Patent Publication Number: US-2012029296-A1

Title: Expandable cannula assemblies for use in percutaneous surgical procedures

Description:
BACKGROUND 
     The present application relates to cannula assemblies and methods for using same in performing surgery in a patient, and more particularly, but not exclusively, relates to cannula assemblies that provide expandable working channels. 
     Traditional surgical procedures for pathologies located within the body involve extensive and lengthy cutting, removing, and or repositioning skin and tissue surrounding the surgical site in order for the surgeon to access the surgical site. This type of approach may cause trauma, damage, and scarring to the tissue, and also presents risks that the tissue will become infected and that a long recovery time will be required after surgery for the tissue to heal. In some cases, these invasive procedures lead to permanent scarring and pain that can be more severe than the pain leading to the surgical intervention. 
     Minimally invasive surgical techniques are particularly desirable in, for example, spinal and neurosurgical applications because of the need for access to locations deep within the body and the presence of vital intervening tissues. The development of percutaneous minimally invasive spinal procedures has yielded a major improvement in reducing recovery time and post-operative pain because they require minimal, if any, muscle dissection and can be performed under local anesthesia. These benefits of minimally invasive techniques have also found application in surgeries for other locations in the body where it is desirable to minimize tissue disruption and trauma. However, current techniques for minimally invasive surgery can require numerous steps before access is gained to the surgical site which can lead to prolonged retraction of tissues and increased complexity and duration of the surgical procedure, amongst other things. Thus, there remains a need for further improvements in the devices, instruments, assemblies, apparatuses, systems and methods for performing minimally invasive and other surgical techniques. 
     SUMMARY 
     One nonlimiting embodiment of the present application is directed to an expandable cannula assembly for use in percutaneous surgical procedures. The cannula assembly includes an elongate body extending along a longitudinal axis between a first end and a second end. The elongate body includes a pair of elongate members that cooperate to define a working channel extending from a proximal end to a distal end and being expandable from a first, unexpanded configuration to a second, expanded configuration. The elongate members are displaceable away from one another along the longitudinal axis from the proximal end to the distal end of the working channel in order to expand the working channel from the first configuration to the second configuration. In one aspect of this embodiment, the elongate members engage with one another to lock the working channel in the second configuration and prevent movement of the elongate members toward one another. 
     In another embodiment, an expandable cannula assembly includes an elongate body extending along a longitudinal axis between a first end and a second end. The elongate body includes a first member partially enclosing a first hollow interior and a second member partially enclosing a second hollow interior. The hollow interiors cooperate to define a working channel extending between a proximal end and a distal end. The working channel is expandable along the longitudinal axis from a first, unexpanded configuration where the second member is positioned in the first hollow interior to a second, expanded configuration by laterally displacing the first and second members away from one another from the proximal end to the distal end. In one aspect of this embodiment, at least a portion of the second member extends outwardly from the first hollow interior of the first member. 
     In yet another embodiment, a cannula assembly includes an elongate body extending along a longitudinal axis between a first end and a second end. The elongate body includes a first elongate member and a second elongate member cooperating to define a working channel extending from a proximal end to a distal end and being expandable from a first, unexpanded configuration to a second, expanded configuration. The first and second elongate members are displaceable from one another along the longitudinal axis from the proximal end to the distal end of the working channel to expand the working channel to the second configuration. In one aspect of this embodiment, the working channel includes a length between the proximal end and the distal end that is greater than a maximum dimension across the working channel in the second configuration. Still, in another aspect of this embodiment, the first elongate member includes a first portion extending about a second portion of the second elongate member from the proximal end to the distal end of the working channel. 
     In a further embodiment, a method includes providing a first cannula extending between opposite first and second ends and including a pair of elongate members cooperating to define a first working channel extending between a proximal end and a distal end and being expandable from a first, unexpanded configuration to a second, expanded configuration. A first one of the elongate members is positioned in a second one of the elongate members when the first working channel is in the first configuration. The method also includes positioning the first cannula with the first working channel in the first configuration at a location adjacent to a surgical site and displacing the elongate members laterally away from one another from the proximal end to the distal end of the first working channel to expand the first working channel to the second configuration. Displacing the elongate members includes radially expanding the second one of the elongate members to facilitate expulsion of a portion of the first one of the elongate members from the second one of the elongate members. Still, other methods for using expandable cannula assemblies are also provided. 
