Patent Publication Number: US-2013245703-A1

Title: Method and apparatus for sacroiliac joint fixation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims a priority benefit to U.S. Provisional Application No. 61/606,199, filed Mar. 2, 2012, the entire disclosure of which is hereby incorporated by reference in its entirety and should be considered a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to medical devices and, more particularly, to methods and apparatus for sacroiliac joint stabilization. 
     2. Description of the Related Art 
     The sacroiliac joint is the joint between the sacrum and the ilium of the pelvis. Strong ligaments connect the sacrum to the ilium. The sacrum supports the spine and is supported on each side by an ilium. The sacroiliac joint is a synovial joint with cartilage and irregular elevations and depressions that produce interlocking of the two bones. 
     Pain in the sacroiliac joint can be caused by a number of conditions, including fracture or dislocation of the pelvis, degenerative arthritis, sacroiliitis (inflammation of the sacroiliac joint), or osteitis condensans ilii. One method for treatment of sacroiliac joint dysfunction is fusion of the sacroiliac joint. Fusion can be accomplished in a number of ways, for example an anterior approach, a posterior approach, or percutaneous screw fixation. The anterior approach can involve an incision along the iliac crest to the anterior superior iliac spine, followed by stripping the iliacus muscle to gain access to the sacroiliac joint. This approach poses a danger of damaging the L5 nerve root which is positioned near the sacroiliac joint. The posterior approach can use any of a number of different incisions, followed by stripping the gluteus maximus off the ilium to gain access to the joint. Both the anterior and posterior approaches pose risk of infection, and require relatively large incisions, resulting in unsightly scarring. 
     Percutaneous sacroiliac joint fusion can reduce the size of necessary incisions and lower the risk of infection through the minimally invasive introduction of joint fixation elements. Such methods typically include various fixation systems that are used for the stabilization of the sacroiliac joint. These fixation systems may include a variety of longitudinal elements such as screws which span the sacroiliac joint and are affixed to the sacrum through the ilium. These systems may be affixed to one side of the patient or to both sides. 
     Notwithstanding the variety of efforts in the prior art, there remains a need for a fixation device for sacroiliac joint stabilization with improved locking force, which resists migration and rotation, and which can be easily and rapidly deployed. 
     SUMMARY OF THE INVENTION 
     There is provided in accordance with one aspect of the present invention, a sacroiliac joint fixation device. The device includes an elongate body having a proximal end and a distal end, a distal anchor on the distal end, and a retention structure on the body, proximal to the anchor. A proximal anchor is movably carried by the body. At least one complementary retention structure is included on the proximal anchor, and is configured to permit proximal movement of the body with respect to the proximal anchor, but to resist distal movement of the body with respect to the proximal anchor. A flange is configured to receive the proximal anchor, the proximal anchor and flange having complementary surface structures to permit angular adjustment with respect to the longitudinal axis of the proximal anchor and the body, and the longitudinal axis of the flange. 
     In according with another aspect of the present invention, a method of providing sacroiliac joint fixation is disclosed. A fixation device that comprises a body having a distal anchor and a proximal anchor can be provided. The distal anchor can be advanced through an ilium of a pelvis and into a sacrum. The fixation device can then be rotated to engage the distal anchor with the sacrum. Next, the fixation device can be axially shortened by reducing the distance between the distal anchor and the proximal anchor, such that a locking element on the proximal anchor engages at least one retention structure on the body thereby applying compression between the sacrum and the ilium. 
     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 drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front oblique view of a portion of a sacroiliac joint having a exemplary embodiment of a fixation device implanted therein. 
         FIG. 2  is a side perspective view of an exemplary fixation device similar to that of  FIG. 1 . 
         FIG. 3  is a side elevational view of the fixation device of  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken through line  4 - 4  of  FIG. 3 . 
         FIG. 4A  is an enlarged view of portion  4 A of  FIG. 4 . 
         FIG. 4B  is an enlarged view of portion  4 B of  FIG. 4  with the fixation device in a first position. 
         FIG. 4C  is an enlarged view of portion  4 C of  FIG. 4  with the fixation device in a second position. 
         FIG. 5  is a cross-sectional view taken through line  5 - 5  of  FIG. 3 . 
         FIG. 6A  is a side perspective view of another embodiment of a proximal anchor for the bone fixation device of  FIG. 2 . 
         FIG. 6B  is a cross-sectional view of the proximal anchor of  FIG. 6A . 
         FIG. 6C  is a side perspective view of another embodiment of a proximal anchor for the bone fixation device of  FIG. 2 . 
         FIG. 6D  is a cross-sectional view of the proximal anchor of  FIG. 6C . 
         FIG. 6E  is a cross-sectional view of another embodiment of a proximal anchor for the bone fixation device of  FIG. 2 . 
         FIG. 6F  is a cross-sectional view of the proximal anchor of  FIG. 6E . 
         FIG. 7  is a cross sectional view through an angularly adjustable proximal anchor plate. 
         FIG. 8  is a front perspective view of the proximal anchor plate of  FIG. 7 . 
         FIG. 9  is a bottom perspective view of a modified embodiment of a bone fixation device. 
         FIG. 10  is an unassembled side perspective view of the bone fixation device of  FIG. 9 . 
         FIG. 11  is an unassembled side view of the bone fixation device of  FIG. 9 . 
         FIG. 12  is a cross-sectional view of the flange and proximal anchor of the bone fixation device of  FIG. 11 . 
