodiment, the flange  244  is provided with a polygonal, and, in particular, a pentagonal or hexagonal circumference.  
      The flange  244  seats against the outer surface of the femur or tissue adjacent the femur. The flange  244  is preferably an annular flange, to optimize the footprint or contact surface area between the flange  244  and the femur. Circular or polygonal shaped flanges for use in femoral head fixation will generally have a diameter of at least about 4 mm greater than the adjacent body  228  and often within the range of from about 4 mm to about 20 mm or more greater than the adjacent body  228 . In a modified embodiment, the flange  244  can be curved to match the curved shape of the femur and further optimize the footprint or contact surface area between the flange  244  and the femur.  
      Tensioning and release of the proximal anchor  36  may be accomplished in a variety of ways, depending upon the intended installation and removal technique. For example, a simple threaded relationship between the proximal anchor  236  and body  228  enables the proximal anchor  236  to be rotationally tightened as well as removed. However, depending upon the axial length of the threaded portion on the pin  228 , an undesirably large amount of time may be required to rotate the proximal anchor  236  into place. For this purpose, the locking structures on the proximal anchor  236  may be adapted to elastically deform or otherwise permit the proximal anchor  236  to be distally advanced along the body  228  without rotation, during the tensioning step. The proximal anchor  236  may be removed by rotation as has been discussed. In addition, any of a variety of quick release and quick engagement structures may be utilized. For example, the threads or other retention structures surrounding the body  228  may be interrupted by two or more opposing flats. Two or more corresponding flats are provided on the interior of the housing  238 . By proper rotational alignment of the housing  238  with respect to the body  228 , the housing  328  may be easily distally advanced along the body  228  and then locked to the body  228  such as by a 90° or other partial rotation of the housing  238  with respect to the body  228 . Other rapid release and rapid engagement structures may also be devised, and still accomplish the advantages of the present invention.  
      In the embodiments illustrated in  FIGS. 15 and 16 , the bone contacting surface  246  of the flange  244  resides in or approximately on a plane which is inclined with respect to the longitudinal axis of the body  228 . Any of a variety of angular relationships between the bone contacting surface  246  of the flange  244  and the longitudinal axis of the body  228  and housing  238  may be utilized, depending upon the anticipated entrance angle of the body  228  and associated entrance point surface of the femur  210 . In general, the longitudinal axis extending through the head  214  and neck  216  of the human femur is inclined at an angle of approximately 126° from the longitudinal axis of the long body  217  of the femur  210 . Angles between the longitudinal axis of body  228  and tissue contacting surface  246  within the range of from about 90° to about 150° will generally be utilized, often within the range of from about 120° to about 150°, for fixed angle fixation devices. Perpendicular flanges (i.e., 90°) are illustrated in  FIG. 15 .  
      The clinician can be provided an array of proximal anchors  236  of varying angular relationships between the bone contacting surface  46  and the longitudinal axis of the body  228  and housing  238  (e.g., 90°, 100°, 110°, 120°, and 130°). A single body  228  can be associated with the array such as in a single sterile package. The clinician upon identifying the entrance angle of the body  228  and the associated entrance point surface orientation of the femur  210  can choose the anchor  236  from the array with the best fit angular relationship, for use with the body  228 .  
      In accordance with an optional feature, illustrated in  FIGS. 16 and 17 , the flange  244  is angularly adjustable with respect to the longitudinal axis of the body  228 . More specifically, in this embodiment, the housing  238  is a separate component from the flange  244 . At its proximal end, the housing  238  includes a semi-spherical or radiused surface  245   a  which forms a part of a head  239 . The a semi-spherical or radiused surfaces  245   a  correspond to corresponding a curved, semi-spherical or radiused surface  245   b  formed on the flange  244 . The surface  245   b  surrounds an aperture  249  in the flange  244 . The aperture  249  is preferably slightly larger than the housing  238  and/or body  228  that extends through the aperture  249  but smaller than the head  239  of the proximal anchor  236 . This arrangement allows the housing  238  and/or body  228  to extend through and pivot with respect to the flange  244  while preventing proximal movement of the proximal anchor  236  with respect to the flange  244 . As such, the angular relationship between the bone contacting surface  246  of the flange  244  and the longitudinal axis of the body  228  can vary in response to the entrance angle. The flange  244  may optionally include additional apertures  247  for use with one or more fixation screws (not shown) for additional security.  
      FIGS.  17 A-C illustrates a modified embodiment of a flange or washer  900 . As with the flange  244  of  FIG. 16 , the washer  900  is configured to interact with the head  239  of the proximal anchor  236 . The washer  900  includes a base  902  and a side wall  904 . The base  902  and side wall  904  define a curved, semi-spherical or radiused surface  245   a  that interacts with the corresponding curved, semi-spherical or radiused surface  245   b  of the head  239 . The surface  245   a  surrounds an aperture  906  formed in the base  902 . As described above, this arrangement allows the housing  238  and/or body  228  to extend through and pivot with respect to the washer  900 .  