     Another embodiment of the present application is a unique system for surgery in a patient. Other embodiments include unique methods, systems, devices, kits, assemblies, equipment, and/or apparatus for use in connection with percutaneous surgical procedures. However, in other embodiments, different forms and applications are envisioned. 
     Further embodiments, forms, features, aspects, benefits, objects and advantages of the present application shall become apparent from the detailed description and figures provided herewith. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of one embodiment of an expandable cannula assembly. 
         FIG. 2  is a section view of the cannula assembly illustrated in  FIG. 1  taken along view line  2 - 2 . 
         FIG. 3  is a perspective view of a separation instrument and the cannula assembly illustrated in  FIG. 1  in an expanded configuration. 
         FIG. 4  is a section view of the cannula assembly illustrated in  FIG. 3  taken along view line  4 - 4 . 
         FIG. 5  is a section view of an alternative embodiment expandable cannula assembly. 
         FIG. 6  is a side view of a delivery cannula positioned at a location adjacent to a surgical site. 
         FIG. 7  is a side view of the cannula assembly illustrated in  FIG. 1  positioned in the delivery cannula of  FIG. 6 . 
         FIG. 8  is a section view taken along view line  8 - 8  of  FIG. 7 . 
     
    
    
     DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any such alterations and further modifications in the illustrated devices and described methods, and any such further applications of the principles of the invention as illustrated herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Instruments, systems, apparatuses, assemblies and methods for performing surgery, including spinal surgeries that include one or more techniques such as laminotomy, laminectomy, foramenotomy, facetectomy, discectomy, interbody fusion, spinal nucleus or disc replacement, and implant insertion including interbody implants and bone engaging fasteners, for example, are provided. The surgery is performed through a working channel or passageway through skin and/or tissue of the patient provided by an expandable cannula assembly. In one form, viewing of the surgical site at the working end of the cannula assembly can be accomplished with naked eye visualization, microscopic viewing devices, loupes, viewing instruments mounted on the cannula, positioned over the cannula, positioned in other portals in the body, and/or through a viewing system such as lateral fluoroscopy. The cannula assembly is movable in situ to increase the size of the working channel to facilitate access to the working space at the distal end of the cannula assembly while minimizing trauma to tissue surrounding the cannula assembly. The cannula assembly can be used with any surgical approach to the spine, including anterior, posterior, posterior mid-line, lateral, postero-lateral, and/or antero-lateral approaches, and in other regions besides the spine. 
     In one embodiment, an expandable cannula assembly for use in percutaneous surgical procedures includes an elongate body extending along a longitudinal axis between a first end and a second end. The elongate body includes a pair of elongate members that cooperate to define a working channel extending from a proximal end to a distal end and being expandable from a first, unexpanded configuration to a second, expanded configuration. The elongate members are displaceable away from one another along the longitudinal axis from the proximal end to the distal end of the working channel in order to expand the working channel from the first configuration to the second configuration. In one aspect, the elongate members engage with one another to lock the working channel in the second configuration and prevent movement of the elongate members toward one another. Still, it should be appreciated that alternative forms, aspects, configurations, arrangements and methods are contemplated with respect to the subject matter disclosed and described herein. 
     Referring now generally to  FIGS. 1-4 , further details regarding cannula assembly  10  will be provided. More particularly, as illustrated in the perspective view of  FIG. 1 , cannula assembly  10  includes an elongate body  12  extending between a proximal end  14  and a distal end  16 . Elongate body  12  includes a pair of elongate members  18 ,  20  that extend between proximal end  14  and distal end  16  and can include beveled ends at distal end  16  to facilitate insertion in an incision, although non-beveled ends are also contemplated. Elongate members  18 ,  20  cooperate to define and surround an expandable working channel  22  illustrated in  FIG. 2  for example. Working channel  22  extends between and opens at distal end  16  of elongate body  12  and to a proximal end  24  positioned distally of proximal end  14  of elongate body  12 . Elongate body  12  generally includes a length LL between proximal end  24  and distal end  16  that facilitates positioning proximal end  24  above the skin of a patient when distal end  16  is positioned adjacent to the targeted surgical site. 
