Patent Publication Number: US-11653962-B2

Title: Apparatus for immobilization and fusion of a synovial joint

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 14/834,040, filed Aug. 24, 2015, which is a continuation of U.S. application Ser. No. 13/594,555, filed Aug. 24, 2012, now U.S. Pat. No. 9,113,972, which claims the benefit of US Provisional Application No. 61/526,807, filed Aug. 24, 2011, each of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the immobilization and fusion of a synovial joint and, more particularly, to apparatus and methods for the immobilization and fusion of the sacroiliac joint. 
     BACKGROUND OF THE INVENTION 
     The sacroiliac joint is a synovial joint between the sacrum, which is the inferior (or caudal) terminus of the spinal column, and the ilium of the pelvis. As with other synovial joints, the sacroiliac joint (or “SI” joint) can degenerate and undergo degenerative arthritic changes due to a number of possible causes, including trauma to the joint or the hypermobility induced in the joint during childbirth. The degenerated SI joint loses stability and experiences non-normal movements, resulting in an inflammatory response and subsequent pain. It has been reported that disorders of the SI joint are the source of back pain for up to 25% of patients (Cohen, 2005). 
     It is accepted practice to treat certain degenerated synovial joints, which normally exhibit low relative motions between the joint surfaces such as the spinal vertebral disc and SI joint, by stabilizing the joint through immobilization and subsequent bony fusion (arthrodesis). It is thought that eliminating relative movement of the joint surfaces will eliminate subsequent inflammation and pain. Surgical arthrodesis of the SI joint was first reported by Smith-Petersen and Rodgers in 1926 and is still performed today. Arthrodesis is performed by inducing growth of bone between the joint surfaces to fuse the joint, first by removal of all of the soft tissue of the joint to eliminate any barriers to bone tissue formation, followed by scraping of the bony joint surfaces to induce bleeding and the subsequent biological response of bone formation, and often with filling the prepared joint with morselized bone to assist in the bone formation process. Immobilizing the joint as part of the arthrodesis procedure provides immediate stability to the joint and reduction of the pain-generating movement, and allows quicker bone formation. 
     Immobilization of the SI joint has historically been performed by placing one or more, simple threaded screws through the joint, normal to the general plane of the joint. Due to variations in bone density and/or morphology between individual patients, as well as occasional bone removal from the ilium from previous spinal fusion surgeries, simple threaded screws are often insufficient to provide the joint stability required for a successful arthrodesis procedure. There exists a need for improved designs that can offer improved ability to immobilize the SI joint in more patients and reduce the risk of back-out of the fixation device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To understand the present invention, it will now be described by way of example, with reference to the accompanying drawings in which: 
         FIG.  1 A  is a perspective view of a dowel according to the present invention; 
         FIG.  1 B  is a perspective view of an alternate embodiment of a dowel according to the present invention; 
         FIG.  2 A  is an anterior view of a plurality of dowels according to the present invention implanted within the SI joint; 
         FIG.  2 B  is a lateral view of a plurality of dowels according to the present invention implanted within the SI joint; 
         FIG.  3 A  is a perspective view of an alternate embodiment of a dowel according to the present invention; 
         FIG.  3 B  is a perspective view of the dowel of  FIG.  3 A  including a central pin; 
         FIG.  4 A  is an anterior view of a plurality of dowels according to the embodiment shown in  FIG.  3 A  implanted within the SI joint; 
         FIG.  4 B  is a lateral view of the plurality of dowels shown in  FIG.  4 A ; 
         FIG.  5    is a perspective view of a dowel according to the present invention; 
         FIG.  6    is a vertical cross-sectional view of the dowel of  FIG.  5    taken along the longitudinal axis thereof. 
         FIG.  7    is a perspective view of the dowel of  FIG.  5    showing the paddle members extended; 
         FIG.  8    is a side elevation view of the dowel shown in  FIG.  7   ; 
         FIGS.  9 A and  9 B  are anterior and lateral views of a plurality of dowels according to the embodiment shown in  FIG.  5    implanted within the SI joint; 
         FIGS.  10 A and  10 B  are anterior and lateral views of a plurality of dowels according to the embodiment shown in  FIG.  7    implanted within the SI joint; 
         FIG.  11    is a vertical cross-sectional view of the dowel of  FIG.  12    taken along the longitudinal axis thereof; 
         FIG.  12    is a perspective view of an alternate embodiment of a dowel according to the present invention showing the paddle members extended; 
         FIG.  13    is a perspective view of the dowel of  FIG.  12    with the paddle members retracted; 
         FIGS.  14 A and  14 B  are anterior views of a plurality of dowels according to the embodiment of  FIG.  12    implanted within the SI joint showing the paddle members retracted and extended, respectively; 
         FIG.  15    is a vertical cross-sectional view of the dowel of  FIG.  16    taken along the longitudinal axis thereof; 
         FIG.  16    is a perspective view of an alternate embodiment of a dowel according to the present invention showing the paddle members extended; 
         FIG.  17    is a vertical cross-sectional view of the dowel of  FIG.  16    with the paddle members retracted; 
         FIGS.  18 A and  18 B  are posterior views of a plurality of dowels according to the embodiment of  FIG.  16    implanted within the SI joint showing the paddle members extended; 
         FIG.  19    is a perspective view of an alternate embodiment of a dowel according to the present invention; 
         FIG.  20    is a vertical cross-sectional view of the dowel of  FIG.  19   ; 
         FIG.  21    is a vertical cross-sectional view of the dowel of  FIG.  19    with an expansion member inserted within the dowel; 
         FIG.  22    is a vertical cross-sectional view of an alternate embodiment of a dowel similar to the dowel of  FIG.  19   ; 
         FIG.  23    is a vertical cross-sectional view of the dowel of  FIG.  22    with an expansion member inserted within the dowel; 
         FIG.  24    is a perspective view of an alternate embodiment of a dowel according to the present invention; 
         FIGS.  25  and  26    are vertical cross-sectional views of the dowel of  FIG.  24    with the radially expandable mesh in unexpanded and expanded configurations; 
         FIG.  27    is a side elevation view of an alternate embodiment of a dowel according to the present invention; 
         FIG.  28    is an exploded perspective view of the dowel of  FIG.  27   ; 
         FIG.  29    is a side elevation view of an alternate embodiment of a dowel according to the present invention; 
         FIG.  30    is an exploded perspective view of the dowel of  FIG.  29   ; 
         FIG.  32    is a perspective view of an alternate embodiment of a fixation device according to the present invention; 
         FIG.  31    is an exploded perspective view of the fixation device of  FIG.  32   ; 
         FIG.  33    is an anterior view of a plurality of fixation devices as shown in  FIG.  32    implanted within the SI joint; and 
         FIG.  34    is a vertical cross sectional view of the fixation device of  FIG.  32   . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A single-piece, solid non-threaded device or connector for joint fixation in the form of dowel  100  for being implanted singly or in multiples, is shown in  FIGS.  1 A,  1 B,  2 A and  2 B . As shown in  FIG.  1 A , dowel  100  has an elongate body  100   a  having a distal or leading end  102  configured to assist insertion into a pre-drilled hole through the SI joint surfaces. As illustrated, the distal end  102  can be tapered for ease of insertion. The body  100   a  of the dowel  100  has a proximal end  108  configured for receiving an impact force. Referring to  FIG.  3 B , the proximal end  108  has a relatively blunt or flat configuration for being contacted with a mallet or other insertion instrument. The dowel body  100   a  may be configured with cannula  106 , which is a hole bored generally along the longitudinal axis of dowel body  100   a  to facilitate the use of a guide pin (not shown) intended to aid in placement of dowel  100  into a pre-drilled hole through the SI joint surfaces. 
     Dowel body  100   a  can have an outer surface that has a non-circular cross-sectional configuration. In this regard, the elongate body  100   a  can include a longitudinal flat surface  104 . The non-round profile aids in avoiding rotation of the SI joint surfaces around the long axis of the implanted dowel  100 , providing further stabilization of the joint. Dowel body  100   a  may have a single or multiple flat surfaces  104 , producing a generally rectangular cross-sectional shape with two flat surfaces  104 , a generally triangular cross-sectional shape with three flat surfaces  104 , etc. Instead of flat surfaces  104 , dowel body  100   a  may have one or more longitudinal concave surfaces  110  as shown in  FIG.  1 B . These concave surfaces  110  provide additional contact area with the surrounding bone and may offer improved rotational resistance. The flat surfaces  104  and concave surfaces  110  extend along the length of the elongate dowel body  100   a  at least sufficient to engage the bone of the sacrum and ilium. Flat surfaces  104 , concave surfaces  110 , and other non-round surface profiles can be combined to provide specific desired cross-sectional profiles as well as a flexural stiffness for dowel  100  that varies depending on the direction of applied force. 