      With particular reference to  FIG. 17C , in the illustrated embodiment, the aperture  906  is elongated with respect to a first direction d 1  as compared a second direction d 2 , which is generally perpendicular to the first direction d 1 . In this manner, the width w 1  of the aperture in the first direction is greater than the width w 2  of the aperture in the second direction. In this manner, the aperture  906  provides a channel  911  with a width w between the sides  911   a,    911   b  defined with respect to the second direction d 2  that is preferably greater than the maximum width of the tubular housing  238  but smaller than the width of the head  908  such that the proximal anchor  236  can not be pulled through the aperture  906 . The height h of the channel is defined between the sides  911   c,    911   d  in the second direction. As such, the elongated aperture  906  permits greater angular movement in a plane containing the first direction d 1  as portions of the proximal anchor  236  are allowed rotate into the elongated portions of the aperture  906 . The aperture  906  may be elliptical or formed into other shapes, such as, for example, a rectangle or a combination of straight and curved sides.  
      The washer  900  optionally includes a portion that is configured so that the proximal end  243  of the anchor  236  is retained, preferably permanently retained, within the washer  900 . In the illustrated embodiment, the side walls  904  are provided with lips  910 . The lips  910  extend inwardly from the side walls  904  towards the aperture  906  and interact with the proximal end  243  of the head  239  so that the proximal anchor  236  is retained within the washer  900 . Preferably, the washer  900  is toleranced to allow the proximal anchor  236  to freely rotate with respect to the washer  900 . In this manner, the washer  900  and the proximal anchor  236  can move together for convenient transport.  
      As described above, when the body  228 , the proximal anchor  236  and the washer  900  are deployed into a patient, the washer  900  can inhibit distal movement of the body  228  while permitting at least limited rotation between the body  228  and the washer  900 . As such, the illustrated arrangement allows for rotational and angular movement of the washer  900  with respect to the body  228  to accommodate variable anatomical angles of the bone surface. This embodiment is particularly advantageous for spinal fixation and, in particular, trans-laminar, trans-facet and trans-facet-pedicle applications. In such applications, the washer  900  may seat directly against the outer surface of a vertebra. Because the outer surface of the vertebra is typically non-planar and/or the angle of insertion is not perpendicular to the outer surface of the vertebra, a fixed flange may contact only a portion of the outer surface of the vertebra. This may cause the vertebra to crack due to high stress concentrations. In contrast, the angularly adjustable washer  900  can rotate with respect to the body and thereby the bone contacting surface may be positioned more closely to the outer surface. More bone contacting surface is thereby utilized and the stress is spread out over a larger area. In addition, the washer, which has a larger diameter than the body  228 , or proximal anchor described herein, effectively increases the shaft to head diameter of the fixation device, thereby increasing the size of the loading surface and reducing stress concentrations. Additionally, the washer  900  can be self aligning with the outer surface of the vertebra, which may be curved or non-planer. The washer  900  can slide along the surface of the vertebra and freely rotate about the body  228  until the washer  900  rests snugly against the surface of the vertebra for an increased contact area between the bone and the washer  900 . As such, the washer  900  can be conveniently aligned with a curved surface of the vertebra.  
      In another embodiment, the washer  900  has a surface treatment or bone engagement features that can engage with the surface of the bone to inhibit relative movement between the washer  900  and the bone. Although not illustrated, the washer  900  can include a plurality of bone engagement features in the form of one or more spikes (not shown) extending from the surface of the washer  900 . The spikes can contact the surface of the bone to provide additional gripping support, especially when the flange  244  is positioned against, for example, uneven bone surfaces and/or soft tissue. Optionally, the washer  900  can have protuberances, roughened surface, ridges, serrations, or other surface treatment for providing friction between the flange  244  and the surface of the bone. However, it should be appreciated that in modified embodiments the washer  900  may be formed without the bone engagement features or surface treatments. As an independent feature, for example, the washer  900  can be enlarged and includes one or two or more openings for receiving one or set screws (not shown). The setscrews can be passed through the openings to securely fasten the washer  900  to a bone.  
      With reference back to  FIGS. 14 and 15 , the proximal end  230  of the body  228  is preferably additionally provided with rotational coupling  248 , for allowing the body  228  to be rotationally coupled to a driving device. Any of a variety of driving devices may be utilized, such as electric drills or hand tools which allow the clinician to manually rotate the cancellous bone anchor  234  into the head of the femur. Thus, the rotational coupling  248  may have any of a variety of cross sectional configurations, such as one or more flats or splines.  
      In one embodiment, the rotational coupling  248  comprises a proximal projection of the body  228  having a polygonal cross section, such as a hexagonal cross section. The rotational coupling  248  is illustrated as a male component, machined or milled or attached to the proximal end  230  of the body  228 . However, the rotational coupling may also be in the form of a female element, such as a hexagonal or other noncircular cross sectioned lumen extending throughout a proximal portion or the entire length of the body  228 . Although illustrated as solid throughout, the body  228  may be cannulated to accommodate installation over a placement wire as is understood in the art. The cross section of the central cannulation can be made non circular, e.g., hexagonal, to accommodate a corresponding male tool for installation or removal of the device regardless of the location of the proximal break point.  
      The body  228  may be provided with at least one&#39;and preferably two or three or more break points  250  spaced axially apart along the proximal portion of the body  228 . Break points  250  comprise a weakened transverse plane through the body  228 , which facilitate severing of the proximal portion of the body  228  following proper tensioning of the proximal anchor. Break point  250  may be constructed in any of a variety of ways, such as by machining or milling an annular recess into the exterior wall of the body  228 , or created one or more transverse perforations through the body  228  such as by mechanical, laser, or EDM drilling.  