         FIG. 13  is an unassembled bottom perspective view of the bone fixation device of  FIG. 9 . 
         FIG. 14  is an unassembled side perspective view of another modified embodiment of a bone fixation device. 
         FIG. 15  is an unassembled side view of the bone fixation device of  FIG. 9 . 
         FIGS. 16A-H  are various views of the pelvis and sacrum with the fixation device of  FIG. 2  implanted therein. 
     
    
    
     DETAILED DESCRIPTION 
     Although the fixation devices of the present invention will be disclosed primarily in the context of a sacroilial fixation procedure, the methods and structures disclosed herein are intended for application in any of a variety medical applications, as will be apparent to those of skill in the art in view of the disclosure herein. For example, the bone fixation device may be applicable to proximal fractures of the femur, for spinal fixation, and a wide variety of fractures and osteotomies, 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. See e.g., U.S. Pat. No. 6,511,481, which is hereby incorporated by reference herein. A wide variety of phalangeal and metatarsal osteotomies and fractures of the foot may also be stabilized using the bone fixation devices described herein. These include, among others, distal metaphyseal osteotomies such as those described by Austin and Reverdin-Laird, base wedge osteotomies, oblique diaphyseal, digital arthrodesis as well as a wide variety of others that will be known to those of skill in the art. Fractures of the fibular and tibial malleoli, pilon fractures and other fractures of the bones of the leg may be fixated and stabilized with these bone fixation devices with or without the use of plates, both absorbable or non-absorbing types, and with alternate embodiments of the current invention The fixation devices 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 washers, with or without tissue spikes for soft tissue attachment, and other implants may also be attached to bone, using either resorbable or nonresorbable fixation devices depending upon the implant and procedure. The fixation devices may also be used to attach sutures to the bone, such as in any of a variety of tissue suspension procedures. The bone fixation device described herein may be used with or without plate(s) or washer(s), all of which can be either permanent, absorbable, or combinations. 
     Referring to  FIG. 1 , there is illustrated an exemplary embodiment of implanted bone fixation devices  12 . The left ilium and a portion of the sacrum are shown as transparent in order to identify the locations of the fixation devices  12 . In  FIG. 1 , three fixation devices  12  are positioned across the sacroiliac joint. As will be explained in more detail below, the bone fixation device  12  may be used in a variety of techniques to stabilize the sacroiliac joint. 
     Referring to  FIGS. 2-4 , the exemplary fixation device  12  will now be described in detail. The fixation device  12  comprises a body  28  that extends between a proximal end  30  and a distal end  32 . The length, diameter and construction materials of the body  28  can be varied, depending upon the intended clinical application. In embodiments optimized for sacroiliac fixation in an adult human population, the body  28  will generally be within the range of from about 30-120 mm in length and within the range of from about 3-12 mm in maximum diameter. The length of the helical anchor, discussed below, may be about 8-100 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 one embodiment, the body  28  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  12 . 
     The distal end  32  of the body  28  is provided with a cancellous bone anchor or distal cortical bone anchor  34 . Generally for sacroilial fixation, the distal bone anchor  34  is adapted to be rotationally inserted into a portion of the sacrum. In the illustrated embodiment, the distal anchor  34  comprises a helical locking structure  72  for engaging cancellous and/or distal cortical bone. In the illustrated embodiment, the locking structure  72  comprises a flange that is wrapped around an axial lumen. The flange extends through at least one and generally from about 2 to about 50 or more full revolutions depending upon the axial length of the distal anchor and intended application. The flange will can complete from about 2 to about 20 revolutions. The helical flange  72  is provided with a pitch and an axial spacing to optimize the retention force within cancellous bone, to optimize compression. 
     The helical flange  72  of the illustrated embodiment has a generally triangular cross-sectional shape (see  FIG. 4 ). However, it should be appreciated that the helical flange  72  can have any of a variety of cross sectional shapes, such as rectangular, oval 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  72  defines an outer boundary. The ratio of the diameter of the outer boundary to the diameter of the central lumen 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  34 . Another aspect of the distal anchor  34  that can be optimized is the shape of the outer boundary and the central core, which in the illustrated embodiment are generally cylindrical. 
     The distal end  32  and/or the outer edges of the helical flange  72  may be atraumatic (e.g., blunt or soft). This inhibits the tendency of the fixation device  12  to migrate anatomically distally after implantation. Distal migration is also inhibited by the dimensions and presence of a proximal anchor  50 , which will be described below. 
     A variety of other arrangements for the distal anchor  32  can also be used. For example, the various distal anchors described in co-pending U.S. patent application Ser. No. 10/012,687, filed Nov. 13, 2001 can be incorporated into the fixation device  12  described herein. The entire contents of this application is hereby expressly incorporated by reference. In particular, the distal anchor may comprise a single helical thread surrounding a central core, much as in a conventional screw, which has been cannulated to facilitate placement over a wire. Alternatively, a double helical thread may be utilized, with the distal end of the first thread rotationally offset from the distal end of the second thread. The use of a double helical thread can enable a greater axial travel for a given degree of rotation and greater retention force than a corresponding single helical thread. Specific distal anchor designs can be optimized for the intended use, taking into account desired performance characteristics, the integrity of the distal bone, and whether the distal anchor is intended to engage exclusively cancellous bone or will also engage cortical bone. 