     Elongate member  18  includes a first perimeter length around its exterior surface  26  between distal end  16  of elongate body  12  and proximal end  24  of working channel  22  and a second, smaller perimeter length around its exterior surface  26  between proximal end  24  of working channel  22  and proximal end  14  of elongate body  12  along a stem portion  28 . Elongate member  20  also includes a first perimeter length around its exterior surface  30  (shown in  FIG. 2 ) between distal end  16  of elongate body  12  and proximal end  24  of working channel  22  and a second, smaller perimeter length around its exterior surface  30  between proximal end  24  of working channel  22  and proximal end  14  of elongate body  12  along a stem portion  32 . In other non-illustrated forms, it is contemplated that elongate body  12  could be provided without stem portions  28 ,  32  such that the perimeter length around the exterior surfaces  26 ,  30  is the same from proximal end  24  to distal end  16 . In addition, in the form where stem portions  28 ,  32  are not included, it should be appreciated that the proximal and distal ends of elongate body  12  will correspond to the proximal and distal ends of working channel  22 . 
     As illustrated in  FIG. 2 , elongate member  18  includes a sidewall  34  having a generally c-shaped configuration in a plane extending orthogonally to longitudinal axis L. However, it should be appreciated that alternative configurations for sidewall  34  are also contemplated. In the illustrated form, sidewall  34  is generally arcuately rounded between a first end portion  38  and an oppositely positioned second end portion  40  and defines an opening  36  positioned between first and second end portions  38 ,  40  Opening  36  is laterally offset from longitudinal axis L and communicates with a hollow interior  42  that is enclosed and surrounded by sidewall  34  with the exception of opening  36 . Moreover, first and second end portions  38 ,  40  have a linear configuration that extends inwardly into hollow interior  42  relative to the remaining portions of sidewall  34 . Stated alternatively, first and second end portions  38 ,  40  do not continue along the same radial arc as the remainder of sidewall  34 . 
     Elongate member  20  includes a sidewall  44  having a generally u-shaped configuration in a plane extending orthogonally to longitudinal axis L. However, it should be appreciated that alternative configurations for sidewall  44  are also contemplated, including a generally c-shaped configuration for example. In the illustrated form, sidewall  44  includes an arcuately rounded portion  46  extending between and connected to a first linear portion  48  and an oppositely positioned second linear portion  50  by a pair of rounded transition portions  46   a,    46   b,  respectively. Sidewall  44  further defines an opening  52  positioned between first and second linear portions  48 ,  50  at an end positioned opposite of arcuately rounded portion  46 . Opening  52  is laterally offset from longitudinal axis L and communicates with a hollow interior  54  that is enclosed by sidewall  44  with the exception of opening  52 . Moreover, first linear portion  48  includes a receptacle  56  that is positioned opposite of arcuately rounded portion  46  and is configured to receive first end portion  38  of sidewall  34  when working channel  22  is enlarged to the expanded configuration. Similarly, second linear portion  50  includes a receptacle  58  that is positioned opposite of arcuately rounded portion  46  and is configured to receive second end portion  40  of sidewall  34  when working channel  22  is enlarged to the expanded configuration. Receptacles  56 ,  58  generally extend orthogonally to axes upon which the remaining portions of linear portions  48 ,  50  extend. Further details regarding the engagement of first and second end portions  38 ,  40  with receptacles  56 ,  58  will be provided below. 
     Working channel  22  is illustrated in an unexpanded configuration in  FIG. 2 . In this configuration, elongate member  20  is housed within hollow interior  42  of elongate member  18  such that sidewall  34  generally surrounds elongate member  20  and opening  36  is positioned outside or laterally of elongate member  20 . Moreover, transition portions  46   a,    46   b  and linear portions  48 ,  50  of sidewall  44  adjacent to receptacles  56 ,  58  are positioned in engagement with interior surface  27  of sidewall  34  of elongate member  18 . In addition, hollow interiors  42 ,  54  cooperate to define working channel  22  which, in the unexpanded configuration, is surrounded and enclosed by sidewall  44  and a portion of sidewall  34  in a plane extending orthogonally to longitudinal axis L. 