     Dowel body  100   a  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for use in joint stabilization, including (but not limited to) plastics, metals, ceramics, human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. These polymers and composites are often well-suited for joint stabilization due to their modulus of elasticity which is similar to the surrounding bone, which reduces the potential for stress-shielding that might occur with stiffer materials. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  100 . The outer surface of dowel body  100   a  may be modified to enable and/or promote bony tissue in-growth into dowel body  100   a . Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the dowel  100  substrate material to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     Illustrations showing one example of one embodiment of dowel  100  implanted in a patient are shown in  FIGS.  2 A and  2 B .  FIG.  2 A  is an anterior (front) view and  FIG.  2 B  is a lateral (side) view, and are similar to what might be seen on a diagnostic image (x-ray, computed tomography (CT) scan or magnetic resonance image (MRI) scan) of an implanted patient. While this example shows the use of three dowels  100 , more or fewer dowels  100  may be utilized depending on individual patient needs. 
     Another joint fixation device or connector in the form of dowel  200  is provided. Dowel  200  has no external screw threads and a hollow inner chamber  214 , can be implanted singly or in multiples, and is shown in  FIGS.  3 A,  3 B,  4 A and  4 B . As shown in  FIG.  3 A , similar to dowel  100 , dowel  200  has an elongate body  200   a  having a relatively tapered distal end  202  to assist insertion into a pre-drilled hole through the SI joint surfaces and a relatively blunt proximal end  208  configured for contact with a mallet or other insertion instrument. The dowel body  200   a  may be configured with cannula  206 , which is a hole bored generally along the longitudinal axis of dowel  200  to facilitate the use of a guide pin (not shown) intended to aid in placement of dowel  200  into a pre-drilled hole through the SI joint surfaces. 
     The dowel body  200   a  also can have another surface with a non-circular configuration such as due to the presence of longitudinal concave surfaces  210 . The non-round profile aids in avoiding rotation of the SI joint surfaces around the long axis of the implanted dowel  200   a , providing further stabilization of the joint. The dowel body  200   a  may have a single or multiple concave surfaces  210 , producing a generally rectangular cross-sectional shape with two concave surfaces  210 , a generally triangular cross-sectional shape with three concave surfaces  210 , etc. Instead of concave surfaces  210 , dowel body  200   a  may have one or more longitudinal flat surfaces  104  as shown in  FIG.  1 A . The flat surfaces  104  and concave surfaces  210  extend along the length of the elongate dowel body  200   a  at least sufficient to engage the bone of the sacrum and ilium. Flat surfaces  104 , concave surfaces  210 , and other non-round surface profiles can be combined to provide specific desired cross-sectional profiles as well as a flexural stiffness for dowel  200  that varies depending on the direction of applied force. 
     Dowel body  200   a  is configured with side holes  212  that extend transverse to the longitudinal axis of the body  200   a  to open to both the outer surface  210  and the inner chamber  214 . The embodiment shown in  FIGS.  3 A and  3 B  shows three oval shaped or rectangular shaped side holes  212  on each of the sides of dowel body  200   a , but other quantities and other shapes of side holes  212  could be used. Side holes  212  are intended to offer the surgeon the option to pack the inner chamber  214  and side holes  212  of dowel  200  with morselized autologous bone, demineralized donor bone product, or bone analogue, with or without bone growth proteins in order to aid new bone growth in general and interlocking bone growth in particular. 
     Dowel body  200   a  may be configured with an inner chamber  214  and proximal hole  207  to accommodate a central pin  216 . Central pin  216  can be inserted in proximal hole  207  and secured within chamber  214  in order to stiffen dowel  200  if necessary. Central pin  216  and proximal hole  207  may be configured with complementary screw threads, slots, barbs, or other mechanical structural features for retention of central pin  216  within dowel body  200   a . Side holes  212  can be filled with bone or bone analogue, and allow interlocking bone growth even with central pin  216  in place. In one embodiment, dowel body  200   a  made from PEEK, with a large inner chamber  214  and multiple large side holes  212  may benefit from insertion of central pin  216  made from a titanium alloy to provide sufficient overall stiffness to adequately stabilize the SI joint. 
     Dowel  200  and central pin  216  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for use in joint stabilization, including (but not limited to) plastics, metals, ceramics, human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. These polymers and composites are often well-suited for joint stabilization due to their modulus of elasticity which is similar to the surrounding bone, which reduces the potential for stress-shielding that might occur with stiffer materials. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  200  and central pin  216 . The outer surface of dowel  200  and/or central pin  216  may be modified to enable and/or promote bony tissue in-growth into dowel  200  and/or central pin  216 . Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying surface material to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     Illustrations showing one example of one embodiment of dowel  200 , without the central pin  216 , implanted in a patient are shown in  FIGS.  4 A and  4 B .  FIG.  4 A  is an anterior (front) view and  FIG.  4 B  is a lateral (side) view, and are similar to what might be seen on a diagnostic image (x-ray, computed tomography (CT) scan or magnetic resonance image (MRI) scan) of an implanted patient. While this example shows the use of three dowels  200 , more or fewer dowels  200  may be implanted depending on individual patient needs. 
     Another joint fixation device or connector in the form of dowel  300  is provided. Dowel  300  has no external threads and a hollow inner chamber  314 , can be implanted singly or in multiples, and is shown in  FIGS.  5 - 8 ,  9 A,  9 B,  10 A and  10 B . As shown in  FIG.  5    similar to dowels  100  and  200 , dowel  300  has an elongate body  300   a  having a relatively tapered distal end  302  to assist insertion into a pre-drilled hole through the SI joint surfaces and a relatively blunt proximal end  308  configured for contact with a mallet or other insertion instrument. 
     The dowel body  300   a  also can have an outer surface with a non-circular configuration such as due to the presence of longitudinal concave surfaces  310  that extend axially or longitudinally therealong. The non-round profile aids in avoiding rotation of the SI joint surfaces around the long axis of the implanted dowel  300 , providing further stabilization of the joint. The dowel body  300   a  may have a single or multiple concave surfaces  310 , producing a generally rectangular cross-sectional shape with two concave surfaces  310 , a generally triangular cross-sectional shape with three concave surfaces  310 , etc. Instead of concave surfaces  310 , dowel body  300   a  may have one or more longitudinal flat surfaces  104  as shown in  FIG.  1 A . The flat surfaces  104  and concave surfaces  310  extend along the length of the elongate dowel body  300   a  at least sufficient to engage the bone of the sacrum and ilium. Flat surfaces  104 , concave surfaces  310 , and other non-round surface profiles can be combined to provide specific desired cross-sectional profiles as well as a flexural stiffness for dowel  300  that varies depending on the direction of applied force. 
     Dowel body  300   a  is configured with side holes  312  positioned in the central portion of dowel  300  that extend transverse to the length of the body  300   a  to open to both outer surface  310  and the inner chamber  314 . The embodiment shown in  FIGS.  5 ,  6 ,  7  and  8    shows one rectangular shaped side hole  312  on each of the sides of the dowel body  300   a , but other quantities and other shapes of side holes  312  could be used. Side holes  312  are intended to offer the surgeon the option to pack the inner chamber  314  and side holes  312  of dowel  300  with morselized autologous bone, demineralized donor bone product, or bone analogue, with or without bone growth proteins in order to aid new bone growth in general and interlocking bone growth in particular. 
     Dowel  300  is configured with an inner chamber  314  and proximal hole  307  sized to contain compression assembly  318  in a non-operative configuration. Compression assembly  318  consists of an actuator in the form of a central pin  320 , having a region with a right-handed RH screw thread  328  located at the distal tip of central pin  320  and a region with a left-handed LH screw thread  329  located at the proximal tip of central pin  320 . Right-handed RH threaded sleeve  322  has a right-handed screw thread on the internal surface of the sleeve, and is movably attached to the distal tip of central pin  320  such that the threads of RH threaded sleeve  322  engage RH screw thread  328 . Left-handed LH threaded sleeve  323  has a left-handed screw thread on the internal surface of the sleeve, and is movably attached to the proximal tip of central pin  320  such that the threads of LH threaded sleeve  323  engages LH screw thread  329 . Extendable anchor members such as paddles  324  are attached to RH threaded sleeve  322  and LH threaded sleeve  323  such that they are allowed to pivot with respect to the longitudinal axis of dowel  300 , and positioned within projection holes  326 . Projection holes  326  connect outer surface  310  with inner chamber  314 . A cam surface is disposed at the medial edge of the holes  326  and is configured to form an upward-angled medial edge cam surface such that longitudinal movement of paddles  324  toward and along the cam surface causes paddles  324  to pivot away, and extend radially outward, from dowel body  300   a . Paddles  324  are shown in retracted position in  FIGS.  5  and  6   , and in extended position in  FIGS.  7  and  8   . Extension of paddles  324  is accomplished by rotating central pin  320  in a paddle actuating direction using an instrument such as a standard screwdriver designed to engage drive slot  330  in the head of the actuator pin  320 . Drive slot  330  can alternatively be configured to accommodate other engagement instruments such as hex wrenches, box wrenches, Torx wrenches, Allen wrenches, or similar designs. 