     With particular reference to  FIGS. 3 ,  4 , and  4 A, the body  28  comprises a first portion  36  and a second portion  38  that are coupled together at a junction  40 . In the illustrated embodiment, the first portion  36  carries the distal anchor  34  while the second portion  38  forms the proximal end  30  of the body  28 . As will be explained in more detail below, in certain embodiments, the second portion  38  may be used to pull the body  28  and therefore will sometimes be referred to as a “pull-pin”. The first and second portions  36 ,  38  are preferably detachably coupled to each other at the junction  40 . In the illustrated embodiment, the first and second portions  36 ,  38  are detachably coupled to each other via interlocking threads. Specifically, as best seen in  FIG. 4A , the body  28  includes an inner surface  41 , which defines a central lumen  42  that preferably extends from the proximal end  30  to the distal end  32  throughout the body  28 . At the proximal end of the first portion  36 , the inner surface  41  includes a first threaded portion  44 . The first threaded portion  44  is configured to mate with a second threaded portion  46 , which is located on the outer surface  45  of the second portion  38 . The interlocking annular threads of the first and second threaded portions  44 ,  46  allow the first and second portions  36 ,  38  to be detachably coupled to each other. In one modified embodiment, the orientation of the first and second threaded portions  44 ,  46  can be reversed. That is, the first threaded portion  44  can be located on the outer surface of the first portion  36  and the second threaded portion  46  can be located on the inner surface  41  at the distal end of the second portion  38 . Any of a variety of other releasable complementary engagement structures may also be used, to allow removal of second portion  38  following implantation, as is discussed below. 
     In a modified arrangement, the second portion  38  can comprise any of a variety of tensioning elements for permitting proximal tension to be placed on the distal anchor  34  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  36  and extend proximally to the proximal handpiece. In one such arrangement, the first portion  36  can include a releasable connector in the form of a latching element, such as an eye or hook. The second portion  38  can include a complementary releasable connector (e.g., a complementary hook) for engaging the first portion  36 . In this manner, the second portion  38  can be detachably coupled to the first portion  36  such proximal traction can be applied to the first portion  36  through the second portion as will be explained below. Alternatively, the second portion  48  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. It should also be appreciated that the body may be from a single piece as described in U.S. Pat. No. 6,511,481, which has been incorporated by reference herein. 
     As shown in  FIG. 4 , the body  28  is cannulated to accommodate installation over a placement wire as is understood in the art. The cross section of the illustrated central cannulation is circular but in other embodiments may be non circular, e.g., hexagonal, to accommodate a corresponding male tool for installation or removal of the second portion  38  of the body  28  as explained above. In other embodiments, the body  28  may partially or wholly solid. 
     With continued reference to  FIGS. 2-4 , the proximal end  30  of the body  28  may be provided with a rotational coupling  70 , for allowing the second portion  38  of the body  28  to be rotationally coupled to a rotation device. The proximal end  30  of the body  28  may be desirably rotated to accomplish one or two discrete functions. In one application, the proximal end  30  is rotated to remove the second portion  38  of the body  28  following tensioning of the device to anchor an attachment to the bone. Rotation of the rotational coupling  70  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  30  of the body. Thus, the rotational coupling  70  may have any of a variety of cross sectional configurations, such as one or more flats or splines. 
     In one embodiment, the rotational coupling  70  comprises a proximal projection of the body  28  having an axial recess with a polygonal cross section, such as a hexagonal cross section. The rotational coupling  70  is illustrated as a female component, machined or milled or attached to the proximal end  30  of the body  28 . However, the rotational coupling may also be in the form of a male element, such as a hexagonal or other noncircular cross sectioned projection. 
     The proximal end  30  of the fixation device is provided with a proximal anchor  50 . Proximal anchor  50  is axially distally moveable along the body  28 , to permit compression of between the distal and proximal ends  32 ,  30  of the fixation device  12 . As will be explained below, complementary locking structures such as threads or ratchet like structures between the proximal anchor  50  and the body  28  resist proximal movement of the anchor  50  with respect to the body  28  under normal use conditions. The proximal anchor  50  preferably can be axially advanced along the body  28  with and/or without rotation as will be apparent from the disclosure herein. 
     Referring to  FIG. 4 , the proximal anchor  50  comprises a housing  52  such as a tubular body, for coaxial movement along the body  28 . As will be explained in more detail below, in certain embodiments, the housing  50  may have diameter sized to fit through an opening formed in fixation bar or plate. 
     In a final position, the distal end of the housing  52  preferably extends distally past the junction  40  between the first portion  36  and the second portion  38 . The housing  52  is provided with one or more surface structures  54  such as a radially inwardly projecting flange  56  (see  FIGS. 4B and 4C ), for cooperating with complementary surface structures  58  on the first portion  36  of the body  28 . In the illustrated embodiment, the complementary surface structures  58  comprise a series of annular ridges or grooves  60 . The surface structures  54  and complementary surface structures  58  permit distal axial travel of the proximal anchor  50  with respect to the body  28 , but resist proximal travel of the proximal anchor  50  with respect to the body  28 . 
     For example, as best seen in  FIG. 4B , the proximal end of the flange  56  is biased towards the longitudinal axis of the body  28 . As such, when the proximal anchor  50  is urged proximally with respect to the body  28 , the flange  56  engages the grooves or ridges  60  of the complementary surface structures  58 . This prevents proximal movement of the proximal anchor  50  with respect to the body  28 . In contrast, as best seen in  FIG. 4C , when the proximal anchor  50  is moved distally with respect to the body  28 , the flange  56  can bend outwardly away from the body  28  and the ridges  60  so as to allow the proximal anchor  50  to move distally. Of course, those of skill in the art will recognize that there are a variety of other complementary surface structures, which permit one way ratchet like movement. For example, a plurality of annular rings or helical threads, ramped ratchet structures and the like for cooperating with an opposing ramped structure or pawl can also be used. In one embodiment, opposing screw threads are dimensioned to function as a ratchet. 
