Patent Publication Number: US-9839448-B2

Title: Implant placement

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/891,326, filed Oct. 15, 2013, and titled “IMPLANT PLACEMENT,” which is herein incorporated by reference in its entirety for all purposes. 
    
    
     INCORPORATION BY REFERENCE 
     All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     FIELD 
     Embodiments of the invention relate generally to implant placement into bone. More specifically, embodiments of the invention relate to implant placement across the sacro-iliac joint. 
     BACKGROUND 
     Many types of hardware are available both for the fixation of bones that are fractured and for the fixation of bones that are to be fused (arthrodesed). 
     For example, the human pelvic girdle (see  FIGS. 1 and 2 ) is made up of three large bones joined together by three relatively immobile joints. One of the bones is called the sacrum and it lies at the bottom of the lumbar spine, where it connects with the L5 vertebra. The other two bones are commonly called “pelvic bones” and are technically referred to as the right ilium and—the left ilium. The sacrum connects with both pelvic bones at the sacroiliac joint (in shorthand, the SI-Joint). 
     The SI-Joint functions to transfer forces from the spine to the lower extremities, and vice-versa. The SI-Joint has been identified as the pain generator in up to 22% of patients who present with lower back pain. 
     Sacroiliac joint fusion is a surgical procedure that is performed to alleviate pain coming from the SI joint in patients who have failed to receive adequate pain relief with non-surgical treatments of the SI joint. Some conditions of the SI joint that may be treated with SI joint fusion (arthrodesis) are: degenerative sacroiliitis, inflammatory sacroiliitis, iatrogenic instability of the sacroiliac joint, osteitis condensans ilii, or traumatic fracture dislocation of the pelvis. Currently, screws and screws with plates are used as the standard instrumentation for sacro-iliac fusion. Historically, an SI joint fusion consisted of an open surgical approach to the SI joint from an anterior, a posterior, or a lateral direction. The surgeon would then debride (remove) the cartilage from the articular portion of the joint and the interosseous ligament from the fibrous portion of the joint. These open approaches require a large incision and deep soft tissue dissection to approach the damaged, subluxed, dislocated, fractured, or degenerative SI joint. 
     A typical technique for placing implants involves placement of one or multiple implants from a lateral to medial direction across the SI-Joint. These implants are placed with a starting point on the lateral aspect of the ilium. The implants are then directed across the ilium, across the sacroiliac joint and into the sacrum. Regarding implant position, care is taken to avoid impinging on neural and vascular structures, including neural tissue within the neural foraminae or spinal canal. In addition, care must be taken to place the implants across the SI joint and avoid the leading tip(s) of the implant(s) violating the osseous envelope of the sacrum. However, the density of the bone in different portions of the sacrum is not typically a consideration during implant placement. 
     Accordingly, it would be desirable to provide systems and methods for placing multiple implants across the SI-Joint into those portions of the sacrum with relatively higher bone density. Implants placed into denser (stronger) bone will demonstrate improved short-term and long-term mechanical stability of the implant construct spanning the SI-Joint post implantation. 
     SUMMARY OF THE DISCLOSURE 
     The present invention relates generally to implant placement into bone. More specifically, embodiments of the invention relate to implant placement across the sacro-iliac joint. 
     In some embodiments, a method of implanting a plurality of implants in the SI-joint is provided. The method can include inserting a plurality of guide pins into an anterior portion of the SI-joint that is associated with articular cartilage; creating a bore around each of the plurality of guide pins; and inserting an implant into each bore such that each implant is located in an anterior portion of the SI-joint that is associated with articular cartilage. 
     In some embodiments, the method further includes obtaining a lateral view of the pelvis; and identifying a plurality of anatomical landmarks in the lateral view including the alar line, the posterior sacral body line, the anterior sacral body line, and the anterior cortex of the sacral alar, wherein the plurality of guide pins are inserted with reference to at least one anatomical landmark. 
     In some embodiments, at least one guide pin and implant are inserted anterior of the anterior sacral body line in the lateral view. 
     In some embodiments, at least two guide pins and implants are inserted anterior of the anterior sacral body line in the lateral view. 
     In some embodiments, the at least one guide pin and implant that is inserted anterior of the anterior sacral body line in the lateral view is angled with respect to the horizontal axis in the inlet view. 
     In some embodiments, at least two guide pins and two implants are placed parallel to the alar line. 
     In some embodiments, the plurality of guide pins are parallel in the outlet view. 
     In some embodiments, the method further includes identifying a target for the plurality of guide pins in the inlet view, wherein the target is located in the middle of the sacral body. 
     In some embodiments, the method further includes advancing the guide pins towards the target in the inlet view. 
