Patent Publication Number: US-2020275919-A1

Title: Lateral Access System For The Lumbar Spine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of: 
     U.S. Application No. 61/324,185, filed Apr. 14, 2010, entitled LATERAL ACCESS SYSTEM FOR THE LUMBAR SPINE, Attorney&#39;s docket no. MLI-82 PROV, which is pending; and 
     U.S. Application No. 61/442,608, filed Feb. 14, 2011, entitled LATERAL ACCESS SYSTEM FOR THE LUMBAR SPINE, Attorney&#39;s docket no. MLI-98 PROV, which is pending. 
     This application is also a continuation-in-part of: 
     U.S. application Ser. No. 12/640,413, filed Dec. 17, 2009, entitled SYSTEMS AND METHODS FOR DILATION AND DISSECTION OF TISSUES, Attorney&#39;s docket no. INS-7, which is pending. 
     U.S. application Ser. No. 12/640,413 claims the benefit of: 
     U.S. Application No. 61/138,629, filed Dec. 18, 2008, entitled SYSTEMS AND METHODS FOR DILATION AND DISSECTION OF TISSUES DURING LATERAL SPINE ACCESS SURGERY, Attorney&#39;s docket no. INS-7 PROV, which is expired; and 
     U.S. Application No. 61/166,069, filed Apr. 2, 2009, entitled SYSTEM AND METHOD FOR DILATION AND DISSECTION OF TISSUES, Attorney&#39;s docket no. MLI-75 PROV, which is expired. 
     The above-identified documents are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present disclosure relates to orthopaedics, and more particularly, to providing access to a surgical site in the body through the use of an expandable minimally invasive dilation device. 
     2. The Relevant Technology 
     Many spinal orthopaedic procedures including discectomy, implantation of motion preservation devices, total disc replacement, and implantation of interbody devices require unimpeded access to a targeted portion of the spinal column. Providing access to the targeted area may require forming a passageway through muscles, fascia and other tissues. Current surgical access systems utilize a series of sequential dilators, or a mechanical retractor system with at least one dilating cannula. 
     There are several disadvantages associated with sequential dilators. Sequential dilator systems can shear the tissues through which they are advanced. These tissues can include muscle, nerves, blood vessels, and organs. In addition, the tissues at the distal end of the dilators can be crushed against bone or other soft tissues rather than properly separated. As multiple dilators are deployed to enlarge a space, the tissues may be repeatedly injured as each dilator is advanced through the same tissues. 
     Accordingly, there is a need in the art for systems and methods that facilitate access to the spine, while minimizing trauma to surrounding tissues and avoiding time-consuming and unnecessary repetitive steps. Keeping the overall diameter and the number of passes of the cannulas to a minimum may minimize the trauma to the surrounding structures. Such systems and methods can simplify surgical procedures and expedite patient recovery. Ultimately, reducing the invasiveness of the procedure will result in faster recoveries and improved patient outcomes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments and are therefore not to be considered limiting of the scope of the invention as set forth in the claims. 
         FIG. 1A  is a perspective view of a tissue dilation device in a closed configuration and attached to a hub, the device comprising a stylus, a balloon, a plurality of arms surrounding the stylus, an inner mesh, and an outer sheath; 
         FIG. 1B  is a perspective view of the tissue dilation device of  FIG. 1A  in the closed configuration, with the outer sheath not depicted, and dashed lines representing the inner mesh; 
         FIG. 2  is a perspective view of the tissue dilation device of  FIG. 1A , with the balloon inflated and the device in an expanded configuration, and dashed lines representing the outer sheath; 
         FIG. 3  is a perspective view of the tissue dilation device of  FIG. 1A  in the expanded configuration, with the outer sheath not depicted, and dashed lines representing the inner mesh; 
         FIG. 4A  is an enlarged cross-sectional longitudinal view of the distal end of the tissue dilation device of  FIG. 1A  in the expanded configuration; 
         FIG. 4B  is an enlarged cross-sectional end view of the distal end of the of the tissue dilation device of  FIG. 1A  in the expanded configuration, taken along line a-a of  FIG. 4A ; 
         FIG. 5A  is a perspective view of a distal portion of the stylus of  FIG. 1A , with dashed lines representing an inner bore; 
         FIG. 5B  is a perspective view of an outer side of one arm of the plurality of arms of  FIG. 1A ; 
         FIG. 5C  is a perspective view of an inner side of one arm of the plurality of arms of  FIG. 1A ; 
         FIG. 5D  is an enlarged cross-sectional transverse view of the plurality of arms of  FIG. 1A  in the closed configuration; 
         FIG. 6A  is a perspective view of the tissue dilation device of  FIG. 1A  in the closed configuration, with the distal end inserted into a psoas muscle adjacent a vertebra, and the proximal end attached to the hub; 
         FIG. 6B  is a perspective view of the tissue dilation device of  FIG. 6A  in the open configuration, with the stylus, balloon, and inner mesh withdrawn, and an open passageway extending through the hub, dilation device and psoas muscle; 
         FIG. 7  is a side view of a curved tissue dilation device in a closed configuration, the device comprising a stylus, two balloons, and a plurality of curved arms radially surrounding the stylus and the balloons, wherein the arms are releasably secured to the proximal end of the stylus, and the arms are releasably secured to one another via lateral engagement features fastened by a plurality of release wires; 
         FIG. 8  is a top exploded view of the stylus and two arms of the curved tissue dilation device of  FIG. 7 ; 
         FIG. 9  is a partially exploded enlarged view of the distal end of the stylus, balloon and arms of the curved tissue dilation device of  FIG. 7 ; 
         FIG. 10  is a perspective view of the tissue dilation device of  FIG. 7  in an expanded configuration, with a luer attached to the proximal end of the stylus; 
         FIG. 11  is a perspective view of the tissue dilation device of  FIG. 7  in an expanded configuration, with a cannula partially inserted into the device; 
         FIG. 12  is a perspective view of a stylized cross-section of a human body, with a curved tissue dilation device in a closed configuration inserted into a psoas muscle, and connected to a targeting system positioned to target a predetermined location along the spine; 
         FIG. 13  is a perspective view of the body, curved tissue dilation device and targeting system of  FIG. 12 , with the dilation device in an open configuration and the dilation device and targeting system secured to table mounted clamps, and with a cannula partially inserted into the dilation device; 
         FIG. 14  is a perspective view of the body, curved tissue dilation device and targeting system of  FIG. 12 , with the cannula fully inserted into the dilation device; 
         FIG. 15  is a perspective view of the body and targeting system of  FIG. 12 , with an electromyography electrode inserted into the psoas muscle and connected to a neural monitoring system; 
         FIG. 16  is a perspective view of another tissue dilation device in an open configuration; 
         FIG. 17  is a perspective view of a hub assembly of the tissue dilation device of  FIG. 16 ; 
         FIG. 18  is an exploded perspective view of the hub assembly of  FIG. 17 ; 
         FIG. 19  is a top view of a stationary disk of the hub assembly of  FIG. 17 ; 
         FIG. 20  is a top view of a drive disk of the hub assembly of  FIG. 17 ; 
         FIG. 21  is a perspective view of a clamp connection of the hub assembly of  FIG. 17 ; 
         FIG. 22  is a perspective view of a disk clamp of the hub assembly of  FIG. 17 ; 
         FIG. 23A  is a perspective view of an arm clamp of the hub assembly of  FIG. 17 ; 
         FIG. 23B  is another perspective view of the arm clamp of  FIG. 23A  from another direction; and  FIG. 23C  is yet another perspective view of the arm clamp of  FIG. 23A  from yet another direction; 
         FIG. 24A  is a perspective view of an arm assembly of the tissue dilation device of  FIG. 16 ; and  FIG. 24B  is an exploded perspective view of the arm assembly of  FIG. 24A ; 
         FIG. 25A  is a side view of the arm assembly of  FIG. 24A ; and  FIG. 25B  is a cross section view of the arm assembly of  FIG. 24A  taken along line  25 B- 25 B shown in  FIG. 25A ; 
         FIG. 26  is a perspective view of an arm of the arm assembly of  FIG. 24A ; 
         FIG. 27  is a perspective view of an arm lock of the arm assembly of  FIG. 24A ; 
         FIG. 28  is a perspective view of a set of dilators; 
         FIG. 29  is a perspective view of a set of cannulas; 
         FIG. 30A  is a top view of the tissue dilation device of  FIG. 16  in an open configuration, operatively assembled with one of the cannulas of  FIG. 29 ; and  FIG. 30B  is a cross section view of the tissue dilation device and cannula of  FIG. 30A  taken along line  30 B- 30 B shown in  FIG. 30A ; 
         FIG. 31A  is a perspective view of yet another tissue dilation device in a closed configuration; and  FIG. 31B  is a top view of the tissue dilation device of  FIG. 31A  in a closed configuration; 
         FIG. 32A  is a perspective view of the tissue dilation device of  FIG. 31A  in an unlocked open configuration; and  FIG. 32B  is a top view of the tissue dilation device of  FIG. 31A  in an unlocked open configuration; 
         FIG. 33A  is a perspective view of the tissue dilation device of  FIG. 31A  in a locked open configuration; and  FIG. 33B  is a top view of the tissue dilation device of  FIG. 31A  in a locked open configuration; 
         FIG. 34  is an exploded perspective view of the tissue dilation device of  FIG. 31A ; 
         FIG. 35A  is a perspective view of a stationary disk of the tissue dilation device of  FIG. 31A ; and  FIG. 35B  is another perspective view of the stationary disk of  FIG. 35A  from another direction; 
         FIG. 36A  is a perspective view of a drive disk of the tissue dilation device of  FIG. 31A ; and  FIG. 36B  is another perspective view of the stationary disk of  FIG. 36A  from another direction; 
         FIG. 37A  is a perspective view of an arm of the tissue dilation device of  FIG. 31A ; and  FIG. 37B  is another perspective view of the arm of  FIG. 37A  from another direction; 
         FIG. 38  is a perspective view of the tissue dilation device of  FIG. 16  operatively assembled with a retainer; 
         FIG. 39A  is a front view of the tissue dilation device and retainer of  FIG. 38 ; and  FIG. 39B  is a cross section view of the tissue dilation device and retainer of  FIG. 38  taken along line  39 B- 39 B of  FIG. 39A ; 
         FIG. 40A  is a perspective view of the retainer of  FIG. 38 ; and  FIG. 40B  is an enlarged detail view of a portion of the retainer of  FIG. 38 ; 
         FIG. 41A  is a perspective view of the hub assembly of  FIG. 17  operatively assembled with a plurality of arm assemblies and a retainer; and  FIG. 41B  is another perspective view of the hub assembly, arm assemblies, and retainer of  FIG. 41A  from another direction; 
         FIG. 42A  is a front view of the hub assembly, arm assemblies, and retainer of  FIG. 41A ;  FIG. 42B  is a cross section view of the hub assembly, arm assemblies, and retainer of  FIG. 41A  taken along line  42 B- 42 B of  FIG. 42A ;  FIG. 42C  is a cross section view of the hub assembly, arm assemblies, and retainer of  FIG. 41A  taken along line  42 C- 42 C of  FIG. 42A ; and  FIG. 42D  is a cross section view of the hub assembly, arm assemblies, and retainer of  FIG. 41A  taken along line  42 D- 42 D of  FIG. 42A ; 