     Retention structures  58  are spaced axially apart along the body  28 , between a proximal limit  62  and a distal limit  64 . The axial distance between proximal limit  62  and distal limit  64  is related to the desired axial working range of the proximal anchor  50 , and thus the range of functional sizes of the fixation device  12 . Thus, the fixation device  12  of the exemplary embodiment can provide compression between the distal anchor  34  and the proximal anchor  50  throughout a range of motion following the placement of the distal anchor in bone. That is, the distal anchor may be positioned within the cancellous and/or distal cortical bone of the sacrum, and the proximal anchor may be distally advanced with respect to the distal anchor throughout a range to provide compression without needing to relocate the distal anchor and without needing to initially locate the distal anchor in a precise position with respect to the proximal side of the ilium. Providing a working range throughout which tensioning of the proximal anchor is independent from setting the distal anchor allows a single device to be useful for a wide variety of fixation procedures, as well as eliminates the need for accurate device measurement. In addition, this arrangement allows the clinician to adjust the compression force during the procedure without adjusting the position of the distal anchor. In this manner, the clinician may focus on positioning the distal anchor sufficiently within the sacrum to avoid or reduce the potential for distal migration, which may damage the particularly delicate tissue , blood vessels, and/or nerves. 
     In many applications, the working range is at least about 10% of the overall length of the device, and may be as much as 20% or 50% or more of the overall device length. In the context of a sacroilial application, working ranges of up to about 15 mm or more may be provided, since estimates within that range can normally be readily accomplished within the clinical setting. The embodiments disclosed herein can be scaled to have a greater or a lesser working range, as will be apparent to those of skill in the art in view of the disclosure herein. 
     With reference back to  FIGS. 2-4 , the proximal anchor  50  includes a flange  66  that, as will be explained below, may be configured to sit against the outer surface of the ilium and/or a fixation rod or plate. The flange  66  is preferably an annular flange, to optimize the footprint or contact surface area between the flange  66  and the bone or fixation rod or plate. Circular or polygonal shaped flanges for use in sacroilial fixation will generally have a diameter of at least about 3 mm greater than the adjacent body  28  and often within the range of from about 2 mm to about 30 mm or more greater than the adjacent body  28 . 
     With particular reference to  FIGS. 2 and 5 , the fixation device may include an antirotation lock between the first portion  36  of the body  28  and the proximal collar  50 . In the illustrated embodiment, the first portion  36  includes a pair of flat sides  80 , which interact with corresponding flat structures  82  in the proximal collar  50 . One or three or more axially extending flats may also be used. As such, rotation of the proximal collar  50  is transmitted to the first portion  36  and distal anchor  34  of the body  28 . 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  36  of the body  28 . 
     To rotate the proximal collar, the flange  66  is preferably provided with a gripping structure to permit an insertion tool to rotate the flange  66 . 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  44  is provided with a polygonal, and, in particular, a pentagonal or hexagonal recess  84  (see  FIG. 4 ). 
     In a modified embodiment, the housing  52  of the proximal anchor  50  can include one or more one or more barbs that extend radially outwardly from the tubular housing  52 . Such barbs provide for self tightening after the device has been implanted in the patient as described in a co-pending U.S. patent application Ser. No. 10/012,687, filed Nov. 13, 2001, which was incorporated by reference above. The barbs may be radially symmetrically distributed about the longitudinal axis of the housing  52 . Each barb is provided with a transverse engagement surface, for anchoring the proximal anchor  50  in the bone. The transverse engagement surface may lie on a plane which is transverse to the longitudinal axis of the housing  50  or may be inclined with respect to the longitudinal axis of the tubular  50 . In either arrangement, the transverse engagement surface  43  generally faces the contacting surface  68  of the flange  44 . As such, the transverse engagement surface inhibits proximal movement of the proximal anchor with respect to the bone. 
       FIGS. 6A and 6B  illustrate another embodiment of a proximal anchor  100 . This embodiment also includes a tubular housing  102  and a flange  104  that may be configured as describe above with respect to  FIGS. 2-4 . The tubular housing  102  may include an anti-rotational lock, which, in the illustrated embodiment, is in the form of one or more sides  106  that interact with corresponding flat structures formed in the body  28  as described above. 
     In this embodiment, the surfaces structures comprises one or more teeth or grooves  112 , which are configured to engage the complementary surfaces structures on the body  28  (see  FIG. 2 ). One or more slots or openings  110  are formed in the tubular housing  102  to form one or more bridges  112 , which carry the teeth  102 . The anchor proximal anchor  100  may be pushed towards the distal end of the body and the teeth  102  can slide along the and over the complementary surface structures  58  on the body  28 . In the illustrated embodiment, the bridge  113  may flex slightly away from the body  28  to allow such movement. The number and shape of the openings  110  and bridges  112  may be varied depending of the desired flexing of the bridges  112  when the proximal anchor  110  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  100  and the grooves on the body  28  may be configured such that the proximal anchor  100  can be rotated or threaded onto the pin in the distal direct and/or so that that the proximal anchor can be removed by rotation. The illustrated embodiment also advantageously includes visual indicia  114  (e.g., marks, grooves, ridges etc.) on the tubular housing  102  for indicating the depth of the proximal housing  100  within the bone. 