     Cannula assembly  10  further includes a first tab member  60  coupled to stem portion  28  of elongate member  18  and a second tab member  62  coupled to stem portion  32 . However, in certain embodiments, where stem portions  28 ,  32  are not included for example, it is contemplated that tab members  60 ,  62  will not be included by cannula assembly  10 . As illustrated in  FIG. 3  for example, tab member  60  includes an elongate groove  60   c  positioned between a pair of projections  60   a  and a pair of indentations  60   b  and tab member  62  includes an elongate groove  62   c  positioned between a pair of projections  62   a  and a pair of indentations  62   b.  Projections  60   a  can be positioned into indentations  62   b  and projections  62   a  can be positioned in indentations  60   b  to provide a snap-fit engagement such that tab members  60 ,  62  are releasably engageable to one another as illustrated in  FIG. 1  for example. However, it should be appreciated that alternative arrangements are contemplated for releasably engaging tab members  60  and  62 . Examples of such arrangements include ball-detent mechanisms, fasteners, threaded coupling members, clamping members, snap rings, compression bands, and straps, just to name a few possibilities. When tab members  60 ,  62  are engaged with one another, elongate grooves  60   c  and  62   c  cooperate to define an elongate passage  64  that extends through tab members  60 ,  62  and between stem portions  28 ,  32  into communication with proximal end  24  of working channel  22 . In addition, the engagement of tab members  60 ,  62  with one another prevents rotation of elongate members  18 ,  20  relative to one another and, as illustrated in  FIG. 2  for example, maintains alignment of arcuately rounded portion  46  of elongate member  20  with opening  36  of elongate member  18 . 
     Elongate body  12  of cannula assembly  10  is insertable through an incision in skin and tissue of a patient to provide working channel  22  to a surgical site. It is contemplated that elongate body  12  can initially be inserted through skin and tissue in an insertion configuration for working channel  22  such as the configuration illustrated in  FIGS. 1 and 2  for example. Working channel  22  can have a size in the insertion configuration that allows access to the surgical location in the patient&#39;s body to perform one or more steps of the surgical procedure. However, it may be desirable during surgery to provide greater access to the surgical site in the patient&#39;s body beyond what is provided through working channel  22  in its initial insertion configuration. Similarly, after insertion into the patient, working channel  22  can be enlarged by separating elongate members  18 ,  20  from one another along longitudinal axis L. Separation of elongate members  18 ,  20  increases the size of working channel  22  from proximal end  24  to distal end  16 . Further details regarding the expansion of working channel  22  will be provided below in connection with  FIGS. 3 and 4 . 
     More particularly, in order to expand working channel  22 , tab members  60 ,  62  are first separated from one another. While not illustrated, it should be appreciated that in one form tab members  60 ,  62  could be separated from one another by distally moving a tapered separation instrument through elongate passage  64  until projections  60   a  are released from indentations  62   b  and projections  62   a  are released from indentations  60   b.  Once released from engagement with one another, the proximal ends of tab members  60 ,  62  may be pivoted distally away from the longitudinal axis L to position tab members  60 ,  62  in the configuration illustrated in  FIG. 3 . As tab members  60 ,  62  are pivoted in this manner, stem portions  28 ,  32 , which may generally be more flexible than the remaining portions of elongate members  18 ,  20  between distal end  16  and proximal end  24  of working channel  22 , are bent or deformed adjacent to proximal end  24  of working channel  22 . In one form, it is contemplated that a surgeon or other medical professional could separate tab members  60 ,  62  from one another relative to longitudinal axis L as indicated by directional arrows A in  FIG. 3  once they have been pivoted to the illustrated configuration. Alternatively, it is also contemplated that tab members  60 ,  62  could be separated in this manner as they are pivoted away from the longitudinal axis L. 