         FIG. 43  is an exploded perspective view of the hub assembly, arm assemblies, and retainer of  FIG. 41A ; 
         FIG. 44  is a perspective view of an arm of one of the arm assemblies of  FIG. 41A ; 
         FIG. 45A  is a perspective view of the retainer of  FIG. 41A ; and  FIG. 45B  is an enlarged detail view of a portion of the retainer of  FIG. 41A ; 
         FIG. 46A  is a front view of the hub assembly of  FIG. 17  operatively assembled with a plurality of arm assemblies and a retainer;  FIG. 46B  is a cross section view of the hub assembly, arm assemblies, and retainer of  FIG. 46A  taken along line  46 B- 46 B of  FIG. 46A ;  FIG. 46C  is a cross section view of the hub assembly, arm assemblies, and retainer of  FIG. 46A  taken along line  46 C- 46 C of  FIG. 46A ; and  FIG. 46D  is a cross section view of the hub assembly, arm assemblies, and retainer of  FIG. 46A  taken along line  46 D- 46 D of  FIG. 46A ; 
         FIG. 47  is an exploded perspective view of the hub assembly, arm assemblies, and retainer of  FIG. 46A ; 
         FIG. 48A  is a perspective view of an arm of one of the arm assemblies of  FIG. 46A ; and  FIG. 48B  is an enlarged detail of a portion of the arm assemblies of  FIG. 46A ; 
         FIG. 49  is a perspective view of the retainer of  FIG. 46A ; 
         FIG. 50  is a perspective view of a clamp assembly in a closed configuration; 
         FIG. 51  is a perspective view of the clamp assembly of  FIG. 50  in an open configuration; 
         FIG. 52A  is a side view of the clamp assembly of  FIG. 50 ; and  FIG. 52B  is a cross section view of the clamp assembly of  FIG. 50  taken along line  52 B- 52 B of  FIG. 52A ; 
         FIG. 53  is an exploded perspective view of the clamp assembly of  FIG. 50 ; 
         FIG. 54A  is a perspective view of a stationary jaw of the clamp assembly of  FIG. 50 ; and  FIG. 54B  is another perspective view of the stationary jaw of  FIG. 54A  from another direction; 
         FIG. 55A  is a perspective view of a movable jaw of the clamp assembly of  FIG. 50 ; and  FIG. 55B  is another perspective view of the movable jaw of  FIG. 55A  from another direction; 
         FIG. 56  is a perspective view of the tissue dilation device of  FIG. 16  in a closed configuration, operatively assembled to a pair of the clamp assemblies of  FIG. 50 ; 
         FIG. 57  is a perspective view of the tissue dilation device and clamp assemblies of  FIG. 56 , operatively assembled to the smallest one of the dilators of  FIG. 28 ; 
         FIG. 58  is a perspective view of the tissue dilation device and clamp assemblies of  FIG. 56 , operatively assembled to the two smallest dilators of  FIG. 28 ; 
         FIG. 59  is a perspective view of the tissue dilation device and clamp assemblies of  FIG. 56 , operatively assembled to the smallest one of the cannulas of  FIG. 29  and a plurality of long pins; 
         FIG. 60  is a perspective view of the tissue dilation device, clamp assemblies, cannula, and pins of  FIG. 59 , operatively assembled to the three largest dilators of  FIG. 28 ; 
         FIG. 61  is a perspective view of the tissue dilation device and clamp assemblies of  FIG. 56 , operatively assembled to the largest one of the cannulas of  FIG. 29  and a plurality of long pins; 
         FIG. 62  is a perspective view of a portion of a stylized human lumbar spine and pelvis, with a stylus positioned and oriented for a direct lateral approach to an intervertebral disc; 
         FIG. 63  is a perspective view of the spine, pelvis, and stylus of  FIG. 62 , operatively assembled to the tissue dilation device of  FIG. 16 ; 
         FIG. 64  is a perspective view of the spine, pelvis, stylus, and tissue dilation device of  FIG. 63 , operatively assembled to the smallest dilator of  FIG. 28 ; 
         FIG. 65  is a perspective view of the spine, pelvis, stylus, and tissue dilation device of  FIG. 63 , operatively assembled to the two smallest dilators of  FIG. 28 ; 
         FIG. 66  is a perspective view of the spine, pelvis, and tissue dilation device of  FIG. 63 , operatively assembled to the smallest cannula of  FIG. 29  and a plurality of long pins; and 
         FIG. 67  is a perspective view of the spine, pelvis, and tissue dilation device of  FIG. 63 , operatively assembled to the largest cannula of  FIG. 29  and a plurality of long pins; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present disclosure relates to systems and methods for dilating tissues to provide access to intervertebral space or other targeted areas. Those of skill in the art will recognize that the following description is merely illustrative of principles which may be applied in various ways to provide many different alternative embodiments. This description is made for the purpose of illustrating the general principles and is not meant to limit the inventive concepts in the appended claims. 
     The present invention provides access to the spine through the use of a minimally invasive expandable dilation device. The device may be placed within the tissue with a minimal profile, yet has a high expansion ratio, with the result that the expanded device provides an optimally sized passageway allowing access to the targeted spinal area, with minimal impact on surrounding tissues. A single device is advanced into the tissues to be dilated, and expanded from within. Thus additional steps of introducing successive dilators are avoided, along with repetitive damage to the tissues caused by forcing dilator after dilator through the tissues. 
       FIGS. 1-6B  display views of one embodiment of a dilation device  60 . The dilation device  60  comprises an obturator or stylus  70 , a plurality of rigid arms  90 , a balloon  110 , a flexible inner mesh  130 , and an optional, flexible outer sleeve or sheath  140 . A portion of the dilation device may be introduced into a muscle, and the dilation device expanded from a closed configuration to an open configuration to dissect and separate the muscle fibers and form a passage through the muscle. After expansion, the stylus, balloon and inner mesh may be removed, leaving an open passage through the muscle, through which instruments, implants and other materials may be passed to perform one or more surgical procedures. 
     Referring to  FIGS. 1A and 1B , the dilation device  60  is shown in a closed configuration, partially extending through a hub  50 . The hub  50  comprises a hub body  52  and a collet  54 , and is attachable to a surgical table mounted support system (not shown), which may provide stability and support to the hub and dilation device during surgical procedures. In  FIG. 1A , of the device  60  only the optional outer sleeve  140 , and the distal ends of the plurality of arms  90  and the stylus  70  are visible, as the outer sleeve  140  obscures most of the device. The outer sleeve  140  is securely attached to the plurality of arms, and circumferentially securely attached to the collet, forming a barrier around the remainder of the device.  FIG. 1B  depicts the device without the outer sleeve  140 , and with dashed lines representing the inner mesh  130 . The balloon  110  is mounted circumferentially on the stylus  70  toward the distal end of the stylus and extends proximally along a portion of the stylus. The plurality of arms  90  may completely surround the balloon in the closed configuration; hence the balloon is not visible in  FIGS. 1A and 1B . The inner mesh  130  surrounds the balloon, interposed between the balloon and the plurality of arms, and extending from a distal end of the plurality of arms toward the collet  54 . The inner mesh may be attached to the stylus. The maximum outer diameter of the device  60  in the closed configuration, measured normal to the longitudinal axis of the stylus and rigid arms, such as along line a-a, may range from 5 to 15 millimeters. 
     Device  60  may further comprise one or more retention bands  64  which are placed around the plurality of arms when the device is in the closed configuration, to aid in holding the device closed. The bands may comprise a biocompatible polymer, which may be bio-absorbable, and have a generally circular ring shape. The bands may be heat-shrunk about the closed device. During expansion, as the arms move radial-laterally relative to one another, the force of the moving arms will stretch and ultimately break the band(s). Any of the dilation devices disclosed herein may comprise these retention bands. 
       FIG. 2  shows the dilation device  60  in an expanded, or open configuration. The outer sleeve  140  is not depicted but its location is indicated with dashed lines. A distal portion  132  of the inner mesh  130  surrounds the balloon, and in this embodiment the distal end of the mesh is attached to the stylus. A proximal end of the mesh  134  extends through the collet  54 , surrounding the stylus. 
       FIG. 3  shows the dilation device  60  in the expanded configuration, without the outer sleeve, and the inner mesh is indicated by dashed lines.  FIG. 4A  shows an enlarged longitudinal cross-sectional view of a distal portion of the device  60  in the expanded or open configuration.  FIG. 4B  shows a further enlarged cross sectional view of the distal end of the device  60  in the open configuration, taken along line a-a of  FIG. 4A . Each end of the balloon  110  is attached to the stylus  70 . To attain the open configuration, fluid is introduced through the stylus into the balloon  110 , inflating the balloon. As the balloon  110  is actuated by inflation, it expands radially, and an outer surface of the balloon pushes against the plurality of rigid arms  90 , and each individual arm  92  is displaced radially outward and laterally away from the adjacent arms. The inner mesh  130 , which surrounds a body of the balloon, also expands or unfolds radially outward, generally conforming to the shape of the balloon where it is adjacent to the balloon. The outer sleeve  140  also expands or unfolds with expansion of the device. After expansion of the device to the open configuration, the stylus  70  and attached balloon  110  and inner mesh  130  may be withdrawn proximally, leaving an open passageway extending from the hub  50  to the distal ends of the arms  92 , the open passageway lined by the arms and the outer sleeve. The device  60  comprises a substantially cylindrical shape in both the closed configuration, as seen in  FIG. 1A , and the open configuration, as seen in  FIG. 2 . 
       FIGS. 5A through 5D  display details of the stylus  70  and arms  92 . Referring to  FIGS. 3, 4A and 5A , stylus  70  comprises a proximal end  72 , distal end  74 , and a shaft  76  extending between the proximal and distal ends. The stylus may also be an obturator or a hypotube, comprising stainless steel or another biocompatible metal. A stylus tip  78  is located at the distal end and may be formed integrally with the stylus, or formed separately from the stylus and rigidly secured to the stylus. The tip  78  may be blunt, to separate and push aside muscle fibers with minimal trauma to the fibers during advancement of the stylus into the muscle. In some embodiments, the tip may be conical, pointed and/or comprise a cutting edge. In some embodiments, the stylus distal end may further comprise connecting features which cooperate with complementary connecting features on the rigid arms to place the arms in a predetermined longitudinal alignment with the stylus when the connecting features are engaged with one another. The stylus may be partially hollow, having an inner bore  80  extending from an opening at the proximal end, to or toward the distal end. One or more ports  82  may penetrate from the bore  80  to the outside of the shaft  76 , through which fluid may flow to inflate the balloon during dilation. A luer (not shown) may be attached to the proximal end of the stylus, in communication with the bore  80 , for introduction of fluids into the stylus bore. 