     Paddles  324  at the proximal end of dowel  300  are intended to engage the surface of the ilium on the exterior of the SI joint and paddles  324  at the distal end of dowel  300  are intended to engage the surface of the sacrum on the exterior of the SI joint, once dowel  300  has been positioned within the pre-drilled hole through the SI joint surfaces and paddles  324  have been extended via rotation of central pin  320 . Continued rotation of central pin  320  following bone engagement by paddles  324  will urge the joint surfaces toward each other to impart a compression force to the SI joint, with the intent of stabilizing the SI joint by compressing the joint surfaces against each other to minimize motion therebetween. 
     Illustrations showing one example of one embodiment of dowel  300  implanted with paddles in retracted and extended positions are shown in  FIGS.  9 A,  9 B,  10 A and  10 B . While this example shows the use of three dowels  300 , with four paddles  324  at each end of dowel  300 , it should be understood that more or fewer dowels  300  may be implanted depending on individual patient needs and that dowel  300  may be designed with more or fewer paddles at each end. 
     Dowel  300  and the components of compression assembly  318  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  300  and compression assembly  318 . Human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite can be used for dowel  300 . The outer surface of dowel  300  and at least portions of compression assembly  318  may be modified to enable and/or promote bony tissue in-growth into those surfaces. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying material to dowel  300  to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     Another joint fixation device or connector in the form of dowel  400  is provided. Dowel  400  has no external threads and a hollow inner chamber  414 , can be implanted singly or in multiples, and is shown in  FIGS.  11 - 13 ,  14 A and  14 B . As shown in  FIG.  11   , similar to the previously-described dowels  100 - 300 , dowel  400  has an elongate body  400   a  having a relatively tapered distal end  402  to assist insertion into a pre-drilled hole through the SI joint surfaces and a relatively blunt proximal end  408  configured for contact with a mallet or other insertion instrument. 
     The dowel body  400   a  also can have an outer surface with a non-circular configuration such as due to the presence of longitudinal concave surfaces  410  that extend axially or longitudinally therealong. The non-round profile aids in avoiding the rotation of the SI joint surfaces around the long axis of the implanted dowel  400 , providing further stabilization of the joint. The dowel body  400   a  may have a single or multiple concave surfaces  410 , producing a generally rectangular cross-sectional shape with two concave surfaces  410 , a generally triangular cross-sectional shape with three concave surfaces  410 , etc. Instead of concave surfaces  410 , dowel body  400   a  may have one or more longitudinal flat surfaces  104  as shown in  FIG.  1 A . The flat surfaces  104  and concave surfaces  410  extend along the length of the elongate dowel body  400   a  at least sufficient to engage the bone of the sacrum and ilium. Flat surfaces  104 , concave surfaces  410 , and other non-round surface profiles can be combined to provide specific desired cross-sectional profiles as well as a flexural stiffness for dowel  400  that varies depending on the direction of applied force. 
     Dowel body  400   a  is configured with side holes  412  positioned in the central portion of dowel  400  that extend transverse to the length of the body  400   a  to open to both outer surface  410  and the inner chamber  414 . The embodiment shown in  FIGS.  11 ,  12 ,  13 ,  14 A and  14 B  shows one rectangular shaped side hole  412  on each of the sides of the dowel body  400   a , but other quantities and other shapes of side holes  412  could be used. Side holes  412  are intended to offer the surgeon the option to pack the inner chamber  414  and side holes  412  of dowel  400  with morselized autologous bone, demineralized donor bone product, or bone analogue, with or without bone growth proteins in order to aid new bone growth in general and interlocking bone growth in particular. 
     Dowel  400  is configured with an inner chamber  414  and proximal hole  407  sized to contain distraction assembly  418  in a non-operative configuration. Distraction assembly  418  consists of an actuator in the form of a central pin  420 , having a region with a right-handed RH screw thread  428  located distal to the medial portion of central pin  420  and a region with a left-handed LH screw thread  429  located proximal to the medial portion of central pin  420 . Right-handed RH threaded sleeve  422  has a right-handed screw thread on the internal surface of the sleeve, and is movably attached to central pin  420  distal to the medial portion of central pin  420  such that the threads of RH threaded sleeve  422  engage RH screw thread  428 . Left-handed LH threaded sleeve  423  has a left-handed screw thread on the internal surface of the sleeve, and is movably attached to central pin  420  proximal to the medial portion of central pin  420  such that the threads of LH threaded sleeve  423  engages LH screw thread  429 . Extendable anchor members such as paddles  424  are attached to RH threaded sleeve  422  and LH threaded sleeve  423  such that they are allowed to pivot with respect to the longitudinal axis of dowel  400 , and positioned within projection holes  426 . Projection holes  426  connect outer surface  410  with inner chamber  414 . A cam surface is disposed at the medial edge of the holes  426  and is configured to form an upward-angled edge opposite the medial edge cam surface such that longitudinal movement of paddles  424  toward and along the cam surface laterally from the medial location causes paddles  424  to pivot away, and extend outward, from dowel body  400   a . Paddles  424  are shown in extended position in  FIGS.  11  and  12   , and in retracted position in  FIG.  13   . Extension of paddles  424  is accomplished by rotating central pin  420  in a paddle actuating direction using a drive instrument such as a standard screwdriver designed to engage drive slot  430  in the head of the actuator pin  420 . Drive slot  430  can alternatively be configured to accommodate other engagement instruments such as hex wrenches, box wrenches, Torx wrenches, Allen wrenches, or similar designs. 
     Paddles  424  proximal to the medial portion of dowel  400  are intended to engage the surface of the ilium within the interior of the SI joint and paddles  424  distal to the medial portion of dowel  400  are intended to engage the surface of the sacrum within the interior of the SI joint, once dowel  400  has been positioned within the pre-drilled hole through the SI joint surfaces and paddles  424  have been extended via rotation of central pin  420 . Continued rotation of central pin  420  following bone engagement by paddles  424  will urge the joint surfaces apart to impart a distraction force to the SI joint, with the intent of stabilizing the SI joint by distracting the joint and inducing tension in the ligaments connecting the ilium and sacrum. 
     Illustrations showing one example of one embodiment of dowel  400  implanted with paddles in retracted and extended positions are shown in  FIGS.  14 A and  14 B . While this example shows the use of three dowels  400 , with four paddles  424  at each end of dowel  400 , it should be understood that more or fewer dowels  400  may be implanted depending on individual patient needs and that dowel  400  may be designed with more or fewer paddles at each end. 
     Dowel  400  and the components of distraction assembly  418  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  400  and compression assembly  418 . Human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite can be used for dowel  400 . The outer surface of dowel  400  and at least portions of compression assembly  418  may be modified to enable and/or promote bony tissue in-growth into those surfaces. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying material to dowel  400  to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     An alternative joint fixation device or connector described below provides a distraction force to the SI joint for stabilization, similar to dowel  400 , but does so with only one set of paddles instead of two. 
     The alternative joint fixation device or connector can also be in the form of dowel  500 . Dowel  500  has no external threads and a hollow inner chamber  514 , can be implanted singly or in multiples, and is shown in  FIGS.  15 - 17 ,  18 A and  18 B . As shown in  FIG.  15   , similar to previously described dowels, dowel  500  has an elongate body  500   a  having a relatively tapered distal end  502  to assist insertion into pre-drilled holes through the SI joint surfaces and a relatively blunt proximal end  508  configured for contact with a mallet or other insertion instrument. 