       FIGS. 6C and 6D  illustrate another embodiment of a proximal anchor  150 . In this embodiment, the proximal anchor  150  comprises a housing  152  such as a tubular body, for coaxial movement along the body  28 . The proximal anchor  150  also includes a flange  154  that is configured that to set against the outer surface of, for example, a bone or fixation bar or rod. In the illustrated embodiment, the flange  154  defines a contacting surface  156 , which preferably forms an obtuse angle with respect to the exterior of the housing  152 . However, in modified embodiments, the contacting surface  154  may be perpendicular or form an acute angle with respect to the housing  152 . 
     Referring to  FIG. 6D , in the illustrated embodiment, the complementary retention structures  54  comprise one or more inwardly projecting teeth or flanges  158 , for cooperating with the complementary rentention structures  58  on the body  28 . The complementary retention structures  58  of the body preferably comprise a plurality of annular ridges or grooves a first surface and a second surface. The first surface generally faces the proximal direction and is preferably inclined with respect to the longitudinal axis of the body  28 . In contrast, the second surface generally faces the distal direction and lies generally perpendicular to the longitudinal axis of the body  28 . 
     The proximal anchor  150  preferably includes one or more of axial slots  160 . The axial slots  160  cooperate to form lever arm(s) on which the teeth or projections  158  are positioned. Thus, as the anchor  150  is pushed towards the distal end of the body  28 , the teeth  158  can slide along the first surface and ride over the retention structures  58  of the body  28  as the teeth  158  are flexed away from the body  28 . 
     After appropriate tensioning of the proximal anchor  150 , the bone may push on the angled portion contacting surface  156  of the proximal anchor  150 . This force is transmitted to the teeth  158  through the lever arms. As such, the teeth  158  are prevented from flexing away from the body  28 , which keeps the teeth  158  engaged with the retention structures  58  of the body  28 . By increasing the tensioning force, proximal movement of the proximal anchor  150  with respect to the body  28  is resisted. 
     The axial length and width of the slots  160  may be varied, depending upon the desired flexing of the lever arms when the proximal anchor  150  is moved distally over the body  28  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  160  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  160  extend through the flange  154  and at least partially into the housing  152 . 
     In this embodiment, the proximal anchor  150  includes four teeth or flanges  158 , which are positioned near the proximal end of the anchor  150 . In modified embodiments, the proximal anchor  150  may include more or lest teeth and/or the teeth may be positioned more distally or proximally on the anchor  150 . It should also be appreciated that these retention structures may be configured such that the proximal anchor  150  may be proximally and/or distally advanced with rotation by providing for a screw like configuration between the retention structures. 
     Another embodiment of a proximal anchor  180  is illustrated in  FIGS. 6E and 6F . As with the previous embodiment, the proximal anchor  180  may include a tubular housing  152  and a flange  154  with a bone contacting surface  156 . In this embodiment, the complementary structure of the proximal anchor  180  comprises an annular ring  182 , which is positioned within an annular recess  184  that is preferably positioned at the distal end of the tubular housing  152 . The annular recess  184  includes a proximal portion  186  and a distal portion  188 . 
     The proximal portion  186  is sized and dimensioned such that as the proximal anchor  180  is advanced distally over the body  28  the annular ring  182  can ride over the complementary retention structures  58  of the body  28 . That is, the proximal portion  182  provides a space for the annular ring  182  can move radially away from the body  28  as the proximal anchor  180  is advanced distally. Preferably, the annular ring  182  is made from a material that provides sufficient strength and elasticity such as, for example, stainless steel or titanium. The annular ring  182  is preferably split such that it can be positioned over the body  405 . In the illustrated embodiment, the annular ring  182  includes a plurality of teeth  192  although in modified embodiments the annular ring  182  may be formed without the teeth. 
     The distal portion  188  of the recess  184  is sized and dimensioned such that after the proximal anchor  180  is appropriately tensioned the annular ring  192  becomes wedged between the body  28  and an angled engagement surface of the distal portion  188 . In this manner, proximal movement of the proximal anchor  180  with respect to the body is prevented. Although not illustrated, it should be appreciated that in modified embodiments, the ring  192  can be formed without a gap. Other embodiments and further details of the proximal anchor described above can be found in U.S. patent application Ser. No. 09/990,587, filed Nov. 19, 2001, which is hereby incorporated by reference herein. 
     With reference back to  FIGS. 2-4 , in the illustrated embodiment, the contacting surface  68  of the flange  44  is tapered and generally faces the outer surface of the ilium, fixation rod, and/or plate. In other embodiments, the bone contacting surface  69  can reside in or approximately on a plane, which is perpendicular with respect to the longitudinal axis of the body  28 . In other embodiments, other angular relationships between the bone contacting surface  68  of the flange  66  and the longitudinal axis of the body  28  and housing  52  may be utilized, depending upon the anticipated entrance angle of the body  28  and associated entrance point surface of the ilium. 
     The clinician may be provided an array of proximal anchors  50  of varying angular relationships between the contacting surface  68  and the longitudinal axis of the body  28  and housing  52  (e.g., 90°, 100°, 110°, 120°, and 130°). A single body  28  can be associated with the array such as in a single sterile package. The clinician upon identifying the entrance angle of the body  28  and the associated entrance point surface orientation can choose the anchor  50  from the array with the best fit angular relationship, for use with the body  28 . 