     Referring to  FIGS. 3 through 5D , the plurality of rigid arms  90  may comprise four or more individual arms  92 . Each arm  92  may be identical to each other arm, and comprises a proximal end  94 , a distal end  96 , a first lateral edge  98  and a second lateral edge  100  opposite the first lateral edge. A shaft  102  extends between the proximal and distal ends, bounded laterally by the lateral edges  98 ,  100 . An outer surface  104  which may be convexly curved covers one side of the arm, while an inner surface  106  which may be concavely curved covers the opposite side. Each entire arm  92  may be curved about its longitudinal axis, such that when the arms are positioned in a closed configuration so that their lateral edges are adjacent one another in contacting alignment as in  FIGS. 1B, 5D, and 6A , a closed cylinder is formed. The inner diameter of the closed cylinder is sized to receive the stylus  70  and the uninflated balloon  110 . The arms may include holes or other features used in secure attachment of the outer sleeve to the arms via sutures, pins, or other attachment mechanisms. In the embodiment shown in FIGS.  1 B- 3 , the arms extend along only a portion of the stylus. In other embodiments, the arms may be longer, and can extend the entire length of the stylus and/or extend out of the hub  50 . Each arm  92  may flare or curve outward at its distal end, which may aid in keeping tissues retained during dilation or expansion. 
     The arm distal end  96  may comprise an arm connection feature which is shaped to engage with a corresponding stylus connection feature to place the arm in a predetermined longitudinal alignment with the stylus. With reference to  FIGS. 4A, 5A and 5B , the arm connection feature may comprise a curved distal edge  97 . The stylus connection feature may comprise a segment of an overhanging lip  79 . The lip  79  comprises a circular flange on the stylus tip  78 , which may project outward from the stylus tip. When an arm curved distal end  97  is positioned in abutment with a correspondingly curved segment of the lip  79  such that the entire curved distal end is in contact with the lip segment, the arm is placed in a predetermined longitudinal alignment with the stylus. 
     The arm lateral edges  98 ,  102  may comprise complementary engagement features which cooperate with the engagement features on adjacent arms to place the arms in contacting longitudinal alignment with one another along their first and second lateral edges when the arms are in the closed configuration. In one embodiment, the engagement features may comprise planar portions wherein the first lateral edge comprises a planar surface  108  which engages a complementary planar surface on the adjacent second lateral edge. In another embodiment, the engagement features comprise tongue-in-groove features wherein the first lateral edge comprises a tongue while the second lateral edge comprises a groove shaped to receive the tongue. In yet another embodiment, the engagement features may comprise alternating edge extensions with bores shaped to receive a longitudinal member such as a wire or suture, so that the edges may be temporarily laced together. In the closed configuration the longitudinal member extends through the bores and the arms are retained in contacting longitudinal alignment; when the longitudinal member is removed the arms are free to disengage and move apart from one another. 
     The arms may be at least partially radiolucent, so as not to compromise visualization of procedures during use of the device with fluoroscopy. Alternatively, the arms may be at least partially radiopaque, to assist with positioning and location of the system under fluoroscopy. The arms may comprise metals such as aluminum, stainless steel, titanium, and other biocompatible metals. The arms may also comprise high density plastics such as Delrin, Radel, Udel, poly ether ether ketone (PEEK), polycarbonate, and acrylonitrile butadiene styrene (ABS), among others. Barium sulphate may be added to constituent plastic materials to provide increased radiopacity. 
     With reference to  FIG. 4A , the balloon  110  comprises a proximal end  112 , a distal end  114 , and a substantially cylindrical balloon body  116  extending therebetween. At the proximal  112  and distal  114  ends, the balloon is circumferentially attached to the stylus through adhesives or other bonding methods such that when fluids are introduced into a balloon lumen  115 , the fluids cannot escape at the points of attachment to the stylus  70 . As fluid is introduced into the balloon  110  from the stylus  70  through the ports  82 , the balloon may inflate proximally to distally. As the proximal end of the balloon inflates, the arms  92  may be pushed slightly distally, then radially outward as the inflation continues distally. The proximal ends  94  of the arms may be pushed radially outward before, or at the same time, as the distal ends  96  of the arms  92 . 
     The balloon may be opaque or translucent, and the balloon may be compliant or non-compliant. A compliant balloon may allow for an even distribution of force on the rigid arms and ultimately the surrounding tissue. A non-compliant balloon may allow for an uneven distribution of force and as such may be well suited for dissection of tissues. The shape of the balloon may be optimized to best suit the physiology and tissue it will dissect. For example, a round balloon may produce uniform force distribution and create a localized open space. An elongated balloon may produce distal expansion to create space at the distal end of the device. The balloon may comprise polyethylene, polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), nylon, Dacron, polyurethane, or other compliant or non-compliant polymers. 
     The inner mesh  130  may be fixed to the stylus at a location distal to the distal end of the balloon, extending to or toward the proximal end of the stylus. The inner mesh  130  is generally tubular and flexible, able to conform to the shape of the balloon, and may be permeable or non-permeable. The inner mesh may be of an indeterminate shape or a pre-formed shape. The mesh may comprise polypropylene, polyethylene (PE), polyethylene terephthalate (PET), poly ether ether ketone (PEEK), nylon, ultra-high molecular weight polyethylene (UHMWPE), or any other biocompatible polymer, or a combination thereof. In some embodiments, the inner mesh may be formed such that as a portion of the inner mesh is expanded by the balloon or dilation member, the length of the inner mesh is foreshortened. 
     In some embodiments, the dilation device  60  may further comprise an outer mesh or sheath  140  which may circumferentially surround the rigid arms and stylus, to further retain and protect bodily tissues during dilation. In other embodiments, the outer sheath may be positioned inside the arms, but outside of and circumferentially surrounding the inner mesh, balloon and stylus. The outer sheath may prevent pinching of tissues and/or migration of tissues between the rigid arms during the dilation process. The outer sheath is securely attached to the arms, whether inside or outside, by adhesive, suturing, and/or a mechanical fastening device such as a pin. The sheath  140  may be generally tubular in form, with open distal and proximal ends. At or toward its distal end, the sheath may be attached to the plurality of arms. At its proximal end, the sheath may be circumferentially attached to the collet  54 , via an o-ring or another fastener. In some embodiments, the outer sheath comprises a mesh interwoven with a secondary material that is conductive. The conductive nature of the mesh may be used to oblate tissue or used in a more diagnostic mode, such as detecting nerve tissue in conjunction with an electromyography (EMG) device. The outer sheath may comprise the same materials as the inner mesh. 
       FIG. 6A  shows the device  60  in the closed configuration, advanced into a muscle, while  FIG. 6B  shows the device  60  in the open configuration, dilating the muscle to provide a passageway through the muscle. In  FIGS. 6A and 6B , the outer sheath  140  is positioned inside the plurality of arms  90 . Referring to  FIG. 6A , the device  60  has been partially advanced into the psoas muscle adjacent the spine. The stylus  70  and closed plurality of arms  90  have penetrated the muscle, dissecting the muscle fibers. As the device is introduced, it may be rotated such that the outer surfaces  104  of the arms  92  are placed in a preferred orientation relative to the muscle fibers, so that the fibers may be primarily pushed aside, instead of torn, during dilation. For example, placing the device so that the outer surfaces  104  of the arms  92  are at approximately 45° relative to the longitudinal axis of the muscle fibers may be preferable, as shown in  FIGS. 6A and 6B . Fluid is introduced into the bore of the stylus, where it passes through the ports  82  into the balloon  110 , inflating the balloon and causing it to expand radially. As the balloon expands, the surrounding inner mesh  130  expands, and the arms  92  are forced radially outward and are radial-laterally displaced from one another, as seen in  FIGS. 6B and 4B . The surrounding muscle fibers are dissected and the muscle is dilated, creating a passageway through the muscle to the spine. 
     After the balloon  110  has been inflated a desired amount, the stylus, balloon and inner mesh may be removed from the device  60 , leaving the expanded arms  90  and outer mesh  140  surrounding an open passageway  62 . Before or after removal of the stylus, balloon and inner mesh, a rigid cannula may be longitudinally inserted into the passageway  62 , inside the arms  90  and outer mesh  140 , to further hold the passageway open; the cannula forming an inner wall of the passageway. Instruments, implants and other materials may be passed through the passageway to perform surgical procedures. In the open configuration, the maximum outer diameter of the device  60 , measured normal to the longitudinal axis of the stylus and rigid arms such as along line a-a in  FIG. 4A , may range from 25 to 40 millimeters. An expansion ratio of the device may be measured as the ratio of maximum outer diameter of the device in the open configuration to the maximum outer diameter of the device in the closed configuration. The expansion ratio of device  60  may range from 2.5 to 8.0. In some embodiments, the expansion ratio may range from 3.0 to 7.5; in other embodiments, the expansion ratio may range from 4.0 to 7.0; while in other embodiments, the expansion ratio may range from 5 to 6.5. In a preferred embodiment, the expansion ratio may be at least 6.0. 
       FIGS. 7-10  illustrate another embodiment of an minimally invasive expandable dilation device. Dilation device  160  comprises a curved stylus  170 , a plurality of rigid curved arms  190  radially arrayed about the stylus, and two balloons which are circumferentially attached to the stylus. Dilation device  160  may be used in a postero-lateral approach to dilate and form a passageway through the psoas muscle in order to obtain access to an intervertebral space. In other embodiments, one balloon may be attached to the curved stylus, or a plurality of balloons. In yet other embodiments, at least one cannula may be introduced into the space defined by the arms to expand the arms apart, instead of one or more balloons. Device  170  may be used to create a curved passageway through the psoas muscle, and/or to create a curved passageway through another muscle or set of tissues. 
     Referring to  FIG. 7 , the dilation device  160  is shown in a closed configuration, with the plurality of curved arms  190  enclosing and obscuring the balloons. A central longitudinal space is circumferentially surrounded by the arms. The plurality of curved arms  190  comprises four individual curved arms  192 ,  194 ,  196 ,  198 . Each arm is releasably secured to the distal end of the stylus via an attachment mechanism  200 . Each arm is also releasably secured to the lateral edge of the adjacent arms via lateral engagement features  220 . A release wire  226  secures each arm to its adjacent neighbor by extending through an arm bore  224  which extends the length of the arms, from the proximal end to the distal end. In this embodiment, the arms are not identical to one another but each shaped so that when fitted together the arms form a closed curved cylinder about the curved stylus. For example, arms  194  and  196  may be shorter than arms  192  and  198 , and have a slightly smaller radius of curvature than arms  192  and  198 . The maximum outer diameter of the device  160  in the closed configuration, measured normal to the longitudinal axes of the stylus and rigid arms, such as along line b-b, may range from 5 to 15 millimeters. 
