Abstract:
A dilator that facilitates implantation of an interspinous spacer is provided. The dilator includes a proximal portion and a tapered distal portion interconnected by an elongated body portion. The tapered distal portion is ideally suited for splitting ligamentous tissue for creating a posterior midline pathway through the supraspinous ligament as well as for distracting the adjacent spinous processes. Two oppositely located and longitudinally extending channels or grooves are formed in the outer surface of the dilator for stabilizing the dilator with respect to the spinous processes. An accompanying cannula together with the dilator form a system for the distraction of the adjacent spinous processes, stabilization of the spinous processes with respect to the system and creation of a working channel for the implantation of an interspinous spacer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/358,010 entitled “Dilator” filed on Jan. 22, 2009, which claims priority to and the benefit of and is a continuation-in-part of U.S. Provisional Patent Application Ser. No. 61/062,448 entitled “Dilator” filed on Jan. 23, 2008 which is incorporated herein by reference in its entirety. U.S. patent application Ser. No. 12/358,010 also claims priority to and is a continuation-in-part of U.S. patent application Ser. No. 11/582,874, now U.S. Pat. No. 8,128,662, entitled “Minimally invasive tooling for delivery of interspinous spacer” filed on Oct. 18, 2006 which is incorporated herein by reference in its entirety. Each of the above applications is incorporated by reference in its entirely. 
    
    
     BACKGROUND 
     A variety of retractors and dilation systems have been used to provide a traditional “open” or “mini-open” approach to the posterior spine, as well as for providing the more modern “minimally invasive” and “percutaneous” access to the spine. The “open” or “mini-open” approaches to the spine typically require larger incisions. These larger incisions readily provide visual and instrument access to the surgical site; however, larger incisions generally result in greater damage to muscle tissue, blood loss, long healing times accompanied by prolonged pain and significant scarring. 
     The development of minimally invasive, percutaneous procedures has provided a major improvement in reducing recovery time and post operative-pain. In minimally invasive, percutaneous techniques patient trauma is minimized by creating a relatively smaller incision, followed by the introduction of a series of successfully larger dilators installed in sequence to dilate the soft tissues and increase the effective size of the incision. In some cases, a guide wire is used to first access the surgical site and then cannulated dilators are installed over the wire. Following installation of the largest dilator deemed necessary, a cannula or retractor is advanced over the largest dilator for providing a working channel from the skin of the patient to the working space adjacent to the spine. Surgery is performed or an implant is inserted through a surgical port or cannula inserted into the dilated incision. 
     Instead of cutting a larger opening, sequential dilation splits the surrounding tissue to create a larger opening. Splitting the muscle fibers apart, rather than cutting the muscle causes less damage to the tissue and leads to faster recovery times and reduced patient discomfort. Also, sequential dilation provides an advantage in that it allows the surgeon to make an initially small incision, then gradually increase the size of the opening to the minimum size required for performing the surgical procedure, thus reducing tissue damage and speeding patient recovery time. 
     Certain spinal procedures, such as those developed by VertiFlex, Inc. and described in U.S. patent application Ser. No. 11/314,712 entitled “Systems and methods for posterior dynamic stabilization of the spine” filed on Dec. 20, 2005 and U.S. patent application Ser. No. 11/582,874 entitled “Minimally invasive tooling for delivery of interspinous spacer” filed on Oct. 18, 2006 and U.S. patent application Ser. No. 11/593,995 entitled “Systems and methods for posterior dynamic stabilization of the spine” filed on Nov. 7, 2006, U.S. patent application Ser. No. 12/148,104 entitled “Interspinous spacer” filed on Apr. 16, 2008, U.S. patent application Ser. No. 12/217,662 entitled “Interspinous spacer” filed on Jul. 8, 2008, U.S. patent application Ser. No. 12/220,427 entitled “Interspinous spacer” filed on Jul. 24, 2008, U.S. patent application Ser. No. 12/205,511 entitled “Interspinous spacer” filed on Sep. 5, 2008, U.S. patent application Ser. No. 12/338,793 entitled “Interspinous spacer” filed on Dec. 18, 2008, U.S. patent application Ser. No. 12/354,517 entitled “Interspinous spacer” filed on Jan. 15, 2009, all of which are incorporated herein by reference in their entireties, access the surgical site through tissue and through the supraspinous ligament, for example, for the insertion of a device, such as an interspinous spacer. Whereas the procedure may be performed in an open, mini-open or minimally invasive, percutaneous approach, penetrating the supraspinous ligament can be challenging as the ligamentous tissue is not only strong but also slippery. However, penetrating the supraspinous ligament particularly lends itself well to sequential dilation as the ligament is formed of a cord of substantially uniformly oriented fibrous strands that are advantageously capable of being split apart rather than transversely cut for minimizing trauma and increasing patient recovery time. Furthermore, approaching the interspinous process space through the supraspinous ligament, like the VertiFlex device, advantageously avoids the multifidus muscle and thereby preserves its critical function as a stabilizer of the lumbar spine. Because of the difficulties associated with penetrating ligament, there is a special need for a dilator and/or dilator system designed for accessing a surgical site through ligament such as the supraspinous or interspinous ligament. The current invention provides a dilator and dilator system for establishing an opening through ligament that may also be used in conjunction with minimally invasive, percutaneous procedures. 