     As tab members  60 ,  62  are separated from one another as indicated by directional arrows A, elongate members  18 ,  20  are moved away from one another and transition portions  46   a,    46   b  of elongate member  20  adjacent proximal end  24  of working channel  22  are brought into further engagement with interior surface  27  of elongate member  18 . As this occurs, elongate member  18  is radially expanded about longitudinal axis L, thereby increasing the distance across opening  36  and allowing a portion of elongate member  20  to be expelled or displaced from hollow interior  42  of elongate member  18 . Upon continued lateral displacement of elongate member  20  from hollow interior  42 , first and second end portions  38 ,  40  of sidewall  34  of elongate member  18  are received in receptacles  56 ,  58  as illustrated in  FIG. 4  for example. 
     Once working channel  22  is expanded adjacent to proximal end  24 , a separation instrument  70  can be inserted into working channel  22  and moved distally therethrough in order to displace elongate member  20  from hollow interior  42  such that working channel  22  has an expanded configuration from proximal end  24  to distal end  16 . Moreover, in this configuration, first and second end portions  38 ,  40  are received in receptacles  56 ,  58  from proximal end  24  to distal end  16  such that a portion of elongate member  18  extends about or is positioned around elongate member  20  from proximal end  24  to distal end  16 . As illustrated in  FIG. 4  for example, when working channel  22  is expanded it generally includes a curvilinear configuration and is enclosed and surrounded by elongate members  18 ,  20  in a plane extending orthogonally to longitudinal axis L. Moreover, in its expanded configuration, the maximum dimension D across working channel  22  is generally less than length LL of elongate body  12 , although it should be appreciated that variations in the relationship of dimension D and length LL are also contemplated. Furthermore, it should be appreciated that working channel  22  will generally have a continuous size and shape from proximal end  24  to distal end  16  when it is in the expanded configuration. 
     While not previously discussed, it should be appreciated that the engagement between first and second end portions  38 ,  40  and receptacles  56 ,  58  maintains working channel  22  in the expanded configuration and prevents elongate members  18 ,  20  from moving toward one another. However, it is contemplated that linear portions  48 ,  50  of elongate member  20  could be squeezed or moved together in order to disengage first and second end portions  38 ,  40  from receptacles  56 ,  58 . Once disengaged, elongate members  18 ,  20  could be forced together to radially expand elongate member  18  and allow elongate member  20  to be repositioned into hollow interior  42 . In addition, separation instrument  70  generally includes a handle portion  72  coupled with a head portion  74  which, in the illustrated form, is provided with an external configuration that generally corresponds to the internal configuration of working channel  22  in its expanded configuration. However, it should be appreciated that alternative configurations for head portion  74  are also contemplated provided that such configurations accomplish full expansion of working channel  22 . For example, in one form, head portion  74  could simply be provided with a height that corresponds to the maximum dimension D across working channel  22  in its expanded configuration to ensure that working channel  22  is fully expanded as separation instrument  70  is moved distally therethrough. In another form, it is contemplated that head portion  74  could be an implant that is releasably engaged with handle portion  72  and expands working channel  22  as it is inserted therethrough to the surgical site. 
     In an alternative form for expanding working channel  22 , it is contemplated that separation of elongate members  18 ,  20  could be accomplished by separation instrument  70  alone without separating tab members  60 ,  62  from one another to initially align first and second end portions  38 ,  40  with receptacles  56 ,  58  adjacent to proximal end  24  of working channel  22 . For example, it is contemplated that head portion  74  of separation instrument  70  could he provided with a tapered configuration that can be inserted into proximal end  24  of working channel  20  once tab members  60 ,  62  have been pivoted away from longitudinal axis L. Similarly, in this configuration the tapered head portion  74  could be moved distally to displace elongate members  18 ,  20  from one another. 
     In a further aspect, it should be appreciated that tab members  60 ,  62  can be engaged with one or more operating room support structures when they are pivoted away from longitudinal axis L as illustrated in  FIG. 3 . More particularly, in one form the support structures can be provided with projections and indentations that can correspondingly engage with projections  60   a,    62   a  and indentations  60   b,    62   b,  although other variations for engaging tab members  60 ,  62  with the support structures are contemplated. Examples of operating room support structures include, without limitation, support arms, braces and other linkage members which are coupled to an operating table or bed and movable to position assembly  10  relative to a surgical site of the patient. 