     Referring to  FIG. 8 , a perspective top down view shows the stylus  170  and two arms  194 ,  196 . Mounted on a stylus shaft  172  are two uninflated balloons  240 ,  242 . Each balloon extends longitudinally along a portion of the stylus, and is secured to the stylus at each balloon end. The stylus comprises an inner bore  174  which extends along a length of the stylus, and is in communication with two ports  176 . The bore and ports provide a passageway for fluid to inflate the balloons  240 ,  242 . At a distal end  177  of the stylus is a stylus tip  178  which may be formed integrally with, or separately from the stylus. The stylus tip has a point  180  which may be blunt in order to more gently push aside tissues during insertion of the stylus into body tissues and muscles. The stylus tip may have a distal conical surface which also aids in atraumatically parting tissues and muscle fibers. The maximum diameter of the stylus tip may be greater than the shaft of the stylus, as in  FIG. 8 ; in other embodiments the maximum diameter of the stylus tip may be the same or less than the stylus shaft. 
     With reference to  FIGS. 8 and 9 , the stylus tip  178  has four discrete connecting features  182  located adjacent the distal end of the stylus. Each connecting feature  182  is a peg protruding radially from the stylus, each peg having an ovoid or egg shape with one rounded end slightly larger than the other. This shape ensures a close fitting with a complementarily shaped receiving hole  204  on the end of each arm. When the arms are fitted on the pegs as in  FIG. 7 , the arms are longitudinally aligned with the stylus in a predetermined longitudinal alignment in which the arms cannot move laterally relative to one another until the connection features are detached; e.g., the hole  204  is taken off the peg  182 . Other connecting features are contemplated, including but not limited to pegs and corresponding holes which are round, oval, rectangular, or multi-sided; or other complementary protrusion and slot combinations. The receiving hole may be open on both ends or may be a recess or cavity with an opening on one side shaped to receive the peg. In an alternative embodiment, the pegs may be located on the arms, and the receiving hole or cavity on the stylus or stylus tip. 
     Each arm  192 ,  194 ,  196 ,  198  comprises a distal end  206 , a proximal end  208 , and an arm shaft  210  bounded laterally by a first lateral edge  212  and a second lateral edge  214 . Each lateral edge  212 ,  214  comprises one or more recessed portions  216  which are distributed alternately with projecting portions  218 . Thus when two arms are fitted together laterally, the projecting portions  218  on one arm fit into the recessed portions  216  on the adjacent arm. An arm bore section  222  extends longitudinally along each lateral edge, through the entire length of each projecting portion  218 . When two arms are fitted together laterally, one continuous arm bore  224  is formed from the alternating arm bore sections  222  which are now axially aligned with one another. The release wire  226 , seen in  FIG. 7 , can be inserted along the length of each arm bore  224  to effectively pin or lace the arms securely together. After advancement of the closed device  160  into the tissues and prior to expansion of the device, the release wire(s)  226  may be withdrawn so that the arms may move apart from one another with the expansion force. Other lateral engagement features are contemplated, including but not limited to tongue-in-groove features, corresponding tab and slot features, or press-fit features. Such features may be disengaged by removal of a pin, suture or wire such as release wire  226 , or may have a friction fit in which the features are detached from one another by sufficient expansive force provided by expansion of the dilating member. 
     Referring to  FIGS. 8 and 9 , details of the arm and stylus distal ends are shown. The distal end  206  of each arm  192 ,  194 ,  196 ,  198  may include an offset  228 , in which the distal end is offset from the shaft  210 . The offset  228  allows the arms to fit more precisely together when the device  160  is in the closed configuration, and also provides a stop surface for a distal end of the release wire  226 . The distal end  206  may also include a waist  230  and an adjacent flared portion  232 . Together, the waist  230  and flared portion  232  form a concavely curved area at the distal end of the arm, which may aid in holding back or retaining tissues dissected and pushed aside by the stylus tip  178  during advancement of the device into muscle and other tissues, and may aid in holding back or retaining tissues moved apart during dilation or expansion of the device  160 . The flared portions  232  may act as a retainer to prevent tissues from slipping back over the distal ends of the arms once the tissues have been dissected apart from one another. The inner surface of the flared portion  232  may also be shaped to complementarily mate with the outer surface of the stylus tip  178 , a portion of which may flare outward. 
     An alternative embodiment of the dilation device may include a stylus and arms with corresponding connecting features such as the peg/hole system set forth above, but no lateral engagement features on the arms. Another embodiment may include lateral engagement features on two or more arms, but no corresponding connecting features between the arms and the stylus. Yet another embodiment may comprise neither distal connection features nor lateral engagement features. It is appreciated that additional embodiments may include any combination of the features described herein. 
     Dilation device  160  may further comprise an inner mesh positioned longitudinally between the balloons and the plurality of arms in the same manner as inner mesh  130  set forth in the previous embodiment. The device may also further comprise an outer sleeve securely attached to the arms and positioned longitudinally either inside or outside the plurality of arms, in the same manner as outer sleeve  140  set forth in the previous embodiment. The mesh and sleeve may comprise the same materials as set forth previously for inner mesh  130 . 
     Referring to  FIG. 10 , dilation device  160  may be expanded by introduction of a fluid into the stylus  170 . Prior to expansion, any release wires may be withdrawn from the arm bores. A luer  244  at the distal end of the stylus provides means for introduction of the fluid, such as saline, into the stylus bore. The fluid is forced distally along the stylus and escapes through ports  176 , inflating balloons  242  and  240 . The proximally located balloon  242  may inflate in advance of the distally located balloon  240 , and this may push the arms slightly distally, then radially outward as both balloons inflate. As the arms  192 ,  194 ,  196 ,  198  move radially outward, the arm connection features  204  are disengaged from the stylus connection features  182  by the expansion force provided by the inflation of the balloons. Similarly, the lateral engagement features  216 ,  218  disengage and the arms may move radial-laterally apart with the expansion of the balloons. After inflation of the balloon has provided sufficient expansion of the device to dilate the surrounding tissue a desired amount, the introduction of fluid may be ceased, and the stylus  170  with the attached balloons  240 ,  242  may be removed, leaving a passageway through the surrounding tissue. In the open configuration, the maximum outer diameter of the device  160 , measured normal to the longitudinal axis of the stylus and rigid arms, such as again along line b-b, may range from 25 to 40 millimeters. The expansion ratio of device  160  may range from 2.5 to 8.0. In some embodiments, the expansion ratio may range from 3.0 to 7.5; in other embodiments, the expansion ratio may range from 4.0 to 7.0; while in other embodiments, the expansion ratio may range from 5 to 6.5. In a preferred embodiment, the expansion ratio may be at least 6.0. 
     Referring to  FIG. 11 , a cannula may be inserted between the stylus  170  and the curved arms  190  to keep the device in the open configuration and prevent migration of tissues into the central longitudinal space  162 . A cannula such as arcuate cannula  246  may be inserted along a curved path over the stylus  170  before withdrawal of the stylus and balloons from the device  160 , as shown in  FIG. 11 . Alternately, the cannula may be inserted along the insides of the arms after withdrawal of the stylus and balloons. It is appreciated that the cannula is not passed along the outside of the device, which could crush or injure of the adjacent tissues. Instead, the cannula is advanced along the inner sides of the expanded arms, and within the optional outer sleeve. Following insertion of the cannula and withdrawal of the stylus, balloons, and optional inner mesh, instruments, implants and other materials may be passed through the cannula to perform surgical procedures at the end of the passageway formed by the expanded device. The cannula may be docked to a skeletal structure such as a vertebra, and/or to a surgical table support system, to provide stability during surgical procedures. 
       FIGS. 12-14  illustrate a dilation device inserted through an opening in the skin and through a psoas muscle to create a passageway to an intervertebral location, from a postero-lateral approach. The device  260  is transformable from a closed configuration in which each arm is in contacting longitudinal alignment with two other of the arms along their lateral edges, and an open configuration in which the arms are radially displaced from the stylus and laterally displaced from one another. Dilation device  260  comprises a stylus  270 , a plurality of arms  280 , a tubular sleeve  290 , and dilating member which is a cannula  300 . In embodiments such as this wherein the dilating member is not a balloon, the entire stylus including the distal tip may be cannulated to allow for flushing of the site, and/or passage of a k-wire. 
     Referring to  FIG. 12 , dilation device  260  is shown in the closed configuration, inserted through an incision in the skin  10  and through the psoas muscle  12 . The sleeve  290  is secured to the inner surfaces of the arms  280 , and at its proximal end, to a collet portion  54  of hub  50 . The stylus  270  is also releasably clamped to the hub  50 . The hub  50  is secured to a targeting system  20  which is fully described in U.S. patent application Ser. No. 12/357,695, filed on Jan. 22, 2009 and entitled Spinal Access Systems and Methods, the entirety of which is herein incorporated by reference. It is appreciated that hub  50  and/or the targeting system may be secured to other support systems such as surgical table support systems known in the art, to provide stability during device insertion and dilation, and during other surgical procedures. Targeting system  20  comprises a housing  22 , a targeting post  24 , a micrometer  26  and a swing arm  28 . The targeting post  24  may be advanced through the skin and fascia to a desired depth and location adjacent the spine  14 , and a targeting depth stop  29  may regulate the depth of the targeting post. The micrometer  26  may be used to finely adjust the position of the targeting post. The offset arm or swing arm  24  connects the housing  22  to the hub  50 . The swing arm  24  may be raised or lowered, rotating about the axis of the housing  22 , to raise or lower the hub  50  and the associated dilation device  270 . In  FIG. 12 , the swing arm  24  has been lowered sufficiently to guide the device  260  along an arcuate curved path into the psoas muscle. 
     Toward the proximal end of the plurality of arms  280 , each arm comprises a longitudinal slot which extends from the proximal end distally along a portion of the arm. This slot may provide a slight amount of flexibility to the arm proximal ends as the cannula  300  is inserted to initiate transformation of the device  260  from the closed to the open configuration. The slots may also be guides, cooperating with pins or protrusions on the cannula or on a separate guiding ring to guide insertion of the cannula into the device. 
     Referring to  FIG. 13 , the dilation device  260  is shown in the open configuration. The hub  50  and the housing  22  are connected to polyaxially adjustable table mounted clamps  30 ,  32 . A post  34  is anchored in a pedicle, and a slidable clamping sphere  36  is positioned on the post. Optionally, the targeting system may be clamped to the post  34 , in place of or in addition to the table mount clamp. The stylus  270  has been withdrawn from the dilation device  260 , and a distal end  302  of the cannula  300  has been partially advanced into the system, inside the plurality of arms  280  and the sleeve  290 . A proximal end  304  of the cannula is docked to the hub  50 . The cannula  300  has a larger diameter than the plurality of arms  280  in the closed configuration. As the cannula  300  is advanced along a curved path inside the space within the closed arms, the arms  280  are forced radial-laterally apart, opening up the attached tubular sleeve  290 , and creating a passageway through the tissues and psoas muscle. 