     The dowel body  500   a  also can have an outer surface with a non-circular configuration such as due to the presence of longitudinal concave surfaces  510  that extend axially or longitudinally therealong. The non-round profile aids in avoiding the rotation of the SI joint surfaces around the long axis of the implanted dowel  500 , providing further stabilization of the joint. The dowel body  500   a  may have a single or multiple concave surfaces  510 , producing a generally rectangular cross-sectional shape with two concave surfaces  510 , a generally triangular cross-sectional shape with three concave surfaces  510 , etc. Instead of concave surfaces  510 , dowel body  500   a  may have one or more longitudinal flat surfaces  104  as shown in  FIG.  1 A . The flat surfaces  104  and concave surfaces  510  extend along the length of the elongate dowel body  500   a  at least sufficient to engage the bone of the sacrum and ilium. Flat surfaces  104 , concave surfaces  510 , and other non-round surface profiles can be combined to provide specific desired cross-sectional profiles as well as a flexural stiffness for dowel  500  that varies depending on the direction of applied force. 
     The outer geometry of dowel  500  is configured with a reduction in the outer dimension of dowel  500  at step  532 , to provide engagement with the surface of the sacrum within the interior of the SI joint sufficient to prevent further movement of dowel  500  through the pre-drilled hole in the sacrum during insertion. This action is enabled by first drilling a smaller hole through the SI joint, sized to allow only the smaller distal portion of dowel  500  to step  532 , then drilling a larger hole only through the ilium, sized to allow insertion of the entire dowel  500 . 
     Dowel body  500   a  is configured with side holes  512  positioned in the central and distal portions of dowel  500  that extend transverse to the length of the body  300   a  to open to both outer surface  510  and the inner chamber  514 . The embodiment shown in  FIGS.  15 ,  16 ,  17 ,  18 A and  18 B  shows two rectangular shaped side holes  512  on each of the sides of the dowel body  500   a , but other quantities and other shapes of side holes  512  could be used. Side holes  512  are intended to offer the surgeon the option to pack the inner chamber  514  and side holes  512  of dowel  500  with morselized autologous bone, demineralized donor bone product, or bone analogue, with or without bone growth proteins in order to aid new bone growth in general and interlocking bone growth in particular. 
     Dowel  500  is configured with an inner chamber  514  and proximal hole  507  sized to contain distraction assembly  518  in a non-operative configuration. Distraction assembly  518  consists of an actuator in the form of a central pin  520 , having a region with a screw thread  529  located proximal to the medial portion of central pin  520 . Threaded sleeve  523  has a screw thread on the internal surface of the sleeve, and is movably attached to central pin  520  proximal to the medial portion of central pin  520  such that the threads of threaded sleeve  523  engages screw thread  529 . Extendable anchor members such as paddles  524  are attached to threaded sleeve  523  such that they are allowed to pivot with respect to the longitudinal axis of dowel  500 , and positioned within projection holes  526 . Projection holes  526  connect outer surface  510  with inner chamber  514 . A cam surface is disposed at the medial edge of the holes  526  and is configured to form an upward-angled edge opposite the medial edge cam surface such that longitudinal movement of paddles  524  toward and along the cam surface laterally from the medial location causes paddles  524  to pivot away, and extend outward, from dowel body  500   a . Paddles  524  are shown in extended position in  FIGS.  15  and  16   , and are shown in retracted position in  FIG.  17   . Extension of paddles  524  is accomplished by rotating central pin  520  in a paddle actuating direction using a drive instrument such as a standard screwdriver designed to engage drive slot  530  in the head of the actuator pin  520 . Drive slot  530  can alternatively be configured to accommodate other engagement instruments such as hex wrenches, box wrenches, Torx wrenches, Allen wrenches, or similar designs. 
     Paddles  524  proximal to the medial portion of dowel  500  are intended to engage the surface of the ilium within the interior of the SI joint once dowel  500  has been positioned within the pre-drilled holes through the SI joint surfaces and paddles  524  have been extended via rotation of central pin  520 . Continued rotation of central pin  520  following bone engagement by paddles  524  will urge the joint surfaces apart to impart a distraction force to the SI joint, since step  532  is engaged with the surface of the sacrum within the interior of the SI joint and prevents continued movement of dowel  500  through the pre-drilled hole in the sacrum, with the intent of stabilizing the SI joint by distracting the joint and inducing tension in the ligaments connecting the ilium and sacrum. 
     Illustrations showing one example of one embodiment of dowel  500  implanted with paddles in extended positions are shown in  FIGS.  18 A  (sacrum transparent) and  18 B (sacrum opaque). While this example shows the use of three dowels  500  with four paddles  524 , it should be understood that more or fewer dowels  500  may be implanted depending on individual patient needs and that dowel  500  may be designed with more or fewer paddles. 
     Dowel  500  and the components of distraction assembly  518  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  500  and compression assembly  518 . Human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite can be used for dowel  500 . The outer surface of dowel  500  and at least portions of compression assembly  518  may be modified to enable and/or promote bony tissue in-growth into those surfaces. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying substrate material to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     The next alternative joint fixation connector is in the form of a hollow dowels  600 ,  601  each having proximal and/or distal zones that expand radially upon insertion of an actuator in the form of a central pin. The expanded zone(s) serve to more fully engage the surrounding bone of the ilium and sacrum for positional fixation of the dowels  600 ,  601 . Two dowels  600  and  601  are described below; the first dowel  600  with only a proximal zone that can expand radially, and a second dowel  601  with the ability to expand radially in both proximal and distal zones. It should be understood that a dowel able to expand radially in only the distal zone is contemplated even though it is not described herein. 
     Dowel  600  has an elongate body  600   a  with no external screw threads and a hollow inner chamber  614  located at the proximal end of dowel body  600   a , that is implanted singly or in multiples, and is shown in  FIGS.  19 - 21   . Similar to previously described dowels, dowel body  600   a  has a relatively tapered distal end  602  to assist insertion into a pre-drilled hole through the SI joint surfaces and a relatively blunt proximal end  608  configured for contact with a mallet or other insertion instrument. Dowel body  600   a  is configured with central bore  604  which extends along the longitudinal axis of dowel  600  from inner chamber  614  through the center of distal end  602 . Alternatively, central bore  604  may extend from inner chamber  614  to cannula  606  located at the tip of distal end  602 . Cannula  606  would generally have a smaller diameter than central bore  604  to facilitate the use of a guide pin (not shown) intended to aid in placement of dowel  600  into a pre-drilled hole through the SI joint surfaces. 
     Dowel body  600   a  has a generally cylindrical primary shape, determined by arcuate surface  605 , and has one or more longitudinal ridges  634  extending out radially from and axially along the surface  605  to provide positional stability when implanted. Ridges  634  extend along the length of dowel  600  at least sufficient to engage the bone of the sacrum and ilium. In the embodiment shown, ridges  634  produce a cross-sectional profile for dowel body  600   a  that is generally square in shape. The use of alternate primary shapes (via alternate surfaces  605 ) or alternate number or shape of ridges  634  may produce cross-sectional shapes that are generally round, oval, triangular, square, etc. Surface  605  and ridges  634  can be configured and combined to provide specific desired cross-sectional profiles as well as a flexural stiffness for dowel  600  that varies depending on the direction of applied force. 
     Dowel body  600   a  is configured with slots  636  that traverse the entire thickness of dowel  600  from inner surface to outer surface  605  thereof. Slots  636  extend along the length of dowel  600  from proximal end  608  to holes  638 . The portion of dowel body  600   a  located between adjacent slots  636  defines expandable arm  640 . Holes  638  at the ends of the slots  636  are intended for mechanical stress relief when expandable arm  640  is forced into radial expansion. The position of holes  638  along the length of dowel body  600   a  determines the length of expandable arms  640 , which may extend nearly the entire length of dowel  600 . Holes  638  may be arranged to be equally spaced circumferentially about the dowel body  600   a  as shown in this embodiment, but may also be placed closer or farther apart to create expandable arms  640  with varying widths. The body  600   a  has internal screw threads  616  formed along the central bore  604  extending from holes  638  to distal end  602  configured to engage the screw threads of central pin  620 , and has a major diameter no larger than the diameter of central bore  604  extending from inner chamber  614  to holes  638 . 
     While four slots  636  are incorporated in this embodiment, it should be understood that a single slot  636  is sufficient to allow expansion of dowel  600  and may be used in an alternate embodiment. Likewise, two, three or more than four slots  636  may also be used in an alternate embodiment, and slots in any number need not align with the longitudinal axis in order to provide the expandability feature, e.g., angled or spiral slots. 