     In some embodiments, the implants are non-colinear in the lateral view. 
     In some embodiments, the implants have an elongate body with a longitudinal axis and a rectilinear cross-sectional profile transverse to the longitudinal axis. 
     In some embodiments, the implants are screws. 
     In some embodiments, a system for fixation of the SI-joint is provided. The system can include a first elongate implant that is inserted into the SI-joint through a cephalad portion of the articular cartilage in the SI-joint; a second elongate implant that is inserted into the SI-joint through a middle portion of the articular cartilage in the SI-joint, wherein the second elongate implant is caudal and anterior to the first elongate implant; and a third elongate implant that is inserted into the SI-joint through a caudal portion of the articular cartilage in the SI-joint, wherein the third elongate implant in caudal to the second elongate implant. 
     In some embodiments, the first elongate implant is inserted into the SI-joint caudal to the alar line and between the posterior sacral body line and the anterior sacral body line in a lateral view. 
     In some embodiments, the second elongate implant is inserted into the SI-joint caudal to the alar line and anterior to the anterior sacral body line in a lateral view. 
     In some embodiments, the third elongate implant is inserted into the SI-joint anterior to the anterior sacral body line in a lateral view. 
     In some embodiments, the first elongate implant is positioned horizontally in an inlet view and the second elongate implant is angled with respect to the first elongate implant in the inlet view. 
     In some embodiments, the first elongate implant, the second elongate implant, and the third elongate implant each have an elongate body with a longitudinal axis and a rectilinear cross-sectional profile transverse to the longitudinal axis. 
     In some embodiments, the first elongate implant, the second elongate implant, and the third elongate implant each are screws. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
         FIGS. 1 and 2  illustrate anterior and posterior views of the pelvic girdle. 
         FIG. 3  illustrates the position of a patient on a Jackson table for CT imaging of the pelvic girdle. 
         FIGS. 4A-4C  are three imaging views of the pelvis, a lateral view of the sacrum shown in  FIG. 4A , an inlet view of the pelvis shown in  FIG. 4B , and an outlet view of the pelvis shown in  FIG. 4C . 
         FIG. 5  illustrates an annotated lateral radiographic view of the lower lumbar spine and sacrum that shows various anatomical landmarks. 
         FIGS. 6A and 6B  illustrate landmarks in the lateral view that can be used to position and orient the C-arm to obtain a true lateral view. 
         FIGS. 7A and 7B  illustrate locations on the patient&#39;s skin that may be marked to provide visual guidance or mapping that correlates with the position of various osseous landmarks identified using the lateral radiographic view. 
         FIGS. 8A and 8B  illustrate how the skin marks in  FIGS. 7A and 7B  may be used as a guide to align the C-arm in the outlet view and the inlet view. 
         FIGS. 9A and 9B  illustrate anatomical features and landmarks of the outlet view and the inlet view, respectively. 
         FIGS. 9C and 9D  illustrate a lateral view and an inlet view that shows that the anterior cortex of the sacral ala is visible as the most anterior aspects of the sacrum. 
         FIGS. 10A and 10B  are lateral views that illustrate an embodiment of an implant target zone that results in the implants passing through the articular or hyaline cartilage portion of the SI-joint rather than the fibro cartilaginous portion of the SI-joint. 
         FIG. 10C  illustrates a first implantation site placed with reference to both the alar line and the posterior sacral body line. 
         FIG. 10D  illustrates a second implantation site that can be placed with reference to the alar line and the location of the first implantation site such that the second implantation site is inferior (caudal) and anterior to the first implantation site and placed in a middle portion of articular cartilage, near the vertex of the boomerang. 
         FIG. 10E  illustrates a third implantation site that can be placed with reference to the anterior sacral body line and the second implantation site, so that the third implantation site is positioned in an inferior (caudal) portion of the articular cartilage. 
         FIGS. 11A-11C  illustrate the orientation of a first guide pin inserted into the first implantation site in the lateral view, the inlet view, and the outlet view. 
         FIGS. 12A-12C  illustrate the orientation of a second guide pin inserted into the second implantation site in the lateral view, the inlet view, and the outlet view. 
         FIGS. 13A-13C  illustrate the orientation of a third guide pin inserted into the third implantation site in the lateral view, the inlet view, and the outlet view. 
         FIGS. 14A-14C  illustrate the orientation of the three guide pins in the lateral view, the inlet view, and the outlet view. 
         FIGS. 15A-15C  illustrate another embodiment of the third guide pin position and orientation when the third implantation site is located in between the posterior sacral body line and the anterior sacral body line in the inlet view. 
         FIGS. 16A and 16B  illustrate an assembly for receiving the guide pin. 