     In accordance with a modified arrangement, illustrated in  FIGS. 7 and 8 , the proximal anchor  50  may be used with a washer  66 ′ that is angularly adjustable with respect to the longitudinal axis of the body  28 . More specifically, in this embodiment, the proximal anchor  50  and the washer  66 ′ include corresponding semi-spherical or radiused surfaces  45   a  and  45   b.  The surface  45   b  surrounds an aperture  49  in the washer  66 . This arrangement allows the proximal anchor  50  to extend through and pivot with respect to the washer  66 ′. As such, the angular relationship between the bone contacting surface  68 ′ of the washer  66 ′ and the longitudinal axis of the body  28  can vary in response to the entrance angle. 
       FIGS. 9-13  illustrate another embodiment of a bone fixation device  200  with an angularly adjustable proximal anchor  202 . In this embodiment, similar reference numbers are used to identify components that are similar components described above. 
     The bone fixation device  200  comprises a body  28  that extending between a proximal end  30  and a distal end  32 . The distal end  32  of the body is provide with a bone anchor  34  as described above. The illustrated body  28  is cannulated; however, it should be appreciated that in modified embodiments the body  28  can be solid. The proximal end of the anchor is provided with a hexagonal recess, which can be used in combination with a rotational tool to rotate the body  28 . Of course, modified embodiments may use a variety of different male or female anti-rotational couplings. 
     The illustrated fixation device includes an annular flange  202  and proximal anchor  204 . As with the proximal anchor described above, the proximal anchor  204  defines a housing  206  that is axially distally moveable along the body  28 . Complementary locking structures  54 ,  58  on the housing  206  and the body  28  such as threads or ratchet like structures resist proximal movement of the anchor  204  with respect to the body  28  under normal use conditions. In some embodiments, the complementary locking structures  54 ,  48  may permit the anchor  204  to be axially advanced along the body  28  by rotation. In other embodiments, the complementary locking structures  54 ,  58  may permit the anchor  204  to be axially advanced along the body  24  without rotation. The illustrated proximal anchor  204  also includes a gap  205  such that the illustrated anchor  204  forms a split ring collar. In modified embodiments, the proximal anchor  204  can be formed without the gap  205 . 
     The proximal anchor  204  preferably includes a smooth and more preferably rounded or spherical outer surface portion  208 , which is configured to fit within a corresponding smooth and preferably rounded recessed portion  210  in the flange  202 . As such, as shown in  FIG. 12 , when the proximal anchor  204  is positioned in the flange  202 , the flange  202  resists distal movement of the proximal anchor  204  while permitting at least limited rotation of between the proximal anchor  204  and the flange  202 . As such, the illustrate arrangement allows for angular movement of the flange  202  with respect to the anchor  204  to accommodate variable anatomical angles of the bone surface. In such applications, the flange  202  may seat directly against the outer surface of the ilium. Because the outer surface of the ilium is typically non-planar and/or the angle of insertion may not be perpendicular to the outer surface of the ilium, a fixed flange may contact only a portion of the outer surface of the ilium. This may cause the ilium to crack due to high stress concentrations. In contrast, the angularly adjustable flange  202  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 flange  202 , which has a larger diameter than the proximal anchor  50 , effectively increases the shaft to head diameter of the fixation device, thereby increasing the size of the loading surface and reducing stress concentrations. 
     In the illustrated embodiment, the flange  202  includes a plurality of bone engagement features  212 , which in the illustrated embodiment comprises one or more spikes  212  positioned on a contacting surface  216  of the flange  202 . The spikes  212  provide additional gripping support especially when the flange  202  is positioned against, for example, uneven bone surfaces and/or soft tissue. However, it should be appreciated that in modified embodiments the flange  202  may be formed without the bone engagement features  212 . Other structures for the bone engagement feature  212  may also be used, such as, for example, ridges, serrations etc. The illustrated embodiment also includes a tapered upper surface  214  that in certain embodiments may be flat. 
       FIGS. 14 and 15  illustrate a modified embodiment of the angularity adjustable fixation device  200 . In this embodiment, the proximal anchor  204 ′ includes an upper portion  211  and a lower portion  213 . The upper portion  211  is configured as described above with respect to the housing. The lower portion in the illustrated embodiment is generally tubular and a generally smaller diameter than the upper portion. The lower portion includes complementary retention structures  54  and generally provides the fixation device with a greater range of adjustable compression and additional retention structures as compared to the previous embodiment. 
       FIGS. 16A-H  are various views of the pelvis and sacrum with the fixation device of  FIG. 2  implanted therein.  FIGS. 16A-B  illustrate top views, while  FIGS. 16C-E  illustrate front views, and  FIGS. 16F-H  illustrate rear views. In some of these Figures, one or both of the ilium  300  and the sacrum  302  are illustrated as transparent, in order to more clearly identify the position of the fixation devices  12 . In the illustrated embodiment, three fixation devices  12  are implanted. Each fixation device  12  is inserted through the ilium  300  and into the sacrum  302 . The distal anchor  34  is disposed within the sacrum  302 . Following implantation and compression, the proximal anchor  50  is disposed against the surface of the ilium  300 . Although the illustrated embodiment utilizes three fixation devices  12 , in other embodiments the number of devices can vary. For example, in some embodiments one fixation device can be employed, while in others two can be used. In various embodiments, three, four, or more fixation devices can be used. 