     Elongate members  18 ,  20  can be provided with sufficient rigidity between distal end  16  and proximal end  24  of working channel  22  to separate and maintain separation of tissue when tissue is retracted by moving elongate members  18 ,  20  away from one another. For example, elongate members  18 ,  20  can include a thickness which provides sufficient rigidity to resist bending or bowing under the forces exerted on it by the retracted tissue. Also, the arcuately shaped portions of sidewalls  34 ,  44  can assist in providing a sufficient section modulus or moment of inertia in the direction of movement of elongate members  18 ,  20  to resist bending, bowing and/or deflection forces applied during such movement. Furthermore, it is contemplated that elongate members  18 ,  20  can be made from any biocompatible material, including but not limited to non-reinforced polymers, carbon-reinforced polymer composites, shape-memory alloys, titanium, titanium alloys, cobalt chrome alloys, stainless steel and others as well. 
     An alternative embodiment cannula assembly  110  is illustrated in section view in  FIG. 5 . Assembly  110  is generally the same as assembly  10 , but includes an alternatively configured arrangement for maintaining the spacing between elongate members  118 ,  120  when working channel  122  is in the expanded configuration. More particularly, elongate member  118  generally includes a c-shaped configuration in a plane extending orthogonally to longitudinal axis L. Interior surface  124  of elongate member  118  also includes a first plurality of teeth  126  positioned adjacent end portion  128  and a second plurality of teeth  130  positioned adjacent end portion  132 . Elongate member  120  also generally includes a c-shaped configuration in a plane extending orthogonally to longitudinal axis L. In addition, elongate member  120  further includes a first laterally extending flange portion  134  positioned adjacent to end portion  136  and a second laterally extending flange portion  138  positioned adjacent to end portion  140 . As will be discussed in greater detail below, flange portions  134 ,  138  are configured to engage with teeth  126 ,  130 , respectively, to control the positioning of elongate members  118 ,  120  relative to one another. 
     More particularly, as illustrated in  FIG. 5  for example, when flange portions  134 ,  138  engage with teeth  126 ,  130 , elongate member  120  can be incrementally moved away from elongate member  118  to expand working channel  122  until flange portions  134 ,  138  contact stop portions  142 ,  144 , which extend beyond teeth  126 ,  130  and limit further separation of elongate member  120  from elongate member  118 . However, unless elongate member  120  is radially compressed, the engagement of flange portions  134 ,  138  with teeth  126 ,  130  prevents movement of elongate member  120  toward elongate member  118 . More particularly, flange portions  134 ,  138  of elongate member  120  slip past teeth  126 ,  130  of elongate member  118  when elongate member  120  is moved away from elongate member  118 . However, if elongate member  120  is forced toward elongate member  118 , flange portions  134 ,  138  interlock with teeth  126 ,  130  and prevent movement of elongate member  120  toward elongate member  118 . In other forms, it is contemplated that the positioning of the flange portions and series of teeth could be interchanged between elongate members  118 ,  120 . 
     While not previously discussed, it should be appreciated that working channel  122  of assembly  110  can be expanded in a manner similar to that described above with respect to working channel  22  of assembly  10 . For example, assembly  110  could be inserted with working channel  122  in an unexpanded configuration followed by expansion using a separation instrument to separate elongate members  118 ,  120  from one another either alone or in combination with initiating separation of elongate members  118 ,  120  by moving the tab members away from one another as discussed above with respect to assembly  10 . 
     One particular application for cannula assemblies  10 ,  110  is in spinal surgery. For example, with further reference to  FIGS. 6-8 , a method of using assembly  10  relative to a disc D positioned between adjacent vertebral bodies V 1  and V 2  will be described. It should be appreciated however that the described method may also be used in connection with assembly  110 . In addition, while assembly  10  is described as being used in connection with a surgical procedure performed on or in relation to disc D between adjacent vertebral bodies V 1  and V 2 , it should be appreciated that use of assemblies,  10 ,  110  at other locations along the spinal column and at other anatomical locations besides the spinal column are contemplated. 