     Referring to  FIG. 14 , the dilation device  260  is shown in the open configuration, and the distal end of the cannula  300  has been fully advanced along the plurality of arms and through the psoas muscle. The swing arm  28  has been partially rotated about the axis of the housing  22 , thus lowering the hub and the docked cannula to fully advance the cannula along the curved path, and decreasing the angle between the swing arm  28  and the targeting post  24 . The clamp arms  30 ,  32  have been adjusted to stabilized the device at the fully advanced position. Other instruments, implants, and materials may be passed through the passageway formed by the cannula. 
       FIG. 15  illustrates an electromyography (EMG) electrode  310  inserted into the psoas muscle. The electrode  310  is connected to a neural monitoring system  320  which can detect the presence of nervous tissue. Prior to insertion of a dilation device such as devices  60 ,  160 , or  260 , the electrode  310  may be advanced into the muscle or tissue to be dilated, and energized, or activated, to detect the presence of a nerve. The electrode may then be deactivated and/or removed, and the dilation device inserted into the muscle or tissue. If a nerve is sensed along a particular path or trajectory, the dilation device may be inserted along a different path or trajectory, in order to avoid the nerve. The electrode and neural monitoring system may be used prior to insertion of the dilation device, after insertion of the device but prior to dilation, and following dilation, to detect and avoid nervous tissue. The electrode may also be activated intermittently during advancement of the dilation device. In alternative embodiments of the device, the rigid arms, if metallic, may be used as the electrode. As well, transmissive tape and/or paint can be applied to the surface of the arms to create a surface capable of transmitting voltage. 
       FIGS. 16-27  illustrate another dilation device  400 . Dilation device  400  may also be described as a dilation assembly since it includes several component parts in an operative arrangement.  FIG. 16  shows dilation device  400  in an open configuration. Dilation device  400  includes a hub assembly  500  and a plurality of arm assemblies  1300 . Dilation device  400  is configured to accept up to five arm assemblies  1300 , although alternate embodiments may include two or more arm assemblies. The arm assemblies  1300  are carried by the hub assembly  500 . In this embodiment, the arm assemblies  1300  are evenly spaced in a circular or pentagonal pattern around the hub assembly  500 , although non-circular or non-symmetric arrangements are contemplated. The pattern of arm assemblies  1300  synchronously expands and contracts relative to the hub assembly  500 . In other words, each arm assembly  1300  is radially movable relative to a center of the pattern. The center of a pattern is a point which is, in some sense, in the middle of the pattern. Other embodiments may provide for independent adjustments in different directions, such as independent adjustment in the anterior-posterior, medial-lateral, and/or cephalad-caudal directions. 
     A portion of the dilation device  400  may be introduced into a muscle, other tissue, or natural passageway in a closed configuration (illustrated in  FIG. 56 ), in which the arms may be in contacting longitudinal alignment. The dilation device  400  may then be expanded from the closed configuration to the open configuration. If introduced into a muscle, the expansion of dilation device  400  bluntly dissects and separates the muscle fibers. The expansion of dilation device  400  forms an open passage through the muscle, tissue, or natural passageway, through which instruments, implants and other materials may be passed to perform one or more surgical procedures. 
       FIGS. 17-23C  illustrate hub assembly  500  in more detail. With reference to  FIGS. 17-18 , hub assembly  500  includes stationary disk  600 , drive disk  700 , basic disk clamp  800 , slotted disk clamp  900 , socket disk clamp  1000 , clamp connection  1100 , arm clamp  1200 , and associated fasteners such as dowel pins and screws. Drive disk  700  is sandwiched between two stationary disks  600 , and the three disks are secured together by disk clamps  800 ,  900 , and  1000  which connect around the outer diameter of the disks. The stationary disks  600  and disk clamps  800 ,  900 , and  1000  form a stationary frame within which drive disk  700  is freely rotatable. Arm clamps  1200  connect around the inside diameter of the stacked disks in a circular pattern, and are freely linearly translatable relative to the stationary frame along a radius of the circular pattern. The rotation of drive disk  700  and the translation of arm clamps  1200  are linked so that a force input causing translation of arm clamps  1200  will also cause drive disk  700  rotation, and vice versa. In this way, hub assembly  500  provides synchronized radial expansion and contraction of the pattern of arm clamps relative to the disks  600 ,  700 . At least some components of hub assembly  500  may be partially or completely radiolucent. 
     Referring to  FIGS. 18-19 , stationary disk  600  is shaped as a ring or annulus with a central aperture  602 . Stationary disk  600  includes five pairs of linear, radially-extending slots  604  evenly spaced around the disk. Stationary disk  600  also includes five pairs of holes  606  evenly spaced around the disk near the outer diameter. Smaller holes  608  may also be present, one between each pair of holes  606 . One or more additional holes  610  may be present, for example, to reduce weight or provide easy access for cleaning. 
     Referring to  FIGS. 18 and 20 , drive disk  700  is shaped as a ring or annulus with a central aperture  702 . Drive disk  700  includes five pairs of arcuate, obliquely-extending slots  704  evenly spaced around the disk. Drive disk  700  also has four arcuate perimeter slots  708  positioned around the disk in four of five evenly spaced positions. Drive disk  700  includes a tab  710  which protrudes from the outer diameter of the disk. 
     Referring to  FIG. 18 , a basic disk clamp  800 , or bracket, may have an arc shape which is complementary to the outer diameter of stationary disk  600 , drive disk  700 , or both. A concave side of disk clamp  800  includes a channel  802  or groove which is sized to receive the combined thickness of two stationary disks  600  and one drive disk  700  with clearance. Disk clamp  800  also includes a pair of through holes  806  which are sized and positioned to correspond to holes  606  of stationary disk  600 , and also sized and configured to complement fastener  898 . For example, if fastener  898  is a screw, then holes  806  may be threaded with a corresponding internal thread form. Disk clamp  800  may accept up to four fasteners  898 . 
     With continued reference to  FIG. 18 , a slotted disk clamp  900 , or bracket, may share some or all of the characteristics set forth above for disk clamp  800 , and is further characterized by a through slot  908 , as may be seen best in  FIG. 30B . Tab  710  of drive disk  700  protrudes through slot  908  when hub assembly  500  is fully assembled, and provides a force input location to rotate drive disk  700 . 
     Referring now to  FIGS. 18 and 22 , a socket disk clamp  1000 , or bracket, may share some or all of the characteristics set forth above for disk clamp  800 , such as concave channel  1002  or holes  1006 . Additionally, disk clamp  1000  includes a central protrusion  1010  extending from a convex side of the bracket. A socket  1012  extends at least partially into protrusion  1010 . 
     With reference to  FIGS. 18 and 21 , clamp connection  1100  is most notably characterized by a spherical ball  1102  from which a shaft  1104  protrudes. Ball  1102  may be smooth or textured. For example,  FIG. 21  illustrates a textured ball  1102  which has a regular pattern of grooves  1112 . Other textures are contemplated, such as knurling, dimples, or a sandblasted finish. Shaft  1104  may optionally have threads  1106  and a drive feature  1108  such as a pair of opposed flats  1110 . If threads are present, clamp connection  1100  may thread into a correspondingly threaded socket  1012  in disk clamp  1000 . Alternatively, clamp connection  1100  may be pinned, press fit, welded, or joined to disk clamp  1000  by some other means. Clamp connection  1100  may also be integrally formed with disk clamp  1000  in some embodiments. 
     Referring to  FIGS. 18 and 23A -C, arm clamp  1200 , or bracket, is illustrated with an hourglass profile in the top view, as may be seen in  FIGS. 19 and 23A . Other profiles are contemplated for their function or decorative appeal. Also visible in  FIG. 23A , dovetail slot  1206  is formed in, and extends completely across, one enlarged end of the hourglass shape. A pocket  1208  may be formed in the bottom of dovetail slot  1206 , as may be seen in  FIG. 23C . Arm clamp  1200  also includes a pair of through holes  1204  which are sized and positioned to correspond to slots  604  of stationary disk  600  and slots  704  of drive disk  700 . Holes  1204  are also sized and configured to complement fastener  1298 . For example, if fastener  1298  is a dowel pin, then at least some of the holes  1204  may be dimensioned and toleranced to receive fastener  1298  with a press fit or interference fit. In a side view, arm clamp  1200  includes a channel  1202  or groove which is sized to receive the combined thickness of two stationary disks  600  and one drive disk  700  with clearance. Channel  1202  extends across the narrow portion of the hourglass shape in the illustrated embodiment, with the bottom of the channel  1202  adjacent to the dovetail slot  1206 . Arm clamp  1200  also includes a through hole  1210  which intersects the dovetail slot  1206  opposite the pocket  1208 . Hole  1210  may be sized and configured to complement fastener  1296 . 
     Hub assembly  500  may be assembled by sandwiching a drive disk  700  between two stationary disks  600 . The three disks may optionally be secured together by passing a fastener  698  through each hole  608  in one of the stationary disks  600 , a corresponding slot  708  in the drive disk  700 , and a corresponding hole  608  in the second stationary disk  600 . Alternately, or in combination, the three disks may be secured together with disk clamps  800 ,  900 ,  1000  and fasteners  898 . In this situation, disk clamp  900  may be positioned over the outer perimeter of the three disks so that tab  710  of the drive disk  700  protrudes through slot  908  and holes  906  align with holes  606  of the stationary disks  600 . Disk clamp  900 , stationary disks  600 , and drive disk  700  may be secured together by inserting four fasteners  698  through holes  906 ,  606 . Two disk clamps  1000  may be similarly positioned, one on either side of disk clamp  900 . Four fasteners may be inserted through holes  1006 ,  606  to secure disk clamps  1000 , stationary disks  600 , and drive disk  700  together. Two disk clamps  800  may be similarly secured in the remaining positions with a total of eight fasteners  698  inserted through holes  806 ,  606 . Each arm clamp  1200  may be prepared by inserting a fastener  1296  through hole  1210 , and then attached to the stationary disks  600  and drive disk  700  by positioning the arm clamp  1200  over the edges of the central apertures  602 ,  702  so that holes  1204  align with corresponding slots  604  and  704 , and inserting two fasteners  1298  through holes  1204  and slots  604 ,  704 . In the illustrated embodiment, all of the arm clamps  1200  are oriented with their fasteners  1296  near the same stationary disk  600 , as may be appreciated in  FIG. 17 . 
     Referring to  FIGS. 24A-27 , an arm assembly  1300  may include an arm  1400 , an arm lock  1500 , and a fastener  1498 . 