     In one embodiment of use, a patient with a sacroiliac joint instability is identified. The target entry point on the ilium  300  and a trajectory angle is then localized by intraoperative imaging, for example by fluoroscopy. A small incision is then made in the skin, and the tip of a guide wire or K-wire is driven through the soft tissue of the patient at an advantageous angle down to the target site on the ilium. The skin incision can then be lengthened, as necessary. In some embodiments, the incision may be lengthened to approximately 17 mm, for example. A similar incision can be made in the fascia, using the guide wire as the midpoint of the incision. A first dilator tube can then be passed over the guide wire until the tip of the dilator tube reaches the target point on the ilium  300 . A second dilator tube having a larger diameter can then be passed over the first dilator tube. Advancing the second dilator tube to the target point on the ilium  300  further retracts tissue along the trajectory path. This can be repeated with additional dilator tubes, as necessary, with progressively wider dilator tubes to expand the patient&#39;s soft tissue down to the entry point on the ilium  300 . An outer dilator tube, or cannula, is then left in place. A depth gauge may then be used to verify that the appropriate depth has been reached. 
     In some embodiments, a pre-drill can thereafter be advanced to the ilium  300 , which is then verified by fluoroscopy. A drill can be advanced until it passes through the ilium and into the sacrum  302 . The distal tip of a tap is driven into the sacrum until it reaches the appropriate depth, which can then be verified by fluoroscopy. A bone fixation device  12  is then driven through the ilium  300  and into the sacrum  302  until it reaches the appropriate depth, which can then also be verified by fluoroscopy. Once the distal anchor  34  is in the desired location, proximal traction is applied to the proximal end  30  of body  28 , such as by conventional hemostats, pliers or a calibrated loading device, while distal force is applied to the proximal anchor. In this manner, the proximal anchor is advanced distally with respect to the body until the proximal anchor fits snugly against the outer surface of the ilium or a fixation plate/rod. Appropriate tensioning of the fixation device is accomplished by tactile feedback or through the use of a calibration device for applying a predetermined load on the implantation device. As explained above, one advantage of the structure of the illustrated embodiments is the ability to adjust compression independently of the setting of the distal anchor  34  within the sacrum. Appropriate stabilization of the sacroiliac joint can then be verified by fluoroscopy. 
     Following appropriate tensioning of the proximal anchor, the second portion  38  of the body  28  is preferably detached from the first portion  36  and removed. In the illustrated embodiment, this involves rotating the second portion  38  with respect to the first portion via the coupling  70 . In other embodiments, this may involve cutting the proximal end of the body  28 . For example, the proximal end of the body may be separated by cauterizing. Cauterizing may fuse the proximal anchor  50  to the body  32  thereby adding to the retention force between the proximal anchor  50  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 body. 
     Following or before removal of the second portion  38  of each body  28 , additional fixations devices may be implanted and/or additional stabilization implants (e.g., rods, plates, etc.) may be coupled to the body. The access site may be closed and dressed in accordance with conventional wound closure techniques. 
     In a modified arrangement, the second portion  38  may form part of the driving device, which is used to rotate the proximal anchor  50  and thus distal anchor  34  into the sacrum. The second portion  38  is used to apply proximal traction. After appropriate tensioning, the second portion  38  can be de-coupled from the first portion  36  and removed with the driving device. 
     In the foregoing variation, the second portion  38  may be connected to a rotatable control such as a thumb wheel on the deployment device. A container may be opened at the clinical site exposing the proximal end of the implant, such that the distal end of the second portion  38  may be removably coupled thereto. Proximal retraction of the hand tool will pull the implant out of its packaging. The implant may then be positioned within the aperture in the bone, rotated to set the distal anchor, and the hand piece may be manipulated to place proximal traction on the second portion  38  while simultaneously distally advancing the proximal anchor. Following appropriate tensioning, the second portion  38  may be disengaged from the implant, and removed from the patient. In the example of a threaded engagement, the second portion  38  may be disengaged from the implant by rotating a thumb wheel or other rotational control on the hand piece. In an alternate embodiment, such as where the second portion  38  comprises a pull wire, following appropriate tensioning across the joint, a first end of the pull wire is released such that the pull wire may be removed from the implant by proximal retraction of the second end which may be attached to the hand piece. 
     In some embodiments, the clinician will have access to an array of fixation devices  12 , having, for example, different diameters, axial lengths and, if applicable, angular relationships. These may be packaged one or more per package in sterile or non-sterile envelopes or peelable pouches, or in dispensing cartridges which may each hold a plurality of devices  12 . The clinician can assess the dimensions and load requirements, and select a fixation device from the array, which meets the desired specifications. 
     Myrid variations on the above-noted procedures can be used. For example, in some embodiments a dilator can be introduced directly, without the use of a guidewire. In some embodiments, a self-tapping high-speed drill can be used. The surgery may be performed percutaneously, minimally invasively, mini-open, or open, depending on surgeon preference. 
     The proximal anchor  50  may be carried by the fixation device  12  prior to advancing the body into the sacrum  302 , or may be attached following placement of the body within the sacrum  302 . In one embodiment, stabilization implants (e.g., a fixation plate and/or rod) may be placed over or coupled to the body or the proximal anchor before the proximal anchor is placed on the body. 