         FIG. 17  illustrates a pin depth gage for determining the depth of insertion of the pin. 
         FIG. 18  illustrates removal of a pin sleeve from the guide pin. 
         FIG. 19  illustrates insertion of a drill over the guide pin. 
         FIG. 20  illustrates drilling a bore through the lateral cortex of the sacrum in the outlet view. 
         FIG. 21  illustrates removal of the drill after drilling the bore. 
         FIG. 22A-22C  illustrate alignment of a soft tissue protector with reference to anatomical landmarks so that the faces of the rectilinear implant are properly oriented with respect to the anatomical landmarks. 
         FIG. 23  illustrates shaping the bore with a broach. 
         FIG. 24  illustrates broach advancement across the SI-joint. 
         FIGS. 25A and 25B  illustrate implant insertion over the guide pin. 
         FIG. 26  illustrates implant advancement. 
         FIGS. 27A and 27B  illustrates implant advancement under CT imaging. 
         FIG. 28  illustrates implant placement under CT imaging. 
         FIGS. 29A and 29B  illustrate various types of implants that can be implanted across the SI-joint using the procedures described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Pre-Op Preparation and Patient Setup 
     A computed tomography (CT) scan may be taken of the pelvis and SI-Joints prior to surgery to check for anatomic anomalies and to identify osseous landmarks useful for implant placement. The patient may be placed on a flat radiolucent table, such as a Jackson table, to facilitate intra-operative imaging. Although one C-arm is typically employed, some surgeons may employ two C-arms, with one C-arm set in the lateral position and the other C-arm rotatable between the inlet and outlet positions, as further described below. As illustrated in  FIG. 3 , the patient may be placed on a flat Jackson type imaging table  300  with towel rolls  302  or other support structures placed transversely under the patient&#39;s chest and waist. This position allows the abdomen to hang free resulting in diminished intraabdominal pressure. Pillows  304  or other support structures can be placed under the patient&#39;s feet to relax the knees. After the patient has been properly positioned, a lateral view, an inlet view, and an outlet view may be taken of the SI-Joint and pelvis. The surgeon may identify any abnormalities that could interfere with the procedure and may also confirm that the anatomical landmarks used to guide implant placement are visible. 
     Intraoperative Imaging 
     As illustrated in  FIGS. 4A-4C , three imaging views, a lateral view of the sacrum shown in  FIG. 4A , an inlet view of the pelvis shown in  FIG. 4B , and an outlet view of the pelvis shown in  FIG. 4C , may be used to facilitate safe insertion and placement of instruments, tools and implants during the procedure. These fluoroscopic views allow the surgeon to identify various anatomical structures and landmarks, which may be used to assist the surgeon in placing the implants into a safe and biomechanically advantageous position. These intra-operative radiographic views may also be acquired using intra-operative CT scans and/or x-ray imaging. 
       FIG. 5  illustrates an annotated lateral radiographic view of the lower lumbar spine and sacrum. Various landmarks that may be used to facilitate implant placement are notated on this image. In the lateral view, the sacrum will appear as a curved wedge, with the posterior sacral cortical wall and the anterior sacral cortical wall being readily visible. In standard radiographic views denser tissues such as bone generally show up as white or light grey and the less dense soft tissues generally show up as black or dark grey. The posterior sacral cortical wall and the anterior sacral cortical wall appear as slightly curved dense white lines that define a triangular wedge shape on the lateral view. Other osseous anatomic landmarks useful for orientation and positioning include the sacral ala, the pelvic brim, the greater sciatic notch, the superior endplate of the S1 vertebrae and the inferior endplate of the L5 vertebrae. 
     The C-arm may be positioned to allow a true orthogonal lateral view of the sacrum and lower lumbar spine.  FIGS. 6A and 6B  illustrate landmarks in the lateral view that can be used to position and orient the C-arm to obtain this true lateral view. An initial lateral fluoroscopic image is obtained and then the C-arm positioning is adjusted until certain anatomic landmarks (left and right) are superimposed on the lateral image signaling that the C-arm is aligned correctly. For example, the disk space between the endplates of L5 and S1 may be aligned based on the initial lateral fluoroscopic imaging and subsequent adjusted images until the visualized endplates of L5 and S1 are parallel and in focus, which occurs when the disk space is perpendicular to the image plane. At this point the two sciatic notches, one from the right ilium and one from the left ilium, should overlap on the lateral fluoroscopic images. The alignment is considered final when the left and right iliac cortical densities, which are also known as the alar lines, are directly superimposed on the lateral fluoroscopic image. The C-arm and the table are adjusted in multiple planes until the alignment described above is achieved. 