      In one embodiment, the distal anchor  234  comprises a helical locking structure  260  for engaging cancellous bone, as illustrated in  FIG. 14 . The locking structure  260 , such as a flange, may either be wrapped around a central core  262  or an axial lumen, as discussed below. The flange extends through at least one and generally from about two to about 250 or more full revolutions depending upon the axial length of the distal anchor and intended application. For most femoral neck fixation devices, the flange will generally complete from about 2 to about 20 revolutions. The helical flange  260  is preferably provided with a pitch and an axial spacing to optimize the retention force within cancellous bone, to optimize compression of the fracture. In some applications, it may advantageous for the distal anchor to engage cortical bone. In such applications, the pitch and axial spacing may be optimized for cortical bone.  
      The helical flange  260  of the embodiment illustrated in  FIG. 14  is shaped generally like a flat blade or radially extended screw thread. However, it should be appreciated that the helical flange  260  can have any of a variety of cross sectional shapes, such as rectangular, triangular or other as deemed desirable for a particular application through routine experimentation in view of the disclosure herein. The outer edge of the helical flange  260  defines an outer boundary. The ratio of the diameter of the outer boundary to the diameter of the central core  262  can be optimized with respect to the desired retention force within the cancellous bone and giving due consideration to the structural integrity and strength of the distal anchor  234 . Another aspect of the distal anchor  234  that can be optimized is the shape of the outer boundary and the central core  262 , which in the illustrated embodiment are generally cylindrical with a tapered distal end  232 .  
      The distal end  232  and/or the outer edges of the helical flange  260  may be atraumatic (e.g., blunt or soft). This inhibits the tendency of the fixation device  212  to migrate anatomically proximally towards the hip joint bearing surface after implantation (i.e., femoral head cut-out). Distal migration is also inhibited by the dimensions and presence of the proximal anchor  236 , which has a larger footprint than conventional screws.  
      Referring to  FIG. 18 , a variation of the distal anchor  234  is illustrated. In this embodiment, the distal anchor comprises a double helix structure. Each helix is spirally wrapped about an imaginary cylinder through at least one and preferably from about 2 to about 20 or more full revolutions per inch. In a modified embodiment, each helix is wrapped around substantially cylindrical central core that includes a central lumen that also extends through the body. As with the previous embodiment, the helix structure is preferably provided with pitch and an axial spacing to optimize the retention force within cancellous bone, which optimizes compression. The tip  72  of the elongated body  260  may be pointed or sharp. In one preferred embodiment, the each helix has wrapped about 7 revolutions per each for an overall thread density of about 14 revolutions per inch.  
      In any of the embodiments herein, an antirotation lock may be provided between the distal anchor and the proximal collar or plate, such as a spline or other interfit structure to prevent relative rotation of the proximal and distal ends of the device following implantation.  
      In use, the clinician first identifies a patient having a fracture such as, for example, a femoral neck fracture, which is fixable by an internal fixation device. The clinician accesses the proximal femur, reduces the fracture if necessary and selects a bone drill and drills a hole  280  ( FIG. 14 ) in accordance with conventional techniques. Preferably, the hole  280  has a diameter within the range from about 3 mm to about 8 mm. This diameter may be slightly larger than the diameter of the distal anchor  234 . The hole  280  preferably extends up to or slightly beyond the fracture  224 . In certain embodiments, the clinician may use a bone drill with a counter sink(s) configured for providing a larger diameter recesses for the housing  238  and/or the flange  244  of the proximal anchor  236 .  
      A fixation device  212  having an axial length and outside diameter suitable for the through hole  280  is selected. The distal end  232  of the fixation device  212  is advanced distally into the hole  280  until the distal anchor  234  reaches the distal end of the hole  280 . The proximal anchor  236  may be carried by the fixation device  212  prior to advancing the body  228  into the hole  280 , or may be attached following placement of the body  228  within the hole  280 . Once the body  228  is in place, the clinician may use any of a variety of driving devices, such as electric drills or hand tools to rotate the cancellous bone anchor  234  into the head of the femur.  
      While proximal traction is applied to the proximal end  230  of body  228 , such as by conventional hemostats, pliers or a calibrated loading device, the proximal anchor  236  is advanced distally until the anchor  236  fits snugly against the outer surface of the femur or tissue adjacent the femur. Appropriate tensioning of the fixation device  212  is accomplished by tactile feedback or through the use of a calibration device for applying a predetermined load on the implantation device. One advantage of the structure of the present invention is the ability to adjust compression independently of the setting of the distal anchor  234 .  
      Following appropriate tensioning of the proximal anchor  236 , the proximal extension  230  of the body  228  is preferably cut off or snapped off and removed. Body  228  may be cut using conventional saws, cutters or bone forceps which are routinely available in the clinical setting. Alternatively, the fixation device can be selected such that it is sized to length upon tensioning, so no proximal projection remains. In certain embodiments, the proximal extension  230  of the body  228  may be removed by cauterizing. Cauterizing the proximal extension  230  may advantageously fuse the proximal anchor  235  to the body  228  thereby adding to the retention force between the proximal anchor  235  and the body  228 . Such fusion between the proximal anchor  235  and the body  228  may be particularly advantageous if the pin  228  and the proximal anchor are made from a bioabsorbable and/or biodegradable material. In this manner, as the material of the proximal anchor and/or the pin is absorbed or degrades, the fusion caused by the cauterizing continues to provide retention force between the proximal anchor and the pin.  