     SUMMARY 
     According to one aspect of the invention, a dilator comprising a proximal portion and a distal portion interconnected by an elongated body portion is provided. At least a part of the distal portion has a cross-sectional area decreasing with distance towards the distal end. Two oppositely located channels are formed in the body portion and extend longitudinally into the distal portion. 
     A system comprising a dilator and a cannula is provided. The dilator comprises a proximal portion and a distal portion interconnected by an elongated body portion. At least a part of the distal portion has a cross-sectional area decreasing with distance towards the distal end. Two oppositely located channels are formed in the body portion and extend longitudinally into the distal portion. The cannula includes two oppositely located channels on the outer surface and has a passageway configured to receive the dilator. 
     A method is provided comprising the steps of inserting a dilator into a patient via a posterior midline approach between two adjacent spinous processes and distracting the adjacent spinous processes by advancing the dilator relative to the adjacent spinous processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    is a side view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 1 b    is a top view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 1 c    is a perspective view of a distal end of a dilator according to the present invention. 
         FIG. 1 d    is an end view of a distal end of a dilator according to the present invention. 
         FIG. 1 e    is a cross-sectional view of the distal end of a dilator according to the present invention. 
         FIG. 2 a    is a side view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 2 b    is a top view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 2 c    is a perspective view of a distal end of a dilator according to the present invention. 
         FIG. 2 d    is an end view of a distal end of a dilator according to the present invention. 
         FIG. 2 e    is a cross-sectional view of the distal end of a dilator according to the present invention. 
         FIG. 3 a    is a side view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 3 b    is a top view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 3 c    is a perspective view of a distal end of a dilator according to the present invention. 
         FIG. 3 d    is an end view of a distal end of a dilator according to the present invention. 
         FIG. 3 e    is a cross-sectional view of the distal end of a dilator according to the present invention. 
         FIG. 4 a    is a side view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 4 b    is a top view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 4 c    is a perspective view of a distal end of a dilator according to the present invention. 
         FIG. 4 d    is an end view of a distal end of a dilator according to the present invention. 
         FIG. 4 e    is a cross-sectional view of the distal end of a dilator according to the present invention. 
         FIG. 5 a    is a side view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 5 b    is a top view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 5 c    is a perspective view of a distal end of a dilator according to the present invention. 
         FIG. 5 d    is an end view of a distal end of a dilator according to the present invention. 
         FIG. 5 e    is a cross-sectional view of the distal end of a dilator according to the present invention. 
         FIG. 6 a    is a side view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 6 b    is a top view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 6 c    is a perspective view of a distal end of a dilator according to the present invention. 
         FIG. 6 d    is an end view of a distal end of a dilator according to the present invention. 
         FIG. 7 a    is a side view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 7 b    is a top view of a dilator and an enlarged portion of the distal end of the dilator according to the present invention. 
         FIG. 7 c    is a perspective view of a distal end of a dilator according to the present invention. 
         FIG. 7 d    is an end view of a distal end of a dilator according to the present invention. 
         FIG. 8 a    is a side view of a cannula according to the present invention. 
         FIG. 8 b    is a perspective view of a distal end of a cannula according to the present invention. 
         FIG. 9  is a flow chart of a method of treatment according to one embodiment. 
         FIG. 10  is a side view of an implant. 
     
    
    
     DETAILED DESCRIPTION 
     While the description of the dilator system of this invention will be discussed primarily in relation to spinal surgery, it should be understood that the system will find use in other areas of surgery in which a surgeon wishes to gain access to an internal cavity by cutting the skin and enlarging an incision in a body wall so that surgical instruments can be inserted to perform a desired surgical procedure. For example, the dilator system may be used to create an incision to provide access to the posterior spine through which pedicle screws may be percutaneously installed in one or more selected vertebra. Alternatively, the dilator system may be used to create an incision to access an intervertebral disc space for performance of a minimally invasive discectomy procedure and/or spinal fusion procedure including the implantation of one or more intervertebral or interspinous process implants. 