     As illustrated in  FIGS. 7A and 7B , once a true lateral view has been obtained, the patient&#39;s skin may be marked to provide visual guidance or mapping on the patient&#39;s skin that correlates with the position of various osseous landmarks identified using the lateral radiographic view. For example, the skin may be marked along the sacral ala and pelvic brim to create a first mark  700 , and the skin along the posterior cortex of the sacral body can be marked to create a second mark  702 , as illustrated in  FIGS. 7A and 7B . In addition, a third mark  704  may be placed overlying the anterior cortex of the sacral body. One or more guide pins, which are easily visible under the C-arm image generated by the fluoroscope, can be used to help locate the landmarks, such as the sacral ala and the posterior cortex of the sacral body. Other radiopaque markers may be used in place of the pin to help located the position of the landmarks. The surgeon may lay the pin against the skin over where the underlying osseous landmark is predicted or estimated to be located, and a C-arm image is used to confirm whether the pin is in the correct position. If the pin(s) are correctly positioned over the landmarks, the mark is then drawn on the skin. If the pin(s) are not correctly positioned, the surgeon may reposition the pin(s) and check pin placement under fluoroscopy until the pin(s) are correctly positioned. 
     As illustrated in  FIGS. 8A and 8B , the skin marks  700 ,  702  may be used as a guide to align the C-arm in the outlet view and the inlet view. For example, in some instances, for an outlet view, the C-arm detector  800  is positioned over the patient&#39;s lower back and oriented perpendicular or normal to the mark  702  identifying the posterior sacral body, and for an inlet view, the C-arm detector  800  is positioned over the patient&#39;s legs and oriented parallel to the mark  702  identifying the anterior cortex of the sacral body. The C-arm may then be adjusted in both the outlet and the inlet views until the underlying osseous anatomy is clearly visualized. For example, in the outlet view the C-arm can be adjusted and oriented such that the pubic symphysis and S2 neural foramina are superimposed on the fluoroscopic image. In some embodiments, the neural foramina of S1, S2, S3 or S4 may be aligned with the pubic symphysis. In the inlet view the C-arm can be adjusted and oriented such that the sacral prominence of S1 appears as a thin line on the fluoroscopic image. In addition, in some embodiments, the C-arm detector  800  can be placed under the table (not shown) instead of above the patient. 
       FIGS. 9A and 9B  illustrate anatomical features and landmarks of the outlet view and the inlet view, respectively. The outlet view is an anterior to posterior view of the pelvis and sacrum that provides a clear visualization of the sacral neuro-foramina, such as the S1 foramen and the S2 foramen, as well as other structures such as the inferior endplate L5 and the superior endplate S1. The inlet view is an anterior to posterior view of the pelvis that provides a clear visualization of the ventral (anterior) cortex of the sacrum. To obtain the inlet view, the C-arm of the fluoroscope is tilted towards the patient&#39;s feet until the dense cortical line of the S1-S2 vestigial disk directly overlies the dense cortical line of the sacral promontory. In the inlet view, the sacral canal, the ventral cortex of the sacral body and the sacral alar areas are visualized by the surgeon. Visualization of the osseous anatomy of the sacral ala and the sacral canal allows the surgeon to insert the implants across the SI-Joint, to keep the leading portion of the implants positioned within the sacral ala while avoiding damage to the neural tissues and vascular structures. In addition, left and right oblique inlet and outlet views can be obtained to better visualize the sacroiliac joint and surrounding sacral foramina. With the C-arm in the inlet position, an oblique inlet image can be obtained by rotating the C-arm approximately 15 degrees to either side of the patient such that the C-arm&#39;s beam is approximately parallel to the SI-Joint and the lateral cortical walls of the S1 and S2 foramina. Similarly, with the C-arm in the outlet position, an oblique outlet image can be obtained by rotating the C-arm approximately 15 degrees to either side of the patient such that the C-arm&#39;s beam is approximately parallel to the SI-Joint and the lateral cortical walls of the S1 and S2 foramina. 
     Implant Placement 
     Proper implant placement across the SI-joint is determined by the surgeon selecting and maintaining the correct implant starting position, the correct implant insertion trajectory, and the correct implant length, which may all be determined with appropriate preoperative planning. Preoperative planning includes careful evaluation of osseous anatomy to identify the various landmarks described herein using CT scans and plane radiographs. Safe placement of implants involves placing the implants across the SI joint keeping the sacral portion of the implants within the osseous confine of the sacrum while avoiding the nerve tunnels of the spinal canal and neuroforamina, avoiding malposition of the implant in the dysplastic sacrum, and avoiding in-out-in positioning secondary to the concavity of the ventral surface of the sacral ala. 