       FIGS. 24A-26  illustrate an arm  1400 , or blade, with an elongated shaft  1402  extending between a base  1404  and a tip  1406 . One side of arm  1400  has a full length groove  1408  or depression. The base  1404  includes a dovetail protrusion  1410  on a side opposite the groove  1408 . The dovetail protrusion  1410  is sized and shaped to complement dovetail slot  1206  of arm clamp  1200 . A slot  1412  extends completely across the end of base  1404  between the groove  1408  and dovetail protrusion  1410 . A hole  1414  is formed in the bottom of slot  1412 , parallel to the dovetail protrusion  1410 . Hole  1414  is intersected near its bottom by a through hole  1416 . Hole  1416  is sized and configured to complement fastener  1498 . The tip  1406  includes a waist  1418  and an adjacent flared portion  1420 . Together, the waist  1418  and flared portion  1420  form a concavely curved area on the tip  1406 , which may aid in holding back or retaining tissues dissected and pushed aside by the dilation device  400 . The tip  1406  may include a docking feature (not shown), such as a spike or a pin hole, to permit the arm  1400  to be stabilized against a structure surrounding the operative site. 
     The arms  1400  may be at least partially radiolucent, so as not to compromise visualization of procedures during use of the device with fluoroscopy. Alternatively, the arms  1400  may be at least partially radiopaque, to assist with positioning and location of the system under fluoroscopy. The arms  1400  may be rigid, semi-rigid, or flexible. The arms  1400  may comprise metals such as aluminum, stainless steel, titanium, and other biocompatible metals. The arms  1400  may also comprise high density plastics such as Delrin, Radel, Udel, poly ether ether ketone (PEEK), polycarbonate, and acrylonitrile butadiene styrene (ABS), among others. Barium sulphate may be added to constituent plastic materials to provide increased radiopacity. The arms may conduct light or function as a light guide or light conductor. 
     Referring to  FIGS. 24B, 25B, and 27 , an arm lock  1500 , or latch, has an elongated shaft  1502  extending between a base  1504  and a head  1506 . The base  1504  has a through hole  1508  which is sized and positioned to correspond to hole  1416  of arm  1400 , and also sized and configured to complement fastener  1498 . The arm lock  1500  has a slot  1510  which divides the shaft  1502  and head  1506  into two portions, leaving the base  1504  intact. Each portion of the head  1506  includes a lateral enlargement  1512  which may be tapered on one side and flat on an opposite side. It can be appreciated from  FIG. 25B  that the tapered sides on the two head portions may be oppositely oriented. At least one portion of the head  1506  may include another enlargement  1514 , which may serve as a force input location to deflect the head portion. 
     Arm assembly  1300  may be assembled by sliding the base  1504  of arm lock  1500  into hole  1414  of arm  1400 , rotating arm lock  1500  so that enlargement  1514  is proximate the dovetail protrusion  1410  and a lateral enlargement  1512  is in slot  1412 , and inserting fastener  1498  into holes  1416  and  1508 . 
     Arm assembly  1300  may be connected to hub assembly  500  by sliding the dovetail protrusion  1410  into the dovetail slot  1206  so that the lateral enlargement  1512  engages the pocket  1208 . In some embodiments, the dovetail connection may be configured to permit the arm assembly  1300  to be adjustably connected to the hub assembly  500 . For example, the dovetail slot  1206  may be lengthened and provided with multiple pockets  1208 . This arrangement would permit the dilation device  400  to approach the operative site obliquely while still permitting the tips  1406  of the arms  1400  to intimately engage the structures surrounding the operative site. 
       FIG. 28  illustrates a set of dilators  1600 ,  1660 ,  1670 ,  1680 ,  1690 . Dilator  1600  has an elongated shaft  1602  extending between a base  1604  and a tip  1606 . A through hole  1608  extends the length of dilator  1600 . Hole  1608  may be sized to complement a guide wire, K-wire, Beath pin, stylus, or other small diameter rod. The base  1604  may be smooth or textured. For example,  FIG. 28  illustrates a textured base  1604  which has a regular pattern of grooves  1610 . Other textures are contemplated, such as knurling, dimples, or sandblasting. The tip  1606  includes a tapered region  1612  to reduce the outside diameter toward the end. The other dilators  1660 ,  1670 ,  1680 ,  1690  in the set may share some or all of the characteristics described for dilator  1600 . However, the outside diameter and inside diameter of each dilator may be selected so that all the dilators nest together with clearance. In this situation, the set of dilators may be described as a set of sequential dilators, since each incrementally larger dilator slides over the next smaller size dilator. The dilators may be color coded to communicate their relative size. 
       FIG. 29  illustrates a set of cannulas  1700 ,  1770 ,  1780 ,  1790 . Cannula  1700  has an elongated shaft  1702  extending between a base  1704  and a tip  1706 . A central through hole  1708  extends the length of cannula  1700 . Hole  1708  may be sized to complement one of the dilators  1600 ,  1660 ,  1670 ,  1680 ,  1690 , such as dilator  1660  for example. One or more peripheral through holes  1710  may extend the length of cannula  1700 . One or more external longitudinal grooves  1712  may be positioned to intersect, or interrupt, at least a portion of the length of holes  1710 . For example,  FIG. 29  shows groove  1712  interrupting a middle portion of hole  1710 . Cannula  1700  may also include one or more external longitudinal grooves  1714  which are separated from hole  1710 .  FIG. 29  shows cannula  1700  with a pattern of five evenly spaced grooves  1714  which alternate with five evenly spaced holes  1710 . Cannula  1700  may also include one or more windows  1716  in the bottom of each groove  1714 , as shown best in  FIG. 30B . For example, cannula  1700  has a pattern of twenty closely-spaced windows in the bottom of each groove  1714 . The base  1704  may be smooth or textured. For example,  FIG. 29  illustrates a smooth base  1704 . The tip  1706  includes a tapered region  1718  to reduce the outside diameter toward the end. The other cannulas  1770 ,  1780 ,  1790  in the set may share some or all of the characteristics described for cannula  1700 . However, the inside diameter of each cannula may be selected so that the cannula slides over a corresponding dilator with clearance. For example, cannula  1700  may slide over dilator  1660 , cannula  1770  may slide over dilator  1670 , cannula  1780  may slide over dilator  1680 , and cannula  1790  may slide over dilator  1690 . Furthermore, the inside diameter of at least one of the cannulas in the set may be selected to correspond to the outer dimension of an instrument or implant which must fit through the cannula. For example, the inside diameter of cannula  1700  may be selected to correspond to the outer dimension of a spinal discectomy instrument, while the inside diameter of cannula  1780  may be selected to correspond to the outer dimensions of a spinal implant, such as a fusion cage. Clearly, when such selections are made for the cannulas, corresponding selections may be necessary in the dilators. The cannulas may be color coded to communicate their relative size or their corresponding dilator. In some embodiments, one or more of the cannulas may be tapered so that the inside diameter at the tip  1706  corresponds to the outside diameter of a dilator, but the inside diameter at the base  1704  is larger. In still other embodiments, one or more of the cannulas may be shorter than the arms  1400 . The cannulas may be so short as to resemble a ring. Such a cannula may be inserted among the tips  1406  of the arms  1400 . 
     The dilators  1600 ,  1660 ,  1670 ,  1680 ,  1690  and/or cannulas  1700 ,  1770 ,  1780 ,  1790  may be at least partially radiolucent. Alternatively, the dilators and/or cannulas may be at least partially radiopaque. The dilators and/or cannulas may comprise metals such as aluminum, stainless steel, titanium, and other biocompatible metals. The dilators and/or cannulas may also comprise high density plastics such as Delrin, Radel, Udel, poly ether ether ketone (PEEK), polycarbonate, and acrylonitrile butadiene styrene (ABS), among others. The dilators and/or cannulas may be fabricated by machining, molding, casting, or other manufacturing operations. Barium sulphate may be added to constituent plastic materials to provide increased radiopacity. The dilators and/or cannulas may be reusable or disposable. The dilators and/or cannulas may conduct light or function as a light guide or light conductor. The dilators and/or cannulas may accept a reusable light ring, collar, or cap; fiber optic cables; or other light source. For example, a reusable light collar may snap onto a base  1704  of a cannula  1700  and key with the grooves  1714 , distributing light throughout a clear cannula. In another example, a disposable battery powered light source may be at least partially insertable into one or more of the holes  1710 , distributing light to the tip  1706  of the cannula. 
       FIGS. 30A-30B  illustrate dilation device  400  operatively assembled with cannula  1700 . The dilation device  400  is in an open configuration and cannula  1700  is in the middle of the circular pattern of arm assemblies  1300 . Each arm  1400  rests in a corresponding groove  1714 , and a lateral enlargement  1512  of each arm lock  1500  rests in a corresponding window  1716 . Groove  1714  and lateral enlargement  1512  may cooperate to guide insertion of the cannula  1700  into the dilation device  400 . It can be appreciated that the orientation of the tapered and flat sides of the lateral enlargement  1512  and the inherent resilience or spring-like property of the slotted shaft  1502  act to readily permit passage of the cannula  1700  in one direction, and act to prevent passage of the cannula  1700  in an opposite direction. Referring to  FIG. 30B , cannula  1700  will pass readily to the right, but is prevented from passing to the left. In this way, cannula  1700  may be prevented from unintended expulsion due to the resistance of the tissues in which it resides. 
       FIGS. 31A-37B  illustrate another dilation device  1800 . Dilation device  1800  may also be described as a dilation assembly since it includes several component parts in an operative arrangement.  FIGS. 31A-31B  show dilation device  1800  in a closed configuration.  FIGS. 32A-32B  show dilation device  1800  in an open unlocked configuration.  FIGS. 33A-33B  show dilation device  1800  in an open locked configuration. Dilation device  1800  includes a stationary disk  1900 , a drive disk  2000 , and three arms  2100 . Alternate embodiments may include two or more arms. The arms  2100  are carried by the stationary disk  1900  and the drive disk  2000 . In this embodiment, the arms  2100  are evenly spaced in a circular or triangular pattern around the disks  1900 ,  2000 , although non-circular or non-symmetric arrangements are contemplated. The pattern of arms  2100  synchronously expands and contracts relative to the two disks  1900 ,  2000 . In other words, each arm  2100  is radially movable relative to a center of the pattern. 
     A portion of the dilation device  1800  may be introduced into a muscle, other tissue, or natural passageway in the closed configuration, and the dilation device  1800  expanded from the closed configuration to the open configuration. If introduced into a muscle, the expansion of dilation device  1800  bluntly dissects and separates the muscle fibers. The expansion of dilation device  1800  forms an open passage through the muscle, tissue, or natural passageway, through which instruments, implants and other materials may be passed to perform one or more surgical procedures. 