      Following trimming the proximal end  230  of body  228 , the access site may be closed and dressed in accordance with conventional wound closure techniques.  
       FIG. 19  also illustrates a fixation device  212  extending through the medial malleolus  326 , across a medial malleolar fracture  330 , and into the tibia  322 . Although  FIG. 19  illustrates fixation of both a lateral malleolar fracture  328  and medial malleolar fracture  130 , either fracture can occur without the other as is well understood in the art. Installation of the fixation devices across malleolar fractures is accomplished utilizing the same basic steps discussed above in connection with the fixation of femoral neck fractures.  
       FIGS. 20-23  illustrate a modified embodiment of a proximal anchor  400 , which can be used with the bone fixation devices described above.  
      With initial reference to  FIGS. 20 and 21 , a proximal end of a fixation device  404  is illustrated. Although the distal anchor of the fixation device  404  is not illustrated, any of the bone anchors previously described or incorporated by reference herein may be used with the illustrated embodiment. Moreover, although the body  405  of the illustrated fixation device  404  is solid, the fixation device can be cannulated as mentioned above.  
      As described above, the proximal end of the body  405  is provided with a plurality of retention structures  406 . The retention structures  406  are spaced apart axially along the fixation device between a proximal limit and a distal limit (not shown). As discussed above, the axial distance between proximal limit and distal limit is related to the desired axial travel of the proximal anchor, and thus the range of functional sizes of the bone fixation. In the illustrated embodiment, the retention structures  406  comprise a plurality of annular ridges or grooves, adapted to cooperate with complementary retention structures  408  on the proximal anchor  400 , which will be described in detail below.  
      The proximal anchor  400  comprises a housing  412  such as a tubular body, for coaxial movement along the body  405 . The proximal anchor  400  also includes a flange  414  that sets against the outer surface of the bone or tissue adjacent the bone as described above. As best seen in  FIG. 21 , the flange  414  defines a bone contacting surface  415 , which preferably forms an obtuse angle with respect to the exterior of the housing  412 .  
      Referring to  FIG. 23 , in the illustrated embodiment, the complementary retention structures  408  comprise at least one inwardly projecting tooth or flange, for cooperating with the complementary retention structures  406  of the fixation device  404 . In the illustrated embodiment, the proximal anchor  400  includes a plurality of teeth or flanges  408 , which are positioned near the proximal end of the anchor  400 .  FIG. 23A  illustrates yet another modified embodiment of the proximal anchor of  FIG. 23 . In this embodiment, a plurality of teeth or flanges  408 ′ are positioned near the distal end of the anchor  400 ′. In the embodiments of  FIGS. 23 and 23 A, it should be appreciated that the retention structures may be configured such that the proximal anchor may be proximally and/or distally advanced with rotation by providing for a screw like configuration between the retention structures. Additionally, the projecting teeth or flanges may be located at any suitable location for engaging the retention structures  406 . Thus, a set or plurality of retention members can be located at any desirable position along the proximal anchor.  
      As mentioned above, the complementary retention structures  406  of the fixation device preferably comprise a plurality of annular ridges or grooves  406 . As shown in  FIG. 23 , the plurality of annular ridges or grooves  406  preferably defines at least a first surface  407  and a second surface  409 . The first surface  407  generally faces the proximal direction and is preferably inclined with respect to the longitudinal axis of the body  405 . In contrast, the second surface  409  generally faces the distal direction and lies generally perpendicular to the longitudinal axis of the body  405 .  
      As shown, in  FIGS. 20 and 21 , the proximal anchor  400  preferably includes one or more of axial slots  416 . The axial slots  416  cooperate to form lever arms  418  (see  FIG. 23 ) on which the teeth or projections  408  are positioned. Thus, as the anchor  400  is pushed towards the distal end of the body  305 , the teeth  408  can slide along the first surface  407  and be lifted over the retention structures  406  of the body  405  as the lever arms  418  are flexed away from the body  405 .  
      After appropriate tensioning of the proximal anchor  400 , the bone pushes on the angled portion bone contacting surface  415  of the proximal anchor  400 . This force is transmitted to the teeth  408  through the lever arms  418 . As such, the teeth  408  are prevented from flexing away from the body  405 , which keeps the teeth  408  engaged with the retention structures  406  of the body  405 . By increasing the tensioning force, the teeth  408  are forced further into the retention structures  406  of the body  406 , thereby increasing the retention force of the proximal anchor  400 . In this manner, the teeth  408  cannot be lifted over the second surface  409  and proximal movement of the proximal anchor  400  is prevented.  
      The axial length and width of the slots  416  may be varied, depending upon the desired flexing of the lever arms  418  when the proximal anchor  400  is moved distally over the body  405  and the desired retention force of the distal anchor when appropriately tensioned. For a relatively rigid material such as titanium, axial lengths and widths of the slots  416  are approximately 0.5 mm for a proximal anchor having a length of approximately 4 mm, an inner diameter of approximately 3 mm. As such, in the illustrated embodiment, the slots  416  extend through the flange  414  and at least partially into the tubular housing  412 .  