       FIGS. 10A and 10B  are lateral views that illustrate an embodiment of an implant target zone  1000  that results in the implants passing through the articular or hyaline cartilage portion of the SI-joint rather than the fibro cartilaginous portion of the SI-joint. The articular cartilage is associated with denser and stronger subchondral bone that can improve implant stability. Therefore, implants can be implanted into the denser and stronger subchondral bone by passing through the articular bone outlined by the implant target zone  1000 . The location of the articular cartilage is shown as a boomerang or V-shaped outline that is located generally along the anterior portion of the SI-joint. The implant target zone  1000  is shown with reference to the various anatomical landmarks in the lateral view, including the alar line  700 , the posterior sacral body line  702 , and the anterior sacral body line  704 . In addition, the anterior portion of the implant target zone  1000  is defined by the anterior cortex of the sacral ala, which is the curved portion of the anterior sacrum that extends from the sacral body to the ilium. In  FIGS. 10A and 10B , the anterior portion of the implant target zone  1000  includes the portion of the implant target zone  1000  that is anterior the anterior sacral body line  704 . In a lateral view and an inlet view, as illustrated in  FIGS. 9C and 9D , the anterior cortex of the sacral ala is visible as the most anterior aspects of the sacrum. In the lateral view illustrated in  FIG. 9C , the anterior cortex of the sacral ala is defined generally by the anterior sacral body line  704 , the greater sciatic notch, and the superior surface of the sacral ala which forms the alar line  700 . Three implantation sites  1002 ,  1004 ,  1006  are shown within the implant target zone  1000  in  FIG. 10B , with two implantations sites  1004 ,  1006  located in the anterior portion of the implant target zone  1000  that corresponds to the anterior cortex of the sacral ala. The anterior cortex of the sacral ala can serve as a landmark for positioning those two implantation sites  1004 ,  1006 . 
     In some embodiments, as shown in  FIG. 10C , a first implantation site  1002  can be placed with reference to both the alar line  700  and the posterior sacral body line  702 . In some embodiments, the first implantation site  1002  can be placed with additional reference to the anterior sacral body line  704 . For example, in some embodiments the first implantation site  1002  can be placed approximately 3 mm to 10 mm from the alar line, D1, and approximately 3 mm to 20 mm from the posterior sacral body line  702 , D2, where the distance from the posterior sacral body line  702  is measured along a line parallel to the alar line  700 . In some embodiments, the first implantation site  1002  can be placed approximately 5 mm from the alar line  700  and about 10 mm from the posterior sacral body line  702 . The goal for the first implantation site  1002  is to place the implant in a superior (cephalad) portion of the articular cartilage by using the alar line  700  and the posterior sacral body line  702  for reference. In some embodiments, the first implantation site  1002  can be placed with additional reference to the anterior sacral body line  704 , such that the first implantation site  1002  is positioned approximately in the midpoint of the posterior and anterior sacral body lines  702 ,  704 . In some embodiments, the first implantation site  1002  is positioned anterior of the midpoint of the posterior and anterior sacral body lines  702 ,  704 . 
     In some embodiments, as shown in  FIG. 10D , the second implantation site  1004  can be placed with reference to the alar line  700  and the location of the first implantation site  1002  such that the second implantation site  1004  is inferior (caudal) and anterior to the first implantation site  1002  and placed in a middle portion of articular cartilage, near the vertex of the boomerang. In some embodiments, the second implantation site  1004  can be located anterior of the anterior sacral body line  704 . In some embodiments, the second implantation site  1004  can be placed approximately 3 mm to 10 mm from the alar line  700 , D3. In some embodiments, the second implantation site  1004  can be placed approximately the same distance from the alar line  700  as the first implantation site  1002 , such that D1 and D3 are approximately equal and a line joining the first implantation site  1002  with the second implantation site  1004  is approximately parallel to the alar line  700 . In some embodiments, the second implantation site  1004  can be positioned about 10 mm to about 40 mm from the first implantation site  1002 , D4. The distance between the first implantation site  1002  and the second implantation site  1004  is generally, at a minimum, at least equal to the diameter of the implant so that the two implants do not overlap. In some embodiments, a guide pin jig can be used to help place the guide pin for the second implantation site  1004 . The guide pin jig can have two holes for receiving guide pins that are separated by a predetermined distance, D4. In some embodiments, the guide pin jig can have a hole for the first guide pin and a slot for receiving the second guide pin, so that the distance between the two guide pins can be varied between about 10 mm to about 40 mm. In some embodiments, the slot can have distance markings showing the distance from the hole for the first guide pin. More generally, the guide pin jig can be used to place the next guide pin by using the position of a previously placed guide pin. 