     Implants are inserted between adjacent spinous processes to distract the spine segments and maintain them in a position to relieve symptoms of spinal stenosis and other conditions that cause pain which is associated with the back. Such implants have a spacer which remains in place between the adjacent spinous processes. An opening is created in the supraspinous and/or interspinous ligament so that the implant (e.g., implant  140  of  FIG. 10  and as described in U.S. Pat. No. 8,128,622) can be inserted. The dilators of the present invention are used to step dilate or gradually dilate body tissue, in particular, the supraspinous and/or interspinous ligament. 
     The dilator system of the present invention includes one or more dilators configured to work independently or in conjunction with one another. When used in conjunction with one another a first dilator is generally smaller in outer diameter or cross-sectional area than that of a second dilator which typically is also cannulated so that the second dilator fits over the first dilator to dilate tissue. It should be noted that the second dilator, in one variation, is not cannulated but is sized larger than the first dilator. In such a variation, the first dilator is removed and the second dilator is inserted to expand body tissue. In another variation, the first dilator is cannulated to be placed over a guide wire that is first positioned in the patient. In any of the variations disclosed herein, the first dilator may also be cannulated. Although in some cases two dilators are discussed it should be noted that more than two dilators may be employed in any of the variations disclosed herein. Furthermore, some of the distal ends of the dilators of the present invention are sufficiently sharp or manufactured with integrated knife points to cut tissue without a need for a separate instrument such as a scalpel to create an initial incision in the skin or ligament which is then expanded with the dilators, whereas other dilators of the present invention have a distal end that is too blunt and a separate instrument such as a scalpel is employed to create the first incision in the tissue or ligament. 
     With reference to  FIG. 1 a   , there is shown a dilator  10  according to the present invention. The dilator  10  has an elongated body  12 , a proximal end  14  and a distal end  16 . The dilator  10  includes a pair of channels  18  shown in  FIGS. 1 b , 1 c  and 1 e    that are oppositely located from each other and run parallel to the longitudinal axis of the dilator  10 . The distal end  20  of the channel  18  commences in the distal end  16  and the proximal end  22  of the channel  18  ends in the body  12  portion of the dilator  10 . In one variation, the channel  18  has a flat base between two sidewalls. When inserted in a patient and aligned with the adjacent spinous processes, the channels  18  are advantageous for distracting the spinous processes apart as well as for keeping the dilator  10  in position between the spinous processes while being inserted especially in a “kissing” condition of the spine where the posterior tips of adjacent spinous processes are in close proximity, touch or “kiss”. In one variation, the channels  18  are absent from the dilator  10 . The distal end  16  of the dilator  10  is a tapered portion where the diameter or cross-sectional area is less than the diameter or cross-sectional area of the body portion  12 . In the embodiment shown in  FIGS. 1 a -1 e   , the distal end  16  portion has a cone shape shown in  FIG. 1 c   . An end view of the distal end  16  is shown in  FIG. 1 d    illustrating the tip or point  24  of the cone or bore  24  in a cannulated version of the dilator. When a cross-section of the distal end  16  is taken at a location distal to the channels  18  and perpendicular to the longitudinal axis of the dilator  10  as shown in  FIG. 1 e   , the cross-sectional area  26  of the distal end  16  is circular in shape. The cone-shaped dilator of  FIGS. 1 a -1 e    is generally employed as a first dilator  10  and may be cannulated for passing over a guide wire or if used as a subsequent dilator for passing over a previous dilator. The cone-shaped dilator  10  shown in  FIG. 1  punctures ligament and passes through soft tissue easily and therefore, it can be used as a first dilator in a minimally invasive percutaneous procedure without the need to first create a cut with a separate sharp edge such as a scalpel. A sharper tip formed by a distal end  16  with a more acute angle Θ (see  FIG. 1 b   ) will prevent the tip  24  from slipping off to the sides of the ligament. 