      Another embodiment of a proximal anchor  420  is illustrated in  FIGS. 24-27B . The proximal anchor  420  includes a flange  424  and a tubular housing  426 . In this embodiment, the complementary structure of the proximal anchor  420  comprises an annular ring  430 , which is positioned within an annular recess  432  that is preferably positioned at the distal end of the tubular housing. See  FIGS. 27A and 27B . The annular recess  432  includes a proximal portion  434  and a distal portion  436 .  
      With specific reference to  FIG. 27A , the proximal portion  434  is sized and dimensioned such that as the proximal anchor  420  is advanced distally over the body  405  the annular ring  430  can slide along the first surface  407  and over the complementary retention structures  406  of the body  405 . That is, the proximal portion  434  provides a space for the annular ring to move radially away from the body  405  as the proximal anchor is advanced distally. Preferably, the annular ring  430  is made from a material that provides sufficient strength and elasticity such as, for example, stainless steel or titanium. The annular ring  430  is preferably split such that it can be positioned over the body  405 . Although the ring  430  is illustrated as having a circular cross section, it may alternatively have a non circular cross section such as rectangular or square.  
      With reference to  FIG. 27B , the distal portion  436  is sized and dimensioned such that after the proximal anchor  420  is appropriately tensioned the annular ring  430  becomes wedged between the second surface  409  and an angled engagement surface of the distal portion  436 . In this manner, proximal movement of the proximal anchor  420  is prevented.  
       FIGS. 27C and 28  illustrates a portion of modified embodiment of a proximal anchor that is similar to the embodiment described above with respect to FIGS.  27 A-B. In this embodiment, proximal anchor includes an annular ring  434 ′ that is split (i.e., has a least one gap) and can be interposed between the body  405  and the proximal recess  439 ′ of the proximal anchor. The ring  434 ′ comprises a tubular housing  435 ′ that may be configured to engage with the body  405  and defines a gap or space  431 ′. In one embodiment, the gap  431 ′ is defined by a pair of edges  433   a′,    433   b′.  The edges  433   a′,    433   b′  can be generally straight and parallel to each other. However, the edges  433   a′,    433   b′  can have any other suitable configuration and orientation. For example, in one embodiment, the edges  433   a′,    433   b′  are curved and at an angle to each other. Although not illustrated, it should be appreciated that in modified embodiments, the ring  434 ′ can be formed without a gap. When the ring  434 ′ is positioned along the body  405 , the ring  434 ′ preferably surrounds a substantial portion of the body  405 . The ring  434 ′ can be sized so that the ring  434 ′ can flex or move radially outwardly in response to an axial force so that the ring  434 ′ can be moved relative to the body  405 , as described above. In one embodiment, the tubular housing  435 ′ includes at least one and in the illustrated embodiment four teeth or flanges  437 ′, which are configured to engage the retention structures  406  on the body  405 . In the illustrated embodiment, the teeth or flanges include a first surface that generally faces the proximal direction and is inclined with respect to the longitudinal axis of the anchor and a second surface that faces distal direction and lies generally perpendicular to the longitudinal axis of the anchor. It is contemplated that the teeth or flanges  437 ′ can have any suitable configuration for engaging with the retention structures of the body  405 .  
      As with the previous embodiment, the proximal anchor includes the annular recess  439 ′ in which the annular ring  434 ′ may be positioned. The body  435 ′ of the ring  434 ′ can be sized to prevent substantial axial movement between the ring  434 ′ and the annular recess  439 ′ ( FIG. 28 ) during use of the proximal anchor. In one embodiment, the width of the annular recess  439 ′ in the axial direction is slightly greater than the width of the annular ring  434 ′ in the axial direction. This tolerance between the annular recess  439 ′ and the annular ring  434 ′ can inhibit, or prevent, oblique twisting of the annular ring  434 ′ so that the body  435 ′ of the ring  434 ′ is generally parallel to the outer surface of the body  405 . Further, the recess  439 ′ is sized and dimensioned such that as the proximal anchor is advanced distally over the body, the annular ring  434 ′ can slide along the first surface and over the complementary retention structures of the body. That is, the recess  439 ′ provides a space for the annular ring to move radially away from the body  405  as the proximal anchor is advanced distally. Of course, the annular ring  434 ′ can be sized and dimensioned such that the ring  434 ′ is biased inwardly to engage the retention structures  406  on the body  405 . The bias of the annular ring  434 ′ can result in effective engagement between the flanges  437 ′ and the retention structures  406 ′.  
      A distal portion  436 ′ of the recess  439 ′ is sized and dimensioned such that after the proximal anchor  420  is appropriately tensioned the annular ring  434 ′ becomes wedged between the body and an angled engagement surface of the distal portion  436 ′. In this manner, proximal movement of the proximal anchor is prevented.  