     In some embodiments, as shown in  FIG. 10E , the third implantation site  1006  can be placed with reference to the anterior sacral body line  704  and the second implantation site  1004 , so that the third implantation site  1006  is positioned in an inferior (caudal) portion of the articular cartilage. In some embodiments, the distance between the third implantation site  1006  and the second implantation site  1004 , D5, can be approximately equal to the distance between the first implantation site  1002  and the second implantation site  1004 , D4. In some embodiments, D5 can be between about 0.5 to 2 times the magnitude of D4. In some embodiments, the third implantation site  1006  can be about 0 to 5 mm anterior of the anterior sacral body line  704 , D6. In some embodiments, the third implantation site  1006  can be about 2 or 3 mm anterior to the anterior sacral body line  704 . As illustrated in  FIG. 10E , the three implantation sites  1002 ,  1004 ,  1006  are located in the anterior articular portion of the SI-joint and are non-colinear. If a line is drawn through the first implantation site  1002  and the third implantation site  1006 , the second implantation site  1004  is offset from the first and third implantation site by a distance, D7, which can be about 1 to 15 mm. 
     The implantation sites described above specify the initial guide pin placement. In addition to the initial placement, guide pin and implant orientation can be specified to optimize the location of the implants across the SI-joint after initial guide pin placement. To orient the guide pins, in addition to the lateral views illustrated in  FIGS. 10A-10E , the outlet and inlet views described above provide additional landmarks that facilitate guide pin orientation. Although the following description describes the various guide pins in terms of a first guide pin, a second guide pin, and a third guide pin, the actual order of insertion can be varied such that any one of the guide pins can be inserted first, second, or third. 
     For example,  FIGS. 11A-11C  illustrate the orientation of the first guide pin  1100  inserted into the first implantation site in the lateral view, the inlet view, and the outlet view. These illustrations and the other illustrations described herein are illustrative, and variations may occur depending on the particular orientations in which the various views are taken. As shown in  FIG. 11A , the first guide pin  1100  is driven substantially axially in the lateral view such that the first guide pin  1100  appears to be driven into the page. In the inlet view shown in  FIG. 11B , the first guide pin  1100  is advanced towards the middle of the space defined by the anterior sacral line  1104  and posterior sacral line  1102 . This target  1106  is illustrated by an X. In some embodiments, the first guide pin  1100  can be advanced proximate the target  1106  since the first guide pin  1100  should not encounter any neural structures in this orientation, allowing a longer implant to be used for the first implant. In addition, in some embodiments, the first guide pin  1100  is advanced substantially horizontally in the inlet view. As illustrated in  FIG. 11C , in the outlet view the first guide pin  1100  is advanced substantially horizontally and is also usually advanced above the S1 foramen  1108 . 
       FIGS. 12A-12C  illustrate the orientation of the second guide pin  1200  inserted into the second implantation site in the lateral view, the inlet view, and the outlet view. As shown in  FIGS. 12A and 12B , the second guide pin  1200  is inserted at an angle α with respect to the horizontal axis of the inlet view. See also  FIGS. 14A and 14B . To determine the correct angle of insertion, the distal end of the second guide pin  1200  is placed or docked at the second implantation site. Then the angle of the second guide pin  1200  is set by swinging out the proximal end of the second guide pin  1200  while keeping the distal end fixed until the second guide pin  1200  is pointed towards target  1106  illustrated in the inlet view of  FIG. 12B , while keeping the second guide pin  1200  substantially horizontal in the outlet view shown in  FIG. 12C . In some embodiments, the guide pin  1200  has a trajectory in the inlet view that is centered on the middle third of the body of S1 and aimed towards the target  1106 . In general, when the implantation site is located anterior of the anterior sacral body line  704  in the lateral view, the guide pin is angled by swinging out the proximal end of the guide pin while keeping the distal end fixed until the guide pin is pointed towards target  1106  illustrated in the inlet view. This allows the guide pin to follow the anterior surface of the ala and sacrum without exiting the anterior surface. In some embodiments, the second guide pin  1200  is also advanced superior (cephalad) the S1 foramen but inferior (caudal) to the first guide pin in the outlet view. When the second guide pin  1200  is advanced superior (cephalad) the S1 foramen  1108 , it can be possible to advance the second guide pin  1200  all the way to target  1106  as above for the first guide pin. In other embodiments, the second guide pin  1200  can be advanced horizontally towards the S1 foramen  1108  in the outlet view, in which case the second guide pin  1200  is advanced up to but not into the S1 foramen  1108 . 