       FIGS. 35A-35B  illustrate a stationary disk  1900  shaped as a ring or annulus with a central aperture  1902 . Stationary disk  1900 , as illustrated, has a concave side  1914  and a convex side  1916 , and may include a peripheral flange  1918 . Stationary disk  1900  includes three slots  1904 . Each slot  1904  may include a radial portion  1906  and a locking portion  1908 . The locking portion  1908  of stationary disk  1900  may be described as a dogleg, an “L”, or a “U”. Other locking portions are contemplated, such as an obliquely angled portion or a tapering portion. One or more additional windows  1910  may be present, for example, to reduce weight or provide easy access for cleaning. Stationary disk  1900  may include one or more peripheral indentations  1912  or other grip features, such as grooves or texturing. 
       FIGS. 36A-36B  illustrate a drive disk  2000  shaped as a ring or annulus with a central aperture  2002 . Drive disk  2000 , as illustrated, has a concave side  2004  and a convex side  2006 . Drive disk  2000  includes three arcuate, obliquely-extending slots  2008  evenly spaced around the disk. 
       FIGS. 37A-37B  illustrate an arm  2100 , or blade, with an elongated shaft  2102  extending between a base  2104  and a tip  2106 . Shaft  2102  has a concave side  2108  and an opposite convex side  2110 . Base  2104  includes a dogleg portion  2112 , or L-shaped portion, which protrudes laterally from shaft  2102 . A pair of bosses  2114  protrude from the dogleg portion  2112  generally parallel to the shaft  2102 . In another embodiment, the bosses  2114  may be replaced by a pin secured in a hole in the L-shaped portion  2112 . 
     Referring to  FIG. 34 , dilation device  1800  may be assembled by inserting a boss  2114  of an arm  2100  into a slot  2008  of the drive disk  2000  so that the dogleg portion  2112  is on the concave side  2004  and the shaft  2102  passes through the central aperture  2002 . The remaining two arms  2100  may be assembled to the drive disk  2000  in a similar fashion. Stationary disk  1900  may be oriented so that the convex side  1916  faces the concave side  2004  and a boss  2114  is in each slot  1904 . Stationary disk  1900  may be axially secured to drive disk  2000  while retaining freedom to rotate relative to drive disk  2000 , at least within angular limits. For example, stationary disk  1900  may be connected to drive disk  2000  by a retaining ring (not shown). 
     Referring to  FIGS. 38-40B , dilation device  400  is shown operatively assembled to a modified dilator  2200 . Compared to dilators  1600 ,  1660 ,  1670 ,  1680 ,  1690  of  FIG. 28 , dilator  2200  has been modified to positively interact with arms  1400  to aid in holding the dilation device  400  closed during insertion into tissue or a natural passageway. 
     Referring to  FIGS. 40A-40B , dilator  2200  has an elongated shaft  2202  extending between a base  2204  and a tip  2206 . A through hole  2208  extends the length of dilator  2200 . Hole  2208  may be sized to complement a guide wire, K-wire, Beath pin, stylus, or other small diameter rod. One or more ribs  2210  extend the length of dilator  2200 . As illustrated in  FIG. 40B , dilator  2200  includes five evenly spaced ribs  2210 . The base  2204  may be smooth or textured. For example,  FIG. 28  illustrates a textured base  2204  which has a regular pattern of indentations  2212 . Other textures are contemplated, such as knurling, dimples, or sandblasting. The tip  2206  includes a tapered region  2214  to reduce the outside diameter toward the end. 
       FIG. 39B  illustrates how the dilator  2200  positively engages the arms  1400  to hold the dilation device  400  closed during insertion. The dilator  2200  fits in the middle of the pattern of arms  1400  with the ribs  2210  between adjacent arms  1400 . The ribs  2210  support the arms  1400  so that the arms  1400  remain straight during insertion. The arms  1400  are prevented from twisting or bending laterally while supported by the ribs  2210 . 
     Referring to  FIGS. 41A-45B , hub assembly  500  is shown operatively assembled to modified arm assemblies  2300  and an arm retention clip  2400 . Compared to arm assembly  1300  of  FIG. 24A , arm assembly  2300  has been modified to positively interact with arm retention clip  2400  to aid in holding the dilation device closed during insertion into tissue or a natural passageway. Arm retention clip  2400  may be connected to hub assembly  500  with arm assemblies  2300  before introducing the dilation device into tissue or a natural passageway, and may be removed after the dilation device is fully introduced. 
       FIG. 44  illustrates an arm  2500 , or blade, with an elongated shaft  2502  extending between a base  2504  and a tip  2506 . Arm  2500  may share some or all of the characteristics of arm  1400  of  FIG. 26 . One side of arm  2500  has a full length groove  2508  or depression, which is interrupted by a deeper groove  2518  centrally located along the shaft  2502  length. The base  2504  includes a dovetail protrusion  2510  on a side opposite the groove  2508 . The dovetail protrusion  2510  may be sized and shaped to complement dovetail slot  1206  of arm clamp  1200 . The tip  2506  includes a waist  2512  and an adjacent flared portion  2514 . Together, the waist  2512  and flared portion  2514  form a concavely curved area on the tip  2506 , which may aid in holding back or retaining tissues dissected and pushed aside by the dilation device. 
       FIGS. 45A-45B  illustrate an arm retention clip  2400  which includes a body  2402 , one or more short prongs  2404 , one or more intermediate length prongs  2406 , and a central shaft  2408 , or stylus. The body may include a protrusion  2410  opposite the prongs  2404 ,  2406  and shaft  2408 . Protrusion  2410  may serve as a handle. Shaft  2408  includes one or more laterally projecting tabs  2412  which may be positioned at a distance from the body  2402 . Tabs  2412  may be sized and shaped, at least in cross section, to complement groove  2518  of arm  2500 . Shaft  2408  may also include a through hole  2414 , or cannulation, which may be sized to receive a guide wire, K-wire, Beath pin, stylus, or other small diameter shaft. 
     When clip  2400  is connected to hub assembly  500  and arm assemblies  2300 , the central shaft  2408  is positioned in the middle of the pattern of arms  2500  ( FIGS. 42B-42C ), the prongs  2406  are positioned outside the pattern of arms  2500  and at least partially between adjacent arms  2500  ( FIG. 42C ), and the prongs  2404  are positioned where adjacent arm clamps  1200  touch ( FIG. 42D ). Furthermore, as may be appreciated from  FIG. 42B , the tabs  2412  are positioned within the corresponding grooves  2518  so that the arms  2500  are supported against lateral bending and torsion deflection from within the pattern of arms. While groove  2518  and tab  2412  are shown with parallel sides, so that tab  2412  may slide laterally in and out of groove  2518 , other shapes are contemplated. For example, groove  2518  and tab  2412  may have a dovetail shape so that tab  2412  may only slide in and out of groove  2518  in an axial direction. Groove  2518  and tab  2412  may be described as connecting features which cooperate to place the arms  2500  in a predetermined longitudinal alignment with the shaft  2408  when the connecting features are mutually engaged. 
     Referring to  FIGS. 46-49 , hub assembly  500  is shown operatively assembled to modified arm assemblies  2600  and an arm retention clip  2700 . Compared to arm assembly  1300  of  FIG. 24A , arm assembly  2600  has been modified to positively interact with arm retention clip  2700  to aid in holding the dilation device closed during insertion into tissue or a natural passageway. Arm retention clip  2700  may be connected to hub assembly  500  with arm assemblies  2600  before introducing the dilation device into tissue or a natural passageway, and may be removed after the dilation device is fully introduced. 
       FIG. 48A  illustrates an arm  2800 , or blade, with an elongated shaft  2802  extending between a base  2804  and a tip  2806 . Arm  2800  may share some or all of the characteristics of arm  1400  of  FIG. 26 . One side of arm  2800  has a full length groove  2808  or depression. The base  2804  includes a dovetail protrusion  2810  on a side opposite the groove  2808 . The dovetail protrusion  2810  may be sized and shaped to complement dovetail slot  1206  of arm clamp  1200 . The tip  2806  includes a waist  2812  and an adjacent flared portion  2814 . Together, the waist  2812  and flared portion  2814  form a concavely curved area on the tip  2806 , which may aid in holding back or retaining tissues dissected and pushed aside by the dilation device  400 . A lateral side of the tip  2806  includes a protruding tab  2820 . An opposite lateral side of the tip  2806  includes a recessed slot  2822  or socket. Slot  2822  is sized and positioned to receive a tab  2820  of an adjacent arm  2800 , as may be appreciated in  FIG. 48B , which shows an enlarged detail view of a pattern of arms  2800  in a partially open configuration. Tab  2820  and slot  2822  may be described as connecting features which cooperate to hold arms  2800  in a predetermined longitudinal alignment. Tab  2820  and slot  2822  may also be described as complementary engagement features which cooperate on adjacent arms to place the arms in contacting longitudinal alignment with one another along their lateral sides when the arms are in the closed configuration. 
       FIG. 49  illustrates an arm retention clip  2700  which includes a body  2702 , one or more short prongs  2704 , and one or more intermediate length prongs  2706 . The body may include a protrusion  2710  opposite the prongs  2704 ,  2706 . Protrusion  2710  may serve as a handle. Clip  2700  may also include a through hole  2714 , or cannulation, which may be sized to receive a guide wire, K-wire, Beath pin, stylus, or other small diameter shaft. 
     When clip  2700  is connected to hub assembly  500  and arm assemblies  2600 , the prongs  2706  are positioned outside the pattern of arms  2500  and at least partially between adjacent arms  2500  ( FIG. 46C ), and the prongs  2704  are positioned where adjacent arm clamps  1200  touch ( FIG. 46D ). Furthermore, as may be appreciated from  FIG. 46B , the tabs  2820  are positioned within the corresponding slots  2822  so that the arms  2800  are self-supported against lateral bending and torsion deflection. 
       FIGS. 50-55B  illustrate clamp  30  in greater detail. Clamp  30  may also be described as a clamp assembly since it includes several component parts in an operative arrangement. Clamp  30  includes a stationary jaw  3100 , a movable jaw  3200 , a lever  3300 , a link  3400 , an optional spring  3500 , a shaft  3600 , and fasteners  3198 ,  3398 .  FIG. 50  shows clamp  30  in a closed configuration.  FIGS. 51-52B  show clamp  30  in an open configuration. Clamp  30  may be designed to releasably lock onto spheres  36 ,  1102 . For example, stationary jaw  3100  may include a partial spherical surface  3102  and movable jaw may include a partial spherical surface  3202 . Surfaces  3102 ,  3202  may be sized to complement spheres  36 ,  1102 . Surfaces  3102 ,  3202  may have non-spherical shapes, for example, conical or stepped cylindrical shapes. Surfaces  3102 ,  3202  may also include macro- or micro-texturing. Clamp  32  may share some or all of the characteristics of clamp  30 . 
     Dilation devices  400 ,  1800 , or hub assembly  500  with arm assemblies  2300  or  2600 , may include one or more nerve retractor ports, which may be described as points of access in the device where a nerve retractor may be insinuated into the device to move a nerve out of the way. 
     Referring to  FIGS. 56-61 , methods of use will be described for dilation device  400 . Similar methods of use may be applicable to dilation device  1800 , or to hub assembly  500  in combination with arm assemblies  2300  or  2600 . 