     An actuator in the form of central pin  620  is configured with a distal threaded section  622 , unthreaded or smooth section  624  and a proximate tapered or conical-shaped section  626 . Threaded section  622  is configured to engage threads  614  along central bore  604 , intermediate section  624  is configured to pass freely through central bore  604 , and conical-shaped section  626  is configured to slidably engage and cam against the surfaces of inner chamber  614  during insertion of central pin  620  into central bore  604 , forcing expandable arms  640  to radially expand.  FIG.  21    illustrates this embodiment with central pin  620  fully inserted into central bore  604 , resulting in dowel  600  with expandable arms  640  in expanded positions. Central pin  620  is configured with slot drive  630 , used for rotating central pin  620  to advance into central bore  604  using a drive instrument such as a standard screwdriver. Slot  630  can alternatively be configured to accommodate other engagement instruments such as hex wrenches, box wrenches, Torx wrenches, Allen wrenches, or similar designs. 
     The alternative dowel  601  has an elongate body  601   a  with slots at both the distal and proximal portions thereof, as shown in  FIGS.  22  and  23   . Dowel body  601   a  has generally the same shape, slot and hole configurations as described for dowel body  600   a  above, including cannula  606  located at the tip of distal end  602 , and a relatively blunt proximal end  608  configured for contact with a mallet or other insertion instrument, and slot  630  for central pin rotation. Dowel body  601   a  has a proximal section having inner chamber  613  and slots  636  extending from proximal end  608  to holes  638 , a distal section having inner chamber  617  configured with a tapered or conical shape that tapers to a closed or substantially closed distal end  602  (see  FIG.  22   ) prior to implantation and slots  637  extending from distal end  602  to holes  639 , and intermediate section with internal threads  615  formed to extend away central bore  605 . 
     Central pin  621  is configured with a distal unthreaded or smooth section  623 , an intermediate threaded section  625  and a proximate tapered or conical-shaped section  627 . Threaded section  625  has threads that are configured to engage threads  615  of central bore  605 , smooth section  623  is configured to slidably engage the surfaces of inner chamber  617  during insertion of central pin  621  into central bore  605 , and conical-shaped section  627  is configured to slidably engage the surfaces of inner chamber  613  during insertion of central pin  621  into central bore  605 . The slidable engagement of smooth section  623  with surfaces along inner chamber  617  and conical section  627  with surfaces along inner chamber  613  during insertion of central pin  621  into central bore  605  forces expandable arms  640  and  642  to radially expand, as shown in  FIG.  23   . Expansion of arms  640  drives the arms to further penetrate into the bone of the ilium, and expansion of arms  642  opens the distal end  602  of body  601   a  so that the arms  642  are driven to further penetrate into the bone of the sacrum. 
     Dowel  600  and central pins  620  and  621  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  600  and central pins  620  and  621 . The outer surface of dowel  600  may be modified to enable and/or promote bony tissue in-growth. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying substrate material to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     The following joint fixation device or connector provides positional fixation similar to dowel  600 , but does so using a radially expandable sleeve such as in the form deformable mesh which expands radially instead of movable arms. The placement of the mesh along the length of the dowel allows the possibility of compression or distraction of the SI joint and is described in detail below. 
     Dowel  700  has no external threads, a hollow inner chamber  514 , and can be implanted singly or in multiples, and is shown in  FIGS.  24 - 26   . In this device, elongate dowel body  700   a  is comprised of three tubular members or constructs: distal tube  760 , medial tube  770  and proximal tube  780 . Distal tube  760  has a relatively tapered distal end  702  to assist insertion into pre-drilled holes through the SI joint surfaces. While not shown in this embodiment, distal tube  760  may be configured with a cannula  706 , which is a hole bored generally along the longitudinal axis of distal tube  760  to facilitate the use of a guide pin (also not shown) intended to aid in placement of dowel  700  into a pre-drilled hole through the SI joint surfaces. 
     The dowel body  700   a  is shown as having an outer surface  705  that is arcuate, and preferably such that the body surface  705  has a circular cross-sectional configuration. While the circular profile is used to illustrate the concept, it should be understood that at least portions of the profile of dowel  700  could be non-circular, such as an oval, triangular, square, or other combinations of various numbers of flat, concave or convex surfaces. The use of non-circular profiles for dowel body  700   a  may be useful for a variety of reasons specific to the intended application, such as imparting varied flexural stiffness along the length of dowel  700 , increasing contact area with surrounding bone and improving positional stability. 
     Medial tube  770  has a single outer diameter  790 , the major outer diameter of dowel body  700   a . Distal tube  760  is configured to have two outer diameters along its length proximal to tapered distal end  702 , with an outer surface portion having the distal-most outer diameter  790  and an adjacent outer surface portion having a smaller proximal-most outer diameter  792  to form a step therebetween via a transversely extending outer shoulder surface. Proximal tube  780  is also configured to have two outer diameters along its length, with an outer surface portion having the proximal-most outer diameter  790  and an adjacent outer surface portion having a smaller distal-most outer diameter  792  to form a step therebetween via a transversely extending outer shoulder surface. 
     Medial tube  770  is configured to have two inner diameters along its length, with an inner medial surface portion having inner diameter  794  and adjacent end surface portions each having a larger inner diameter  796  extending from either end of the medial surface portion of medial tube  770 . This provides the medial tube  770  with a pair of transversely inner shoulder surfaces between the smaller diameter medial surface portion and the larger diameter end surface portions. Larger inner diameter  796  of medial tube  770  is sized to provide a slip fit (slidable contact) with the surface portions of distal tube  760  and proximal tube  780  having outer smaller diameter  792 . The axial length of the surface portions of distal tube  760  and proximal tube  780  having diameter  792  are sufficient for mounting of deformable mesh  750  in its undeformed-shape as manufactured, on the smaller diameter portions of the tubes  760  and  780  the combination of which defines the relaxed assembled state of dowel  700  as shown in  FIGS.  24  and  25   . 
     In this device deformable mesh  750  is tubular in shape as manufactured, and is configured to have an inner diameter to provide a slip fit (slidable contact) with the smaller diameter portions of distal tube  760  and proximal tube  780  having diameter  792 , and has the same general outer diameter  790  of medial tube  770 . The deformable mesh  750  has an axial length sized so that it is retained on the surface portions of distal tube  760  and proximal tube  780  having diameter  792 . More particularly, the distal mesh  750  is retained between distal and intermediate outer shoulder surfaces that extend transverse to smaller diameter surface portion of distal tube  760 . Similarly, the proximate mesh  750  is retained between intermediate and proximate outer shoulder surfaces that extend transverse to smaller diameter surfaces portion of proximal tube  780 . The medial portion of deformable mesh  750  is configured with holes that provide predictable radial expansion of deformable mesh  750  upon compression of the mesh  750 . The holes of deformable mesh  750  may be square in shape as shown in  FIG.  24   , or may alternatively be rectangular, circular, hexagonal or of any other shape that is able to produce predictable radial expansion. While deformable mesh  750  is tubular in shape in its manufactured state in this embodiment, it should be understood that alternative shapes may be used to match the alternative shapes of dowel  700  described above. 
     Distal tube  760 , medial tube  770  and proximal tube  780 , when assembled as shown in  FIGS.  24  and  25    to form dowel  700 , combine to form inner chamber  714  with sliding zones  774  and  776 . The inner surface of distal tube  760  is configured with a screw thread  762 . The inner surfaces of medial tube  770  and proximal tube  780  are smooth; with inner diameters  794  sufficient to accommodate unimpeded insertion and rotation of actuator in the form of central pin  720 . Siding zones  774  and  776  allow for the longitudinal movement of distal tube  760  and proximal tube  760 , respectively, along the longitudinal axis of dowel  700  within medial tube  770 . 
     Central pin  720  is configured for insertion into inner chamber  714  and threaded engagement with distal tube  760  via screw thread  722  of pin  720 . Central pin  720  is configured with drive slot  730 , used for rotating central pin  720  using a drive instrument such as a standard screwdriver. Drive slot  730  can alternatively be configured to accommodate other engagement instruments such as hex wrenches, box wrenches, Torx wrenches, Allen wrenches, or similar designs. Central pin  720  is configured with a relatively blunt proximal end  708  designed for contact with a mallet or other insertion instrument. 