      FIGS.  29 A-D illustrate a modified embodiment of the body  228 ′ and proximal anchor  700 . In this embodiment, the body  228 ′ comprises a first portion  236 ′ and a second portion  238 ′ that are coupled together at a junction  240 ′ ( FIG. 29D ). In the illustrated embodiment, the first portion  236 ′ carries the distal anchor  234 ′ while the second portion  238 ′ forms the proximal end  230 ′ of the body  228 ′. The first and second portions  236 ′,  238 ′ are preferably detachably coupled to each other at the junction  240 ′. In the illustrated embodiment, the first and second portions  236 ′,  238 ′ are detachably coupled to each other via interlocking threads. Specifically, as best seen in  FIG. 29D , the body  228 ′ includes an inner surface  241 ′, which defines a central lumen  242 ′ that preferably extends from the proximal end  230 ′ to the distal end  232 ′ throughout the body  228 ′. At the proximal end of the first portion  236 ′, the inner surface  241 ′ includes a first threaded portion  244 ′. The first threaded portion  244 ′ is configured to mate with a second threaded portion  246 ′, which is located on the outer surface  245 ′ of the second portion  238 ′. The interlocking annular threads of the first and second threaded portions  244 ′,  246 ′ allow the first and second portions  236 ′,  238 ′ to be detachably coupled to each other. In one modified embodiment, the orientation of the first and second threaded portions  244 ′,  246 ′ can be reversed. That is, the first threaded portion  244 ′ can be located on the outer surface of the first portion  236 ′ and the second threaded portion  246 ′ can be located on the inner surface  241 ′ at the distal end of the second portion  238 ′. Any of a variety of other releasable complementary engagement structures may also be used, to allow removal of second portion  238 ′ following implantation, as is discussed below.  
      In a modified arrangement, the second portion  238 ′ can comprise any of a variety of tensioning elements for permitting proximal tension to be placed on the distal anchor  234 ′ while the proximal anchor is advanced distally to compress the fracture. For example, any of a variety of tubes or wires can be removably attached to the first portion  236 ′ and extend proximally to the proximal handpiece. In one such arrangement, the first portion  236 ′ can include a releasable connector in the form of a latching element, such as an eye or hook. The second portion  238 ′ can include a complementary releasable connector (e.g., a complementary hook or eye) for engaging the first portion  236 ′. In this manner, the second portion  238 ′ can be detachably coupled to the first portion  236 ′ such that proximal traction can be applied to the first portion  236 ′ through the second portion as will be explained below. Alternatively, the second portion  238 ′ may be provided with an eye or hook, or transverse bar, around which or through which a suture or wire may be advanced, both ends of which are retained at the proximal end of the device. Following proximal tension on the tensioning element during the compression step, one end of the suture or wire is released, and the other end may be pulled free of the device. Alternate releasable proximal tensioning structures may be devised by those of skill in the art in view of the disclosure herein.  
      With particular reference to  FIGS. 29A-29D , the proximal end  230 ′ of the body  228 ′ may be provided with a rotational coupling  270 ′, for allowing the second portion  238 ′ of the body  228 ′ to be rotationally coupled to a rotation device. The proximal end  230 ′ of the body  228 ′ may be desirably rotated to accomplish one or two discrete functions. In one application of the invention, the proximal end  230 ′ is rotated to remove the second portion  238 ′ of the body  228 ′ following tensioning of the device across a fracture or to anchor an attachment to the bone. Rotation of the rotational coupling  270 ′ may also be utilized to rotationally drive the distal anchor into the bone. Any of a variety of rotation devices may be utilized, such as electric drills or hand tools, which allow the clinician to manually rotate the proximal end  230 ′ of the body. Thus, the rotational coupling  270 ′ may have any of a variety of cross sectional configurations, such as one or more flats or splines.  
      With particular reference to  FIG. 29A , the fixation device may include an antirotation lock between the first portion  236 ′ of the body  228 ′ and the proximal anchor  700 . In the illustrated embodiment, the first portion  236 ′ includes a pair of flat sides  280 ′, which interact with corresponding flat structures  282 ′ in the proximal anchor  700 . One or three or more axially extending flats may also be used. As such, rotation of the proximal anchor  700  is transmitted to the first portion  236 ′ and the distal anchor  234 ′ of the body  228 ′. Of course, those of skill in the art will recognize various other types of splines or other interfit structures can be used to prevent relative rotation of the proximal anchor and the first portion  236 ′ of the body  228 ′. For example, in one embodiment, the first portion  236 ′ may include three flat sides, which interact with corresponding flat structures on the proximal anchor.  
      To rotate the proximal anchor  700 , the flange  708  is preferably provided with a gripping structure to permit an insertion tool to rotate the flange  708 . Any of a variety of gripping structures may be provided, such as one or more slots, flats, bores or the like. In one embodiment, the flange  708  is provided with a polygonal, and, in particular, a pentagonal or hexagonal recess  284 ′. See  FIG. 30A .  
      In  FIGS. 29B and 29C , the proximal anchor  700  is shown in combination with a washer  250 ′ arranged as described above with reference to FIGS.  17 A-C.  
      FIGS.  30 A-F illustrate in more detail the a proximal anchor  700  of FIGS.  29 A-C. This embodiment includes a tubular housing  702  similar to the tubular housing  602  described above with reference to FIGS.  12 A-F. In the illustrated embodiment, the tubular housing  702  is attached to, coupled to, or integrally formed (partially or wholly) with a secondary tubular housing  704 , which includes one or more anti-rotational features  706  (e.g., flat sides) for engaging corresponding anti-rotational features formed on the pin, which can be similar to the first portion  236 ′ (e.g., see description above). The flange or collar  708  is attached, coupled or integrally formed with the proximal end of the secondary tubular housing. The teeth or flanges  610  on the bridges  606  may also be configured such that the proximal anchor may be distally advanced and/or removed with rotation. The illustrated embodiment also advantageously includes visual indicia  712  (e.g., marks, grooves, ridges etc.) on the tubular housing  704  for indicating the depth of the proximal anchor  700  within the bone.  