       FIGS. 13A-13C  illustrate the orientation of the third guide pin  1300  inserted into the third implantation site in the lateral view, the inlet view, and the outlet view. As shown in  FIG. 13A , the third guide pin  1300  is inserted at an angle in the lateral view, which is expected when the implantation site is anterior of the anterior sacral body line  704  as shown. As shown in  FIG. 13B , the third guide pin  1300  is also advanced towards the target  1106  at an angle β to the horizontal axis. As described above, the angle of the guide pin is set by swinging out the proximal end of the guide pin while keeping the distal end fixed until the guide pin is pointed towards target  1106  illustrated in the inlet view of  FIG. 13B , while keeping the guide pin substantially horizontal in the outlet view shown in  FIG. 13C . As shown in  FIG. 13C , the third guide pin  1300  is advanced substantially horizontally in the outlet view such that the third guide pin  1300  is directed towards the S1 foramen  1108 . In this case, the third guide pin  1300  is advanced up to but not within the S1 foramen  1108 . In other embodiments, the third guide pin  1300  can be directed between the S1 foramen  1108  and the S2 foramen  1110 , when the third implantation site is located more inferiorly (caudally) than illustrated in  FIGS. 13A-13C . In this case, the third guide pin  1300  can be advanced past the S1 foramen  1108  and the S2 foramen  1110  to the medial portion of the sacrum, similar to the first guide pin. 
       FIGS. 14A-14C  illustrate the orientation of the three guide pins in the lateral view, the inlet view, and the outlet view. As shown in  FIG. 14B , the first guide pin  1100  is inserted horizontally in the inlet view. The second guide pin  1200  is oriented at an angle α with respect to the first guide pin  1100  or the horizontal axis in the inlet view. Angle α can vary depending on the particular implantation site chosen. In some embodiments, angle α can vary between about 0 degrees to about 45 degrees, or about 0 degrees to about 30 degrees. The third guide pin  1300  is oriented at an angle β with respect to the first guide pin  1100  or the horizontal axis in the inlet view. In some embodiments, angle β can also vary between about 0 degrees to about 45 degrees, or about 0 degrees to about 30 degrees. In some embodiments, as shown, the angle β is less than angle α. In other embodiments, angle α is approximately equal to angle β. As described in more detail below, implants can be advanced over the guide pins, resulting in implants having the same position and orientation as the guide pins across the SI-joint. By having the implants oriented at an angle with respect to each other and not parallel to each other, resistance to lateral separation of the SI-joint is increased. 
       FIGS. 15A-15C  illustrate another embodiment of the third guide pin  1300  position and orientation when the third implantation site is located in between the posterior sacral body line  702  and the anterior sacral body line  704  in the inlet view. In this embodiment, the inlet view may demonstrate that the first and third guide pin are superimposed and the outlet view may demonstrate that the first and third guide pin are substantially parallel. Because the implantation site is located between the posterior sacral body line  702  and the anterior sacral body line  704 , the third guide pin can  1300  be inserted axially (into the page) in the lateral view and horizontally in the inlet view shown in  FIG. 15B .  FIGS. 15A and 15C  illustrates the implantation site is located more inferior (caudal) than shown in  FIGS. 13A-13C , and consequently, the third guide pin  1300  is advanced horizontally between the S1 foramen  1108  and the S2 foramen  1110  in the outlet view. 
       FIGS. 16A and 16B  illustrate an assembly  1600  for receiving the guide pin  1602 . The assembly can include a soft tissue protector  1604 , a drill sleeve  1606  that is removably disposed within the soft tissue protector  1604 , and a pin sleeve  1608  removably disposed within the drill sleeve  1606 . The assembly  1600  can be slid over the guide pin  1602  until bony contact is achieved.  FIG. 17  illustrates a pin depth gage  1700  that is placed against the proximal end of the assembly  1600  so that the proximal end of the guide pin  1602  is aligned with the depth gage markings  1702 . The pin depth can then be read directly from the markings  1702  of the depth gage  1700 , allowing the proper implant length to be selected. In some embodiments, the implant is left proud of the ilium&#39;s cortical wall by about 2 to 5 mm, so the implant size can be determined by adding 2 to 5 mm to the pin depth. 