     With reference to  FIG. 6  for example, an insertion cannula  210  is inserted through an incision in the skin S and advanced to a location adjacent disc D between adjacent vertebral bodies V 1  and V 2 . Cannula  210  generally includes a working channel  212  that extends between and opens at proximal end  214  and distal end  216 . It should be appreciated that disc D can be accessed from any of an anterior, posterior, antero-lateral, postero-lateral or lateral approach. In one form, prior to insertion of cannula  210 , the skin and tissue can be sequentially dilated via a dilation instrument set (not illustrated) which can include guidewires and/or one or more tissue dilators of increasing size. The tissue dilators are inserted one over another to form a pathway through the skin and tissue to the surgical site in the patient. In such procedures, cannula  210  is positioned over an inserted dilator to form a pathway through the skin and tissue adjacent to disc D, and the guidewires and dilators, if used, are removed from cannula  210  to leave working channel  212  open. However, it should be appreciated that insertion and positioning of cannula  210  without guidewires and dilators is also possible and contemplated in connection with the use of assembly  10 . Furthermore, it should also be appreciated that cannula assembly  10  can be positioned directly through the skin and tissue of a patient to a location adjacent a surgical site without the use of cannula  210 . 
     As illustrated in  FIG. 7 , cannula assembly  10  is positioned in working channel  212  of cannula  210  after it has been properly positioned relative to the surgical site. Once positioned in working channel  212  of cannula  210 , cannula assembly  10  still provides access to the surgical site through working channel  22  in its unexpanded configuration. For the entire surgery or for certain procedures during the surgery, it may be desired by the surgeon to increase the size of working channel  22  to facilitate access to the surgical site with differently sized and/or shaped implants or instruments. However, as illustrated in  FIG. 8  for example, working channel  212  is sized relative to elongate member  18  such that it prevents radial expansion of elongate member  18 , which in turn prevents separation of elongate members  18 ,  20  from one another to expand working channel  22 . Similarly, cannula  210  is removed from the incision over cannula assembly  10  in order to allow elongate members  18 ,  20  to he separated along longitudinal axis L as discussed above to increase the size of working channel  22  and provide a working space larger than that provided by working channel  212 . In one form, cannula assembly  10  can be positioned relative to disc D such that elongate members  18 ,  20  can be primarily or predominantly separable in the direction of the spinal column axis since the muscle tissue adjacent the spine has a fiber orientation that extends generally in the direction of the spinal column axis. The separation of elongate members  18 ,  20  can also separate the muscle tissue along the fibers, thus the amount of separation and the resultant tearing and trauma to the muscle tissue can be minimized. It is also contemplated in other techniques employing cannula assembly  10  that working channel  22  can be enlarged primarily in a direction other than along the spinal column axis or in areas other than spine. 
     Upon completion of the surgical procedure, cannula assembly  10  can be disengaged from any operating room support structures, if utilized, and then removed from its location adjacent to the surgical site with working channel  22  in its expanded configuration. However, it is also contemplated that working channel  22  could be reduced to its unexpanded configuration as discussed above before cannula assembly  10  is removed from its location adjacent to the surgical site. 
     Alternative configurations and uses of the cannula assemblies described herein are also contemplated. For example, in one form, one or more additional cannula assemblies could be positioned at one or more other vertebral levels of the spinal column in order to perform a surgical procedure across multiple levels of the spinal column. In addition, the cannula assemblies and methods described herein may also be used in surgical procedures involving animals, or in demonstrations for training, education, marketing, sales and/or advertising purposes. In addition, the cannula assemblies and methods described herein may also be used on or in connection with a non-living subject such as a cadaver, training aid or model, or in connection with testing of surgical systems, surgical procedures, orthopedic devices and/or apparatus. 
     Any theory, mechanism of operation, proof, or finding stated herein is meant to further enhance understanding of the present application and is not intended to make the present application in any way dependent upon such theory, mechanism of operation, proof, or finding. It should be understood that while the use of the word preferable, preferably or preferred in the description above indicates that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the application, that scope being defined by the claims that follow. In reading the claims it is intended that when words such as “a,” “an,” “at least one,” “at least a portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. Further, when the language “at least a portion” and/or “a portion” is used the item may include a portion and/or the entire item unless specifically stated to the contrary. 
     While the application has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the selected embodiments have been shown and described and that all changes, modifications and equivalents that come within the spirit of the application as defined herein or by any of the following claims are desired to be protected.