     Once in place within the pre-drilled hole through the SI joint, dowel  700  is secured in position by rotating central pin  720  to cause distal tube  760  and proximal tube  780  to slide and be retracted within medial tube  770  due to the engagement of screw thread  762  of distal tube  720  with screw thread  722  of central pin  720 , resulting in reduction of the length of dowel  700  and compression of the distal and proximal meshes  750 . More particularly, rotation of the threaded pin actuator  720  causes the small diameter portion of the distal tube  760  to be drawn into the medial tube  770  along slide zone  774  until its end abuts the inner distal shoulder surface of the tube  770 . Rotation of the threaded pin actuator  720  also causes enlarged head of the pin actuator  720  to push the small diameter portion of the proximal tube  780  into the medial tube  770  along zone  776  until its end abuts inner proximal shoulder surface of the tube  770 . As dowel  700  is shortened in axial length, deformable meshes  750  located at the distal and proximal portions of dowel  700  are placed under compressive forces between the corresponding outer shoulder surfaces and relieve these forces through radial expansion. Deformable mesh  750  which has expanded radially will frictionally contact the surrounding ilium and sacrum bones of the SI joint, resulting in fixation of dowel  700  within the SI joint. An illustration of the present embodiment in completely shortened, retracted, or compressed form, with complete expansion of deformable mesh  750  and taking up of sliding zones  774  and  776  by the small diameter portions of tubes  760  and  780  is shown in  FIG.  26   . 
     Other embodiments of dowel  700  may include configurations where sliding zones  774  and  776  are not completely taken up upon complete retraction or compression. Alternate embodiments may also include placement of mesh  750  in distal and proximal regions of dowel  700  that provide for the engagement of the exterior surfaces of the ilium and sacrum of the SI joint to provide compression of the SI joint following expansion of mesh  750 , or placement of mesh  750  in relatively medial locations of dowel  700  that provide for the engagement of the interior surfaces of the ilium and sacrum of the SI joint to provide distraction of the SI joint following expansion of mesh  750 , with compression or distraction capable of providing stability to the SI joint. Other alternate embodiments may include the use of only one mesh  750 , or more than the two locations of mesh  750  that have been described herein. 
     The components comprising dowel  700  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for distal tube  760 , medial tube  770 , proximal tube  780 , mesh  750  and central pin  720 . Human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite can also be used for dowel  700 . At least portions of the outer surfaces of distal tube  760 , medial tube  770 , proximal tube  780  and mesh  750  may be modified to enable and/or promote bony tissue in-growth into those surfaces. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying substrate material to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     The following joint fixation device or connector provides positional fixation once implanted, using two screw threaded sleeves to engage the surrounding bone of the SI joint. This device can be implanted in fully-assembled form or be assembled in situ, wholly or in part. Variations of particular aspects of this device can induce compression or distraction of the SI joint as desired by the surgeon for joint stabilization prior to fusion. 
     The device can be in the form of a dowel  800  that is implanted singly or in multiples, and is shown in  FIGS.  27  and  28   . Dowel  800  is comprised of multiple components or members including proximal pin  820 , distal pin  830 , distal sleeve  840  and proximal sleeve  850 . In alternate embodiments, dowel  800  may also incorporate additional components as needed to facilitate insertion of the components and relative rotation of the threaded components as described below. 
     Distal pin  830  is configured similar to a machine bolt, with a pin head  832  and a pin threaded shank portion  834 . Pin head  832  has an outer diameter less than the diameter of the pre-drilled hole through the SI joint, but larger than the inner diameter of distal sleeve  840 . Pin threaded portion  834  is configured to engage the inner screw thread  844  of distal sleeve  840  and threaded hole or cylindrical cavity  824  of proximal pin  820 . 
     Proximal pin  820  is generally cylindrical in shape and can be configured with a smooth portion  822  having a diameter larger than the inner diameter of sleeve  850  but no greater than the minor diameter of thread  852  of sleeve  850 . The cylindrical cavity within smooth portion  822  is open to the distal end of proximal pin  820 , and is threaded to be configured to engage with threaded portion  834  of distal pin  830 . Proximal pin  820  has a screw threaded portion  826  configured to engage inner thread  856  of proximal sleeve  850 . Proximal end  828  of threaded portion  826  of proximal pin  820  may be configured with a slot used for rotating proximal pin  820  during implantation using an instrument such as a standard screwdriver, or can alternatively be configured to accommodate other engagement instruments such as hex wrenches, box wrenches, Torx wrenches, Allen wrenches, or similar designs. 
     Distal sleeve  840  is configured with outer thread  842  along its outer surface which is intended to engage the bone of the sacrum in the SI joint, and inner thread  844  along its inner surface which is configured to engage threaded portion  834  of distal pin  830 . 
     Proximal sleeve  850  is configured with outer thread  852  along its outer surface which is intended to engage the bone of the ilium in the SI joint, inner thread  856  along its inner surface which is configured to engage threaded portion  826  of proximal pin  820 , and radially enlarged end flange  854  which is intended to avoid over-insertion of dowel  800  by contacting the outer surface of the ilium of the SI joint. Flange  854  may have drive slots, holes, projections or other such features along its radial or proximal surfaces configured for engagement of a drive instrument to allow for rotation of proximal sleeve  850  relative to proximal pin  820 . 
     Dowel  800  can be completely assembled as shown in  FIG.  27    and threaded into a pre-drilled hole through the SI joint. Alternatively, distal pin  830 , distal sleeve  840  and proximal pin  820  can be assembled prior to implantation and threaded into a pre-drilled hole through the SI joint, with proximal sleeve  850  threaded onto distal pin  830  to complete the implantation of dowel  800 . This two-step in situ assembly may allow for a more customized fit of dowel  800  to an individual patient&#39;s anatomical requirements. Furthermore, dowel  800  may be entirely assembled in situ, first by insertion of distal pin  830  through a pre-drilled hole through the SI joint, followed by threaded assembly of distal sleeve  840  over distal pin  830  and engaging the bone of the sacrum, with subsequent threaded assembly of proximal pin  820  over the remaining portion of distal pin  830 , and finally with threaded assembly of proximal sleeve  850  over proximal pin  820  and engaging the bone of the ilium. 
     Sleeve  840  can have a smaller nominal diameter than proximal pin  820 , necessitating a hole through the sacrum that is smaller than that through the ilium. Sleeve  840  can have a diameter such that the distal surface of proximal pin  820  engages with the surface of the sacrum within the interior of the SI joint sufficient to avoid further movement of dowel  800  through the pre-drilled hole in the sacrum during insertion. 
     Proximal pin  820  can be configured with a relatively short axial length of smooth portion  822 , such that continued rotation of proximal sleeve  850  relative to proximal pin  820  following engagement of flange  854  with the outer surface of the ilium causes the inner surfaces of the sacrum and ilium of the SI joint to compress to increase the stability of the SI joint prior to fusion. 
     The components comprising dowel  800  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  800 . In particular, the design of distal pin  830  and proximal pin  820  allows for dowel  800  to be built with an overall stiffness ranging from extremely stiff, through use of a metal such as a titanium alloy, to a stiffness similar to that of bone through use of PEEK. Human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite can also be used for distal sleeve  840  and proximal sleeve  850  of dowel  800 . At least portions of the outer surfaces of distal sleeve  840  and proximal sleeve  850  may be modified to enable and/or promote bony tissue in-growth into those surfaces. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying substrate material to enable bony in-growth such as porous PEEK. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     The following joint fixation device or connector provides positional fixation once implanted, using two screw threaded sleeves which expand to further engage the surrounding bone of the SI joint. This device can be implanted in fully-assembled form or be assembled in situ. The threaded sleeves can be screwed into place during implantation, with the threads engaging the surrounding bone, prior to expansion of the sleeves, or the diameter of the pre-drilled hole through the SI joint can be such that the sleeves do not engage the surrounding bone until expansion of the sleeves. 
     This device can be in the form of a dowel  900  that is implanted singly or in multiples, and is shown in  FIGS.  29  and  30   . Dowel  900  is comprised of multiple components or members, including central pin  920 , distal sleeve  930 , central sleeve  940 , proximal sleeve  950  and tapered tube  960 . In alternate embodiments, dowel  900  may also incorporate additional components as needed to facilitate insertion of the components and relative rotation of the threaded components as described below. 
     Central pin  920  is configured with flared distal tip end  922 , smooth portion  926 , rail  927  and screw threaded portion  924  configured to engage inner screw thread  964  of tapered tube  960 . The flared configuration of distal tip  922  is such that it is operable to expand arms  939  of distal sleeve  930  during implantation of dowel  900 . Rail  927  projects from the surface of central pin  920 , extends from distal tip  922  to threaded portion  924  and is configured to slidably engage corresponding slots in the three sleeves to avoid rotation of central pin  920  during rotation of tapered tube  960  during final assembly and expansion of the distal and proximal sleeves. Distal tip  922  has an outer diameter less than the diameter of the pre-drilled hole through the SI joint, but larger than the inner diameter of distal sleeve  930 . Central pin  920  may be configured with drive slot  928  (indicated but not shown in  FIG.  30   ), used for rotating central pin  920  using an instrument such as a standard screwdriver. Drive slot  928  can alternatively be configured to accommodate other engagement instruments such as hex wrenches, box wrenches, Torx wrenches, Allen wrenches, or similar designs. 