     Next, as illustrated in  FIGS. 18 and 19 , the pin sleeve  1608  can be removed and a drill  1900  with a cannulated drill bit  1902  can be inserted over the guide pin  1602  to prepare a cavity for receiving the implant. Drilling can be monitored under fluoroscopy in the outlet view as illustrated in  FIG. 20 , allowing the user to monitor the depth of the drill and to watch for unwanted pin advancement. Drilling can be used to penetrate through the entirety of the ilium, to cross the SI joint and to penetrate the lateral cortex of the sacrum. Once the drill  1900  has reached the appropriate depth, the drill  1900  can be removed with the assistance of an exchange pin  2100 , which can be inserted through the proximal end of the drill  1900  and drill bit  1902  until the exchange pin  2100  abuts against the guide pin  1602 , as illustrated in  FIG. 21 . The exchange pin  2100  prevents the guide pin  1602  from withdrawing as the drill  1900  is removed. When placing the exchange pin  2100  against the guide pin  1602 , care should be taken to avoid inadvertently advancing the guide pin  1602 . After the drill  1900  is removed, the drill sleeve  1606  can also be removed from the assembly. 
     For an implant having a rectilinear cross-sectional profile or a cross-sectional profile defined by a plurality of apices and a plurality of sides, the soft tissue protector  1604  can have a matching cross-sectional profile, as shown in  FIGS. 22A and 22B . For the first implant at the first implantation site, the soft tissue protector  1604  can be rotated so that one side of the soft tissue protector is parallel to the alar line  700 . In some embodiments, for the second implant at the second implantation site, the soft tissue protector  1604  can also be rotated so that one side of the soft tissue protector is parallel to the alar line  700 . This is appropriate when the second implantation site is located at the same distance from the alar line  700  as the first implantation site, as described above. Such a configuration allows the implant  2200  to be positioned close to the alar line without penetrating through the cortices. For the third implant, the soft tissue protector can be rotated so that one side of the soft tissue protector  1604  is facing the greater sciatic notch  2202 .  FIG. 22C  illustrates the orientation of the faces of the implants  2200  to the anatomical landmarks in the lateral view. 
       FIG. 23  illustrates an embodiment of a broach  2300  and a mallet  2302  that can be used to create a rectilinear bore to match the shape of the implant. The broach  2300  has a distal end having a plurality of cutting edges and surfaces for removing bone. The mallet  2302  can have a slot  2306  in the mallet head  204  for receiving an exchange pin  2100  that allows the mallet  2302  to be used in conjunction with the exchange pin  2100 . The broach  2300  can be inserted over the guide pin  1602  and into the soft tissue protector  1604  until it contacts the bone. As illustrated in  FIG. 24 , the broach  2300  can be tapped with the mallet  2302  across the SI-joint under visualization in the outlet view. The broach  2300  can be removed with the exchange pin  2100  in place in order to prevent the guide pin  1602  from withdrawing as the broach  2300  is removed. 
     After the broach  2300  has been removed, the implant  2200  can be placed over the guide pin  1602  and an impactor  2500  can be used to advance the implant  2200  into the broached bore, as illustrated in  FIGS. 25A and 25B . The alignment of the soft tissue protector  1604  can be checked and adjusted as needed so that one side of the soft tissue protector is aligned with the alar line  700 . In some embodiments, autograft or allograft bone derived materials, for example the bone materials generated by the drill and broach, can be applied to the implant surface before insertion. Other materials for coating the implant  2200  include growth factors such as bone morphogenetic protein. In some embodiments, the implant  2200  can have a tapered distal end  2202  that is inserted into the bore first.  FIG. 26  illustrates a slotted mallet  2302 , as described above, being used to strike and advance the impactor  2500  and implant  2200 . The slot  2306  in the mallet  2302  can receive an exchange pin  2100  and allows the mallet  2302  to be used in conjunction with an exchange pin  2100 , if desired. As illustrated in  FIGS. 27A and 27B , implant advancement can be monitored under fluoroscopy in the outlet view to ensure that the implant avoids the foramen. The implant  2200  is advanced until about 2 to 5 mm of the implant remains proud of the surface of the ilium, as shown in  FIG. 28 . 
     SI-Joint Implants 
     Various implants  2200  can be used for the fixation and fusion of the SI-Joint, as illustrated in  FIGS. 29A and 29B . These include screw like implants that are threaded or unthreaded implants or implants with both threaded and unthreaded portions. In some embodiments, the unthreaded implants can have a rectilinear cross-sectional profile transverse to the longitudinal axis of the implant, such as triangular or rectangular. In other embodiments, the unthreaded implants can have a cross-sectional profile defined by one or more apices joined by one or more sides which can be either rectilinear or curvilinear. Implants with a rectilinear cross-sectional profile or a cross-sectional profile defined by one or more apices can better resist rotation than a screw like implant with a circular cross-sectional profile. 
     Use of a round implant such as a screw type implant may require modifications to portions of the procedure described above including, for example, replacing the soft tissue protector with a standard tubular dilator system, and elimination of the broach and impactor. 
     While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions are incorporated herein by reference in their entirety. For example, features described in one embodiment may be combined with another embodiment. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.