      In one embodiment of use, a fixation device  212 ′ having an axial length and outside diameter suitable for the hole  280  is selected. The distal end  232 ′ of the fixation device  212 ′ is advanced distally into the hole  280  until the distal anchor  234 ′ reaches the distal end of the hole  280 . The proximal anchor  700  may be carried by the fixation device  212 ′ prior to advancing the body  228 ′ into the hole  280 , or may be attached following placement of the body  228 ′ within the hole  280 . Once the body  228 ′ and proximal anchor  700  are in place, the clinician may use any of a variety of driving devices, such as electric drills or hand tools to rotate the proximal anchor  700  and thus cancellous bone anchor  234 ′ into the head of the femur. In modified embodiments, the fixation device is configured to be self-drilling or self tapping such that a hole does not have be formed before insertion into the bone.  
      Once the anchor  234 ′ is in the desired location, proximal traction is applied to the proximal end  230 ′ of body  228 ′, such as by conventional hemostats, pliers or a calibrated loading device, while distal force is applied to the proximal anchor  700 . In this manner, the proximal anchor  700  is advanced distally until the anchor  700  fits snugly against the outer surface of the femur or tissue adjacent the femur and the fracture  24  is completely reduced. Appropriate tensioning of the fixation device  212 ′ is accomplished by tactile feedback or through the use of a calibration device for applying a predetermined load on the implantation device. One advantage of the structure of the present invention is the ability to adjust compression independently of the setting of the distal anchor  234 ′.  
      Following appropriate tensioning of the proximal anchor  700 , the second portion  238 ′ of the body  228 ′ is preferably detached from the first portion  236 ′ and removed. In the illustrated embodiment, this involves rotating the second portion  238 ′ with respect to the first portion via the coupling  270 ′. Following removal of the second portion  238 ′ of each body  228 ′, the access site may be closed and dressed in accordance with conventional wound closure techniques.  
      An advantage certain embodiments of the fixation devices disclosed above is that the proximal anchor provides the device with a working range such that one device may accommodate varying distances between the distal anchor and the proximal anchor. In certain applications, this allows the technician to focus on the proper positioning of the distal anchor with the knowledge that the proximal anchor lies within the working range of the device. With the distal anchor positioned at the desired location, the proximal anchor may then be advanced along the body to compress the fracture and/or provide stability between bones. In a similar manner, the working range provides the technician with flexibility to adjust the depth of the proximal anchor. For example, in some circumstances, the bone may include voids, cysts osteoporotic bone that impairs the stability of the distal anchor in the bone. Accordingly, in some circumstances, the technician may advance the distal anchor and then desire to retract the distal anchor such that it is better positioned in the bone. In another circumstance, the technician may inadvertently advance the distal tip through the bone into a joint space. In such circumstances, the working range of the device allows the technician to reverse and retract the anchor and recompress connection. Such adjustments are facilitated by the working range of the proximal anchor on the body.  
      Preferably, the clinician will have access to an array of fixation devices (e.g., fixation devices  212 ,  212 ′) having, for example, different diameters, axial lengths and angular relationships. These may be packaged one per package in sterile envelopes or peelable pouches, or in dispensing cartridges which may each hold a plurality of devices  212 . Upon encountering a fracture for which the use of a fixation device is deemed appropriate, the clinician will assess the dimensions and load requirements, and select a fixation device from the array which meets the desired specifications.  
      The fixation devices described above may be used in any of a wide variety of anatomical settings beside the proximal femur, as has been discussed. For example, lateral and medial malleolar fractures can be readily fixed using the device of the present invention. Referring to  FIG. 19 , there is illustrated an anterior view of the distal fibula  320  and tibia  322 . The fibula  320  terminates distally in the lateral malleolus  324 , and the tibia  322  terminates distally in the medial malleolus  326 . A fixation device  212  is illustrated as extending through the lateral malleolus  324  across the lateral malleolar fracture  328  and into the fibula  320 . Fixation device  212  includes a distal anchor  34  for fixation within the fibula  320 , an elongate body  228  and a proximal anchor as has been discussed.  
      As mentioned above, the devices describe herein may also be used for spinal fixation. In embodiments optimized for spinal fixation in an adult human population, the body  228  will generally be within the range of from about 20-90 mm in length and within the range of from about 3.0-8.5 mm in maximum diameter. The length of the helical anchor, discussed above, may be about 8-80 millimeters. Of course, it is understood that these dimensions are illustrative and that they may be varied as required for a particular patient or procedure.  
      In spinal fixation applications, the fixation device  212  may be used as a trans-facet screw. That is, the fixation device extends through a facet of a first vertebra and into the facet of a second, typically inferior, vertebrae. This procedure is typically (but not necessarily) preformed with bilateral symmetry. Thus, even in the absence of a stabilizing bar tying pedicle screws to adjacent vertebrae or to the sacrum, and in the absence of translaminar screws that can extend through the spinous process, the fixation devices can be used to stabilize two vertebrae, such as L 3  and L 4  to each other pending the healing of a fusion. In one embodiment, the body  228  of fixation device  228  has a length of approximately 10 mm-30 mm and the diameter of the body is approximately 3 mm to 5.5 mm.  