     Distal sleeve  930  is configured with an outer surface having screw thread  932  along its outer surface which is intended to engage the bone of the ilium in the SI joint, a relatively smooth inner surface  934  having a channel  937 , and through slots  938 . For embodiments of dowel  900  where the bone-contacting sleeves are not intended to be screwed into the surrounding surfaces prior to expansion of the sleeves, surface features to enhance friction other than screw threads may be used, such as circumferential, longitudinal or angled ridges; cross-hatching; roughened surfaces, etc. Slots  938  extend through the entire sleeve wall, originate at the distal end, project along the longitudinal axis and terminate along the wall or body of sleeve  930  before reaching the proximate end thereof. Four slots are incorporated in this embodiment, defining arms  939  which are able to expand radially away from the central axis of sleeve  930 . It should be understood that a single slot  938  is sufficient to allow expansion of distal sleeve  930  and may be used in an alternate embodiment. Likewise, two, three or more than four slots  938  may also be used in an alternate embodiment, and slots in any number need not align with the longitudinal axis in order to provide the expandability feature, e.g., angled or spiral slots. Slots  938  may terminate at a hole passing through the wall of sleeve  930  which acts as a mechanical stress relief during expansion. Channel  937  is configured to slidably engage rail  927  of central pin  920 . 
     Central sleeve  940  has an outer diameter larger than the inner diameter of both sleeve  930  and sleeve  950  but no greater than the minor diameter of thread  932  of sleeve  930  and sleeve  952  of sleeve  950 . The inner diameter of central sleeve  940  is sized to allow central sleeve  940  to move freely over central pin  920  during assembly. The inner surface of central sleeve  940  has an inner channel  947  configured to slidably engage rail  927  of central pin  920 . 
     Proximal sleeve  950  is configured with an outer surface having screw thread  952  along its outer surface which is intended to engage the bone of the ilium in the SI joint, a relatively smooth inner surface  954 , flange  956 , inner channel  957  (indicated but not shown in  FIG.  30   ) and slots  958 . For embodiments of dowel  900  where the bone-contacting sleeves are not intended to be screwed into the surrounding surfaces prior to expansion of the sleeves, surface features of proximal sleeve  950  to enhance friction other than screw threads  952  may be used, such as circumferential, longitudinal or angled ridges; cross-hatching; roughened surfaces, etc. Slots  958  extend through the entire sleeve wall, originate at the proximal surface of flange  956 , project along the longitudinal axis and terminate along the wall or body of sleeve  950  before reaching the distal end thereof. Four slots are incorporated in this embodiment, defining arms  959  which are able to expand radially away from the central axis of sleeve  950 . As with distal sleeve  930 , it should be understood that a single slot  958  is sufficient to allow expansion of distal sleeve  950  and may be used in an alternate embodiment. Likewise, two, three or more than four slots  958  may also be used in an alternate embodiment, and slots in any number need not align with the longitudinal axis in order to provide the expandability feature, e.g., angled or spiral slots. Slots  950  may terminate at a hole passing through the wall of sleeve  950  which acts as a mechanical stress relief during expansion. Flange  956  is intended to avoid over-insertion of dowel  900  by contacting the outer surface of the ilium of the SI joint. Flange  956  may have drive slots, holes, projections or other such features along its radial or proximal surfaces configured for engagement of an instrument for holding proximal sleeve  950  in place while rotating tapered tube  960 . 
     Tapered tube  960  has a screw threaded inner surface  964  configured to engage threaded portion  924  of central pin  920 , and a relatively smooth outer surface with a diameter that increases from the distal end to the proximal end of tapered tube  960 . The tapered configuration of the tube  960  is such that it is operable to expand arms  959  of proximal sleeve  950  during implantation of dowel  900  as shown in  FIG.  29   . The proximal surface of tapered tube  960  may have drive slots, holes, projections or other such features along its radial or proximal surfaces configured for engagement of a rotary drive instrument to allow for rotation of tapered tube  960  relative to proximal pin  920 . 
     Dowel  900  can be completely assembled, but without complete rotation of tapered tube  960  and sleeve arm expansion, and inserted and screwed into a pre-drilled hole through the SI joint using slot  928  of central pin  920 . Tapered tube  960  is screwed into final position, radially expanding arms  939  of distal sleeve  930  and arms  959  of proximal sleeve  950  by compression of the tapered portions of central pin  920  and tapered tube  960  into their corresponding sleeves, as shown in  FIG.  29   . Alternatively, the pre-drilled hole through the SI joint may be large enough to avoid engagement of dowel  900  with the surrounding bone prior to expansion of arms  939  and  959 , allowing dowel  900  to be positioned without needing to screw it in, followed by screwing tapered tube  960  into final position to radially expand arms  939  and  959  to engage the surrounding bone of the SI joint. 
     Alternatively, dowel  900  may be entirely assembled in situ, first by insertion of central pin  920  through a pre-drilled hole through the SI joint having a diameter sufficiently large to enable placement of each sleeve without rotation, followed by insertion of distal sleeve  930 , central sleeve  940 , and proximal sleeve  950  through the hole in the SI joint and over central pin  920  and alignment of channels  937 ,  947  and  957  with rail  927 . Once in place, tapered tube  960  is screwed into final position to radially expand arms  939  and  959  to engage the surrounding bone of the SI joint. 
     Distal sleeve  930  can have a smaller diameter than central sleeve  940 , necessitating a hole through the sacrum that is smaller than that through the ilium. Distal sleeve  930  can have a diameter such that the distal surface of central sleeve  940  engages with the surface of the sacrum within the interior of the SI joint sufficient to prevent further movement of dowel  900  through the pre-drilled hole in the sacrum for added security of placement during insertion. 
     The components comprising dowel  900  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for dowel  900 . In particular, the design of central pin  920  and central sleeve  930  allows for dowel  900  to be built with an overall stiffness ranging from extremely stiff, through use of a metal such as a titanium alloy, to a stiffness similar to that of bone through use of PEEK. Human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite can also be used for distal sleeve  930  and proximal sleeve  950  of dowel  900 . At least portions of the outer surfaces of distal sleeve  930  and proximal sleeve  950  may be modified to enable and/or promote bony tissue in-growth into those surfaces. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying substrate material to enable bony in-growth such as porous PEEK. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     The following joint fixation device or connector is comprised of two threaded bone-contacting sleeves and an expansion member, which when fully assembled in situ engages and expands into the bone of a synovial joint, such as the ilium and sacrum, to secure the fixation device in place and thereby keep the device from retracting from the bone. This concept also provides the capability for SI joint compression or distraction while simultaneously stabilizing the joint. Although this embodiment is described with reference to the SI joint, it is understood that this embodiment may have applicability in other synovial joints. 
     This device can be in the form of a multipiece fastener or dowel  1000  that is implanted singly or in multiples, and is shown in  FIGS.  31 ,  32 ,  33 , and  34   . As shown in  FIGS.  31  and  32   , dowel  1000  is comprised of an expansion member in the form of central pin  1020 , a first expandable fastener in the form of outer sleeve  1040 , and a second expandable fastener in the form of inner sleeve  1030 . Advantageously, the inner sleeve  1030  is configured to both fix the adjacent first and second bones of synovial joint in conjunction with the outer sleeve  1040 , as well as expand an expandable portion of the outer sleeve  1040  to firmly fix the outer sleeve to the first bone. The central pin is preferably configured to expand an expandable portion of the inner sleeve  1030  to firmly fix the inner sleeve to the second bone of the synovial joint. A longitudinal cross-sectional view of the assembled dowel is shown in  FIG.  34   . 