     Referring to  FIG. 56 , dilation device  400  is shown in the closed configuration. Clamp  30  has been locked to a sphere  1102 . Clamp  32  has been positioned on a sphere  1102 , but is illustrated in the open configuration. The shafts  3600  of clamps  30 ,  32  may be locked to an external support (not shown), such as a surgical table mounted support system, which may provide stability and support to the hub and dilation device during surgical procedures Optionally, prior to inserting dilation device  400  through tissues or into a natural passageway, a stylus, guide wire, K-wire, Beath pin, or other small-diameter shaft or tube may be inserted through the tissues or passegeway. The stylus tip position and shaft trajectory may be verified, for example, with imaging or other detection means to ensure the desired operative site is targeted. In this situation, dilation device  400  may be inserted over the stylus by sliding the stylus into a central space in the middle of the pattern of arms. Whether or not a stylus was used to locate the operative site and trajectory, an arm retention means may be employed during insertion of dilation device  400  or other dilation devices. For example, a retention band  64  may be applied around the waist  1418  of arms  1400 . In another example, a dilation device may be introduced with an arm retention clip  2400  or  2700  connected to the device. In this situation, the retention clip  2400  or  2700  is removed after the dilation device is satisfactorily introduced. Some embodiments may include a lock or clip to immobilize the drive disk  700  with the dilation device  400  in the closed configuration, such as by interacting with the tab  710 . Other embodiments may replace the manually operated tab  710  with a geared linkage which may make the dilation device  400  operate more smoothly. 
     Referring to  FIG. 57 , dilation device  400  is shown with dilator  1600  inserted in the middle of the pattern of arms. The stylus, if used, is received in hole  1608  of dilator  1600 . As the dilator  1600  is inserted into dilation device  400 , the arms  1400  are pushed radially outward from the center of the pattern. The bases  1404  of the arms  1400  may be pushed radially outward before, or at the same time as, the tips  1406 . In other words, the arms  1400  may be straight, or they may toe out or toe in. The surrounding tissue or passageway is pushed radially outward by the arms. No shearing, pulling, or dragging force is exerted on the tissue, at least in the direction of advancement of the dilator  1600 . Thus, the potentially problematic effects of sequential dilation directly against tissue are avoided. Furthermore, since the dilator  1600  pushes against the arm assemblies  1300  instead of tissue, it may require significantly less effort, and therefore less time, to insert each dilator. Alternatively, dilation device  400  may be initially introduced with dilator  2200  inserted in the middle of the pattern of arms; an optional stylus may be used as well. 
     Referring to  FIG. 58 , dilation device  400  is shown after insertion of dilator  1660  over dilator  1600 . As dilator  1660  is inserted into dilation device  400 , the arms are pushed radially outward farther from the center of the pattern, and the surrounding tissue is urged radially outward as a result. 
     Additional dilators may be sequentially inserted into the dilation device  400  to increase the diameter of the working passageway. However, it may be possible to perform at least a portion of a surgical procedure through a relatively small-diameter cannula such as cannula  1700  in order to minimize insult to the surrounding tissues, at least by reducing the length of time those tissues are maximally dilated. Cannula  1700  may be introduced into the dilation device over dilator  1660 , taking care to align the grooves  1714  with the lateral enlargements  1512 . As cannula  1700  is inserted into dilation device  400 , the arms are pushed radially outward farther from the center of the pattern, and the surrounding tissue is urged radially outward as a result. Thus cannula  1700  may function as a dilator. After cannula  1700  is inserted, it is prevented from expulsion by the action of the lateral enlargements  1512  in the windows  1716 . Cannula  1700  may be further stabilized by inserting fasteners  1798  through holes  1710  and into the structures surrounding the operative site. Fasteners  1798  may be hex-headed tip-threaded pins, as shown, which may thread into bone. After cannula  1700  is adequately stabilized, the stylus (if used) and dilators  1600 ,  1660  may be removed to open up the interior of the cannula  1700  for the passage of instruments or implants. 
     Referring to  FIG. 59 , dilation device  400  is shown with cannula  1700  inserted in the middle of the pattern of arms and fixed in place with pins  1798 . Cannula  1700  may provide an adequate working space for instruments or implants. For example, cannula  1700  may be an appropriate size for the passage of minimally invasive discectomy instruments. Alternatively, dilator  1660  may function as a cannula. 
     Dilator  1670  may be inserted over cannula  1700  to further dilate the tissues. As dilator  1670  is introduced between cannula  1700  and the arm assemblies  1300 , the lateral enlargements  1512  may be automatically pushed out of the windows  1716 , disengaging the cannula  1700  from the dilation device  400 . Should even more dilation be required, dilators  1680 ,  1690  may be used sequentially over dilator  1670 . 
     Referring to  FIG. 60 , dilation device  400  is shown with cannula  1700  still fixed in place with pins  1798 , and with dilators  1670 ,  1680 ,  1690  nested over cannula  1700 . 
     Cannula  1790  may be introduced into the dilation device over dilator  1690 , taking care to align the cannula grooves  1714  with the lateral enlargements  1512 . As cannula  1790  is introduced into the dilation device  400 , the arms are pushed farther laterally. Thus cannula  1790  may function as a dilator. After cannula  1790  is inserted, it is prevented from expulsion by the action of the lateral enlargements  1512  in the windows  1716 . Cannula  1790  may be further stabilized by inserting fasteners  1798  through holes  1710  and into the structures surrounding the operative site. After cannula  1790  is adequately stabilized, the cannula  1700  and dilators  1660 ,  1670 ,  1680  may be removed to open up the interior of the cannula  1790  for the passage of instruments or implants. Alternatively, cannula  1770  may be introduced after dilator  1670 , or cannula  1780  may be introduced after dilator  1680 . In still other alternatives, dilators  1670 ,  1680 ,  1690  may function as cannulas. 
     Referring to  FIG. 61 , dilation device  400  is shown with cannula  1790  inserted in the middle of the pattern of arms and fixed in place with pins  1798 . Cannula  1790  may provide an adequate working space for instruments or implants. For example, cannula  1790  may be an appropriate size for the passage of a spinal implant, such as a total disc implant or fusion cage. 
       FIGS. 62-67  illustrate steps in a method of use for dilation device  400  in the spine. Similar methods of use may be applicable to dilation device  1800 , or to hub assembly  500  in combination with arm assemblies  2300  or  2600 . 
     In  FIG. 62 , a stylus  71  has been introduced to a lumbar intervertebral disc through a direct lateral approach. In  FIG. 63 , a dilation device  400  has been introduced over the stylus  71 . Dilation device  400  may be stabilized with clamps  30 ,  32  (not shown). Alternatively, the circular nature of the hub assembly  500  of dilation device  400  provides opportunities for it to be used in the practice of targeting the initial stylus to the intervertebral space. In  FIG. 64 , dilator  1600  has been introduced over stylus  71  and within the pattern of arms  1400 . In  FIG. 65 , dilator  1660  has been introduced over dilator  1600 . In  FIG. 66 , cannula  1700  has been introduced and stabilized with fasteners  1798 , and stylus  71  and dilators  1600 ,  1660  have been removed. A discectomy procedure may be performed with minimally invasive instruments working through cannula  1700 . In  FIG. 67 , cannula  1790  has been introduced and stabilized with fasteners  1798 , and cannula  1700  and dilators  1670 ,  1680 ,  1690  have been removed. A spinal implant may be inserted with minimally invasive instruments working through cannula  1790 . At the conclusion of the surgical procedure, cannula  1790  may be released from dilation device  400  by moving tab  710  sideways to expand the pattern of arms, thus disengaging lateral enlargements  1512  from windows  1716 . 
     One way to view the teachings set forth above is to characterize certain structures as connecting means for placing each arm in a predetermined longitudinal alignment with the stylus. In the various embodiments set forth above the connecting means can be said to be elements  79  and  97  as shown in  FIGS. 4A, 5A and 5B ; elements  182  and  204  as shown in  FIGS. 8 and 9 ; elements  1400  and  2210  as shown in  FIGS. 38-40B ; elements  2518  and  2412  as shown in  FIGS. 41A-45B ; and elements  2820 ,  2822 , and  2700  as shown in  FIGS. 46A-49 . Other connecting means are contemplated, including but not limited to pegs and corresponding holes which are round, oval, rectangular, or multi-sided; or other complementary protrusion and slot combinations. The receiving hole may be open on both ends or may be a recess or cavity with an opening on one side shaped to receive the peg. In an alternative embodiment, the pegs may be located on the arms, and the receiving hole or cavity on the stylus or stylus tip. 
     Certain aspects of the teaching set forth above can be characterized as lateral engagement means for placing the arms in contacting longitudinal alignment with one another along their first and second lateral edges. The structure for the lateral engagement means is found in  FIGS. 5C and 5D  in elements  108 ; in  FIGS. 7-9  in elements  216 ,  218 ,  224  and  226 ; in  FIGS. 41A-45B  in elements  2518  and  2412 ; and in  FIGS. 46A-49  in elements  2820 ,  2822 , and  2700 . Other lateral engagement means are contemplated, including but not limited to tongue-in-groove features, corresponding tab and slot features, or press-fit features. Such features may be disengaged by removal of a pin, suture or wire such as release wire  226 , or may have a friction fit in which the features are detached from one another by sufficient expansive force provided by expansion of the dilating member. 
     Some aspects of the teaching set forth above can be characterized as a means for dilation. In the various embodiments set forth above the means for dilation can be said to be element  110  in  FIGS. 3, 4A, and 4B ; elements  240  and  242  in  FIGS. 8-11 ; element  300  in  FIGS. 14 and 15 ; elements  1600 ,  1660 ,  1670 ,  1680 , and  1690  in  FIG. 28 ; elements  1700 ,  1770 ,  1780 , and  1790  in  FIG. 29 ; and element  2100  in  FIGS. 37A-37B . Other dilation means contemplated include expansion instruments such as retractors and other mechanical expanders. 
     Some aspects of the teaching set forth above can be characterized as a means for circumferentially surrounding at least a portion of the dilating member. In the various embodiments set forth above the means for circumferentially surrounding at least a portion of the dilating member can be said to be elements  130  and  140  in  FIGS. 1-4B  and  FIGS. 6A-6B , element  246  in  FIG. 11 , element  290  in  FIGS. 12-14 , element  1400  in  FIG. 26 , element  2500  in  FIG. 44 , and element  2800  in  FIG. 48A . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. It is appreciated that various features of the above-described examples can be mixed and matched to form a variety of other alternatives. For example, the dilating member may comprise a balloon, and/or a cannula. Embodiments may variously include connecting features between the stylus and the plurality of arms, and engagement features between individual arms. It is also appreciated that this system is not limited to creating a passage through a muscle; it may be used to create a passage through any soft tissues, or to dilate and hold open a naturally occurring passage. As such, the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.