      The fixation device  212  may also be used as a trans-laminar facet screw. In this embodiment of use, the fixation device extends through the spinous process and facet of a first vertebra and into the facet of a second, typically inferior, vertebra. As with the previous embodiment, this procedure is typically (but not necessarily) preformed with bilateral symmetry. In one embodiment, the body  228  of fixation device  212  has a length of approximately 50 mm-90 mm and the diameter of the body is approximately 4 mm to 5.5 mm.  
      The fixation device may also be used is used as a facet-pedical screw (e.g., as used in the Boucher technique). In such an embodiment, the fixation device extends through the facet of a first vertebra and into the pedicle a second, typically inferior, vertebra. As with the previous embodiment, this procedure is typically (but not necessarily) preformed with bilateral symmetry. In such an embodiment, the fixation device  212  and the body  228  is approximately 20-40 millimeters in length and 3.0-5.5 millimeters in diameter.  
      FIGS.  31 A-D illustrate another embodiment of a proximal anchor  800 . In this embodiment, the proximal anchor  800  includes a recess  839  configured to receive a split ring  434 ′ as described above with reference to  FIGS. 27C and 28 . As will be explained in detail below, the proximal anchor  800  includes an anti-rotation feature to limit or prevent rotation of the ring  434 ′ within the proximal anchor  800 . In light of the disclosure herein, those of skill in the art will recognize various different configurations for limiting the rotation of the ring  434 ′. However, a particularly advantageous arrangement will be described below with reference to the illustrated embodiment.  
      In the illustrated embodiment, the proximal anchor  800  has a tubular housing  804  that can engage with a body  228  or a first portion  236 ′ of a body  228 ′ as described above. With reference to  FIGS. 31B and 31D , the tubular housing  804  comprises one or more anti-rotational features  806  in the form of a plurality of flat sides that are configured to mate corresponding anti-rotational features  280 ′ or flat sides of the body  228 ′ of the fixation device. As shown in  FIG. 31D , in the illustrated embodiment, the body  228 ′ has three flat sides  280 ′. Disposed between the flat sides  280  are the portions of the body  228 ′ which include the complementary locking structures such as threads or ratchet like structures as described above. The complementary locking structures interact with the ring  434 ′ as described above to resist proximal movement of the anchor  800  under normal use conditions while permitting distal movement of the anchor  800  over the body  228 .  
      As mentioned above, the ring  434 ′ is positioned within the recess  839 . In the illustrated embodiment, the recess  839  and ring  434 ′ are positioned near to and proximal of the anti-rotational features  806 . However, the ring  434 ′ can be located at any suitable position along the tubular housing  804  such that the ring  434 ′ can interact with the retention features of the body  
      During operation, the ring  434 ′ may rotate to a position such that the gap  431 ′ between the ends  433   a′,    433   b′  of the ring  434 ′ lies above the complementary retention structures on the body  228 ′. When the ring  434 ′ is in this position, there is a reduced contact area between the split ring  434 ′ the complementary retention structures thereby reducing the locking strength between the proximal anchor  800  and the body  228 ′. In the illustrated embodiment, for example, the locking strength may be reduced by about ⅓ when the gap  431 ′ over the complementary retention structures between flat sides  280 ′. As such, it is advantageous to position the gap  431 ′ on the flat sides  280 ′ of the body  228 ′ that do not include complementary retention structures.  
      To achieve this goal, the illustrated embodiment includes a pair of tabs  812 ,  814  that extend radially inward from the interior of the proximal anchor  800 . The tabs  812 ,  814  are configured to limit or prevent rotational movement of the ring  434 ′ relative to the housing  804  of the anchor  800 . In this manner, the gap  431 ′ of the ring  434 ′ may be positioned over the flattened sides  280 ′ of the body  228 ′.  
      In the illustrated embodiment, the tabs  812 ,  814  have a generally rectangular shape and have a generally uniform thickness. However, it is contemplated that the tabs  812 ,  814  can be square, curved, or any other suitable shape for engaging with the ring  434 ′ as described herein.  
      In the illustrated embodiment, the tabs  812 ,  814  are formed by making an H-shaped cut  870  in the tubular housing  800  and bending the tabs  812 ,  814  inwardly as shown in  FIG. 31D . As shown in  FIG. 31D , the tabs  812 ,  814  (illustrated in phantom) are interposed between the edges  433   a′,    433   b′  of the ring  434 ′. The edges  433   a′,    433   b′  of the ring  434 ′ can contact the tabs to limit the rotational movement of the ring  434 ′. Those skilled in the art will recognize that there are many suitable manners for forming the tabs  812 ,  814 . In addition, in other embodiments, the tabs  812 ,  814  may be replaced by a one or more elements or protrusions attached to or formed on the interior of the proximal anchor  800 .  
      For the embodiments discussed herein, the pin, together with the distal anchor 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 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. The distal anchor can be separately formed from the pin body 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 is integrally molded with the pin body, if the desired material has appropriate physical properties.  
      Retention structures can also be integrally molded with the pin body. Alternatively, retention structures can be machined or pressed into the pin body 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-actide)  
      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.  
      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. Moreover, although the present invention has been described in terms of certain preferred embodiments, other embodiments of the invention including variations in the number of parts, 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 to form various combinations and sub-combinations. 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.