     Outer sleeve  1040  is configured with radially enlarged proximal annular end flange  1046 , having an outer diameter that is greater than the pre-drilled hole in the ilium sufficient to allow abutment surface  1045  of outer sleeve  1040  to seat against the outer surface of the ilium of the SI joint to avoid over-insertion during implantation. Outer sleeve  1040  includes an outer bone engaging surface including a helical screw threaded portion  1044  that is configured to be inserted in the bone material of the ilium  20  to secure the position of outer sleeve  1040 . Outer sleeve  1040  has at least one slot  1048  positioned to extend axially along threaded portion  1044  extending to the distal end of outer sleeve  1040 , to provide at least one radially expandable portion to allow at least part of threaded portion  1044  of outer sleeve  1040  to expand radially outwardly. The embodiment shown in  FIGS.  31  and  32    has four slots  1048 , creating four arms  1042  that are able to expand radially, although it should be understood that any number of slots may be used and they need not align with the longitudinal axis in order to provide the expandability feature, e.g., angled or spiral slots. The inner annular surface of outer sleeve  1040  defines a longitudinally extending passage that is relatively smooth, and has a diameter that decreases from the proximal portion of outer sleeve  1040  to the distal portion  1049  of outer sleeve  1040 , creating an interference fit with smooth annular proximal portion  1032  of inner sleeve  1030  sufficient to provide for subsequent radial expansion of the threaded portion  1044  upon full insertion of inner sleeve  1030  within outer sleeve  1040 . The outer sleeve  1040  preferably includes a drive structure, such as a hex head, at the distal end for engaging with a rotary driving tool, such as a screw driver, for driving the outer sleeve into the ilium. 
     Inner sleeve  1030  is configured with a radially enlarged proximal annular end flange  1036 , having an outer diameter that is greater than the inner diameter of outer sleeve  1040  sufficient to allow the flange  1036  of the inner sleeve  1030  to seat against an interior shoulder abutment surface  1047  of flange  1046  of outer sleeve  1040 . The inner sleeve  1030  is axially longer than the outer sleeve  1040  and has an outer surface with a smooth unthreaded shaft portion  1032  located proximally for engaging with the interior surface of the outer sleeve  1040  as shown, and helical screw threaded portion  1034  is located distally to extend beyond the distal end of the outer sleeve  1040  when the inner sleeve  1030  is seated therein ( FIG.  32   ) and is configured to engage the bone of the sacrum  10  to secure the position of inner sleeve  1030  and the entire assembled dowel  1000 . The screw threaded shaft portion  1034  is preferably tapered radially outward in a distal to proximal direction. Inner sleeve  1030  has at least one slot  1038  positioned to extend axially along threaded portion  1034  to the distal end of inner sleeve  1030 , providing a radially expandable portion of the threaded portion of inner sleeve  1030  configured to expand in diameter. The embodiment shown in  FIGS.  31  and  32    has four slots  1038 , creating four arms  1031  that are configured to radially expand, although it should be understood that any number of slots may be used, and they need not align with the longitudinal axis in order to provide the expandability feature, e.g., angled or spiral slots. 
     The interior surface of inner sleeve  1030  defines a longitudinally extending passage having a relatively smooth annular distal portion and is internally threaded along a screw threaded proximal portion  1033  configured to engage the external screw threads  1022  of central pin  1020 . As shown in  FIG.  34   , the inner surface of the inner sleeve  1030  includes an abutment surface  1035  extending transverse to the longitudinal axis  30  of the inner sleeve  1030  for engaging with a corresponding abutment surface  1028  of central pin  1020  to provide a limit on how far the central pin may be inserted into the inner sleeve. The diameter of the inner surface of inner sleeve  1030  decreases from the proximal portion of inner sleeve  1030  to the distal portion  1037  of inner sleeve  1030 , creating an interference fit with smooth portion  1024  of central pin  1020  sufficient to provide for subsequent radial expansion of the threaded portion  1034  upon full insertion of central pin  1020  within inner sleeve  1030 . 
     Central pin  1020  is configured with hex-shaped pin head  1026 , screw threaded proximal portion  1022  and relatively smooth annular distal portion  1024 . Pin head  1026  is intended to be engaged by a rotary drive instrument following implantation of outer sleeve  1040 , inner sleeve  1030 , and insertion of central pin  1020  through inner sleeve  1030 . In alternative embodiments, pin head  1026  may have more or less than six flat surfaces, creating other than a hex shape, or have shaped depressions such as a slot, square hole, hex hole, etc. or other internal and/or external configurations known to those skilled in the art that are able to transmit rotational movement from an instrument to central pin  1020 . Threaded proximal portion  1022  is configured to threadingly engage the threaded portion  1033  of the internal surface of inner sleeve  1030 . 
     The SI joint is generally prepared for implantation of dowel  1000  by drilling a first recess sized to accommodate outer sleeve  1040  only through the ilium bone  20  of the SI joint. Next, a second recess is formed in the sacrum bone  10  of the SI joint, sized to accommodate a portion of the inner sleeve  1030  and centered on the first recess. The order in which the recesses are drilled may be reversed, or the recesses may be formed simultaneously. Alternatively, the dowel may include self-tapping threads such that forming recesses in the bone prior to insertion of the dowel is unnecessary. Dowel  1000  is generally implanted by first screwing outer sleeve  1040  into the drilled hole through the ilium of the SI joint until the abutment surface  1045  of flange  1046  is seated against the ilium. Inner sleeve  1030  is then inserted through outer sleeve  1040  and screwed into the drilled hole through the sacrum of the SI joint until flange  1036  is seated against the interior abutment surface  1047  of flange  1046 . Passage of smooth portion  1032  of inner sleeve  1030  through threaded portion  1044  of outer sleeve  1040  causes the arms  1042  at the distal portion of outer sleeve  1040  to radially expand and further tightly engage the bone of the ilium to fix the outer sleeve thereto. Central pin  1020  is then inserted through inner sleeve  1030  and screwed into inner sleeve  1030  until central pin  1020  is fully seated against inner sleeve  1030 . Passage of smooth portion  1024  of central pin  1020  through threaded portion  1034  of inner sleeve  1030  causes the arms  1031  at the distal portion of inner sleeve  1030  to radially expand and further tightly engage the bone of the sacrum to fix the inner sleeve thereto.  FIG.  32    is an illustration of dowel  1000  following complete implantation and in situ assembly. 
     The overall length of outer sleeve  1040  can be varied during manufacture to produce a dowel  1000  that provides either compression or distraction of the SI joint. An outer sleeve  1040  with a short overall length, which limits protrusion of the distal end of the sleeve into the interior of the SI joint, will work in conjunction with inner sleeve  1030  as the inner sleeve  1030  is threaded into the sacrum  10  to pull the sacrum towards the ilium of the SI joint, stabilizing the joint through compression. Alternatively, outer sleeve  1040  can be configured with an overall length that protrudes through the ilium bone  20  and across the SI joint to abuttingly contact the joint surface of the sacrum  10  with an abutment surface  1041  at the distal end of the outer sleeve  1040 . Abutment surface  1041  extends transversely to the longitudinal axis  30  such that the outer sleeve  1040  will abut the surface of the sacrum  10  and not protrude into the sacrum. Preferably, the threaded outer surface  1044  of the outer sleeve  1040  is configured to keep the outer sleeve from self-tapping into the sacrum. Once the abutment surface  1040  contacts the sacrum, further insertion of the outer sleeve will apply force to the sacrum and begin to push the ilium away from the sacrum, distracting the joint and inducing tension in the ligaments connecting the ilium and sacrum. Implantation of inner sleeve  1030  and central pin  1020  as described above provides further fixation and stabilization of the distracted joint. Alternatively, implantation of outer sleeve  1040  alone may supply sufficient fixation and stabilization and avoid the need for implantation of the remaining components described above. 
     Illustrations showing one example of one embodiment of dowel  1000  implanted is shown in  FIG.  33   . While this example shows the use of three dowels  1000 , it should be understood that more or fewer dowels  1000  may be implanted depending on individual patient needs. 
     The components comprising dowel  1000  may be made of any biologically inert material with sufficient strength, toughness and other material properties appropriate for their function and use in joint stabilization, including (but not limited to) plastics, metals and ceramics. Suitable plastics include polyether-ether-ketone (PEEK), polyether-ketone-ketone (PEKK) and other related polyaryls, which may be combined with carbon fiber or other fillers to form composites with enhanced material properties. Suitable metals include titanium and titanium alloys such as Ti6Al4V and Ti6Al7Nb (commonly used in medical device implants) and various stainless steel formulations. Combinations of the materials listed herein can be used to produce mechanical properties most desired for central pin  1020 , inner sleeve  1030  and outer sleeve  1040 . Human cadaver donor bone and bone analogues such as coral-derived hydroxyapatite can also be used for inner sleeve  1030  and outer sleeve  1040 . At least portions of the outer surfaces of inner sleeve  1030  and outer sleeve  1040  may be modified to enable and/or promote bony tissue in-growth into those surfaces. Surface enhancements may include a roughened surface, or application of a porous (open-pore) version of the underlying substrate material to enable bony in-growth such as porous PEEK or sintered titanium. Other surface enhancements can include application of a non-similar material such as hydroxyapatite. 
     While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.