     As noted above, depending upon the sacroiliac joint fixation technique, the distal anchor of one or more bone fixation devices described herein are advanced through the ilium and into a suitable portion of the sacrum. In use, the threads of the fixation device can be placed across the sacroiliac joint, and compression achieved by distally advancing the proximal anchor. This approach not only provides compression across the sacroiliac joint which helps promote fusion, but also provides intraoperative flexibility to stop the distal anchor of the device where necessary and compress to the length to achieve an appropriate fit. The device may be used with or without a washer. 
     In the embodiment of  FIGS. 16A-H , the proximal anchor is typically supported directly against the outer surface of the ilium  300 . Because the outer surface is typically non-planar and/or the insertion angle of the fixation device is not perpendicular to the outer surface, an angularly adjustable flange may be used that can rotate with respect to the body and thereby the bone contacting surface may be positioned more closely to the outer surface of the ilium  300 . This results in more bone contacting surface being utilized and the stress supported by the fixation device is spread out over a larger area of the ilium  300 . 
     The surface of the fixation devices  12  may be treated to promote bone in-growth, and therefore fusion across the sacroiliac joint. These treatments may be placed over the entire length of the device or only on certain portions of the device depending on the specific needs it addresses or the advantages it provides. These treatments can include titanium plasma spray, a coating of hydroxyapatite, resorbable blast media, and others. In addition, bone graft (autogenous, demineralized bone matrix, bone morphogenetic protein, or other) may be be inserted into the pre-drilled hole prior to insertion of the fixation device. In some embodiments, an allograft sleeve may be placed over the fixation device so that the sleeve spans the sacroiliac joint, thereby encouraging bone in-growth and sacroiliac joint fusion. 
     In various embodiments, bone graft material (e.g., autograft, allograft, demineralized bone matrix), bone growth promoters (e.g., bone morphogenic proteins), and/or bone cement may be used in conjunction with the fixation devices described herein. For example, bone graft material, bone growth promoters, and/or bone cement can be introduced into the sacroiliac joint before and/or after insertion of the fixation device(s) across the joint. This may help promote fusion of the joint, and/or to increase fixation. This can be particularly advantageous in cases in which the bone quality is poor, but the approach may be applied to any quality of bone. In some embodiments, the fixation device is cannulated. Accordingly, in such embodiments the bone graft, bone growth promoters, and/or bone cement can be introduced through the interior passageway after insertion of the fixation device. In some embodiments, the fixation device may be cannulated and may also include a plurality of exit holes. For example, a plurality of exit holes may be arranged on the outer surface of the fixation device. The exit holes may be in fluid communication with the interior passageway, such that bone graft material, bone growth promoters, and/or bone cement introduced through the interior passageway can exit through the plurality of exit holes. In some embodiments, one or more of the exit holes may be oriented in a direction transverse to the interior passageway. In some embodiments, the exit holes may be distributed along substantially the entire length of the fixation device. In other embodiments, the exit holes may be limited to one or more regions of the fixation device. For example, the exit holes may be limited to certain regions such that the bone graft material, bone growth promoters, and/or bone cement exits the fixation device in preferential areas to promote fusion, such as at the joint. In some embodiments, the exit holes may be limited to the distal region, such that the exiting bone graft material, bone growth promoters, and/or bone cement improves fixation. 
     The fixation devices described above may be made from either conventional bioabsorbable materials or conventional non-absorbable materials, combinations thereof and equivalents thereof. In addition, natural materials such as allografts may be used. Examples of absorbable materials include homopolymers and copolymers of lactide, glycolide, trimethylene carbonate, caprolactone, and p-dioxanone and blends thereof. The following two blends may be useful: 1) the blend of poly(p-dioxanone) and a lactide/glycolide copolymer, as disclosed in U.S. Pat. No. 4,646,741 which is incorporated by reference and (2) the glycolide-rich blend of two or more polymers, one polymer being a high lactide content polymer, and the other being a high glycolide content disclosed in U.S. Pat. No. 4,889,119 which is incorporated by reference. Additional bioabsorbable materials are disclosed in copending application Ser. No. 09/558,057 filed Apr. 26, 2000, the disclosure of which is incorporated in its entirety herein by reference. 
     The fixation devices may also be made from conventional non-absorbable, biocompatible materials including stainless steel, titanium, alloys thereof, polymers, composites and the like and equivalents thereof. In one embodiment, the distal anchor comprises a metal helix, while the body and the proximal anchor comprise a bioabsorbable material. Alternatively, the distal anchor comprises a bioabsorbable material, and the body and proximal anchor comprise either a bioabsorbable material or a non-absorbable material. As a further alternative, each of the distal anchor and the body comprise a non-absorbable material, connected by an absorbable link. This may be accomplished by providing a concentric fit between the distal anchor and the body, with a transverse absorbable pin extending therethrough. This embodiment will enable removal of the body following dissipation of the pin, while leaving the distal anchor within the bone. 
     The 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 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 components. Alternatively, capillary pathways may be provided throughout the body and collar, such as by manufacturing the anchor and body 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. 
     In the embodiments described above, it should be appreciated that the distal anchor may be configured to be used with a pre-drilled hole and/or self tapping. 
     The components 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 dimensions, configuration and materials will be apparent to those of skill in the art in view of the disclosure herein. In addition, all features discussed in connection with any one embodiment herein can be readily adapted for use in other embodiments herein. The use of different terms or reference numerals for similar features in different embodiments does not imply differences other than those which may be expressly set forth. Accordingly, the present invention is intended to be described solely by reference to the appended claims, and not limited to the preferred embodiments disclosed herein.