     Turning now to  FIGS. 2 a -2 e   , there is shown another variation of a dilator  10  according to the present invention wherein like reference numbers are used to describe like parts. Referring first to  FIG. 2 a   , the dilator  10  has an elongated body  12 , a proximal end  14  and a distal end  16 . The dilator  10  includes a pair of channels  18  shown in  FIGS. 2 b , 2 c  and 2 e    that are oppositely located from each other and run parallel to the longitudinal axis of the dilator  10 . The distal end  20  of the channel  18  commences in the distal end  16  and the proximal end  22  of the channel  18  ends in the body  12  portion of the dilator  10 . When inserted in a patient and aligned with the adjacent spinous processes, the channels  18  are advantageous for distracting the spinous processes apart as well as for keeping the dilator  10  in position between adjacent spinous processes while being inserted especially in a “kissing” condition of the spine where the posterior tips of adjacent spinous processes are in close proximity, touch or “kiss”. In one variation, the channels  18  are absent from the dilator  10 . The distal end  16  of the dilator  10  is a tapered portion where the diameter or cross-sectional area is less than the diameter or cross-sectional area of the body portion  12  and decreases toward the distal end  16 . In the embodiment shown in  FIGS. 2 a -2 e   , the distal end  16  portion has a wedge shape formed by two substantially flat faces  28  that angle towards each other at the distal end  16  and form a line or rectangular tip  24  shown in  FIG. 2 d   . An end view of the distal end  16  is shown in  FIG. 2 d    illustrating the line or rectangular tip  24  of the wedge. A cannulated variation of the dilator  10  is not shown but is within the scope of the present invention. When a cross-section of the distal end  16  is taken at a location distal to the channels  18  and perpendicular to the longitudinal axis of the dilator  10  as shown in  FIG. 2 e   , the cross-sectional area  26  of the distal end  16  is rectangular in shape. The wedge-shaped dilator of  FIGS. 2 a -2 e    is generally employed as a first dilator  10  and may be cannulated for passing over a guide wire or if used as a subsequent dilator for passing over a previous dilator. The distal end  16  is positioned in the patient such that the length of the tip  24  is aligned along the cephalad-caudal direction when puncturing the supraspinous ligament or otherwise aligned substantially parallel to the fibrous strands of the ligament. The wedge-shaped dilator  10  shown in  FIGS. 2 a -2 e    does not puncture ligament as readily as the dilator  10  of  FIGS. 1 a -1 e    and hence, is typically used in conjunction with a scalpel or other sharp edge, for example, to create a small opening in the ligament prior to insertion of the dilator  10  of  FIGS. 2 a -2 e    which then splits the ligament to create a larger opening as it is inserted. For these reasons, the dilator of  FIGS. 2 a -2 e    is generally used as a first dilator in a mini-open or open procedure in which direct visual access is gained and a sharp edge is used to first create a cut. The line or rectangular shaped point  24  is centered as seen in  FIGS. 2 d  and 2 e    and therefore advantageously assists in centering the location of the splitting on the ligament. It should be noted that a sharper tip may be formed by a distal end  16  with a more acute angle Θ (see  FIG. 2 b   ) thereby, creating or approaching a knife-like edge that can pierce the ligament without first using a sharp edge and therefore well suited for truly percutaneous procedures. 
     Turning now to  FIGS. 3 a -3 e   , there is shown another variation of a dilator  10  according to the present invention wherein like reference numbers are used to describe like parts. Referring first to  FIG. 3 a   , the dilator  10  has an elongated body  12 , a proximal end  14  and a distal end  16 . The dilator  10  includes a pair of channels  18  shown in  FIGS. 3 b , 3 c , 3 d  and 3 e    that are oppositely located from each other and run parallel to the longitudinal axis of the dilator  10 . The distal end  20  of the channel  18  commences in the distal end  16  and the proximal end  22  of the channel  18  ends in the body  12  portion of the dilator  10 . When inserted in a patient and aligned with the adjacent spinous processes, the channels  18  are advantageous for distracting the spinous processes apart as well as for keeping the dilator  10  in position between the spinous processes while being inserted especially in a “kissing” condition of the spine where the posterior tips of adjacent spinous processes are in close proximity, touch or “kiss”. In one variation, the channels  18  are absent from the dilator  10 . The distal end  16  of the dilator  10  is a tapered portion where the diameter or cross-sectional area is less than the diameter or cross-sectional area of the body portion  12  and decreases towards the distal end  16 . In the embodiment shown in  FIGS. 3 a -3 e   , the distal end  16  portion has a pyramid shape formed by four substantially flat faces  28  that angle towards each other at the distal end  16  and meet at a tip  24  shown in  FIGS. 3 c  and 3 d   . An end view of the distal end  16  is shown in  FIG. 3 d    illustrating the tip  24  of the pyramid-shaped distal end  16 . A cannulated variation of the dilator  10  is not shown but is within the scope of the present invention wherein the tip  24  would include an opening. When a cross-section of the distal end  16  is taken at a location distal to the channels  18  and perpendicular to the longitudinal axis of the dilator  10  as shown in  FIG. 3 e   , the cross-sectional area  26  of the distal end  16  is substantially square in shape. The pyramid-shaped dilator of  FIGS. 3 a -3 e    is generally employed as a first dilator  10  and may be cannulated for passing over a guide wire or if used as a subsequent dilator for passing over a previous dilator. The pyramid-shaped dilator  10  shown in  FIGS. 3 a -3 e    can puncture ligament and pass through soft tissue and hence, is generally used as a first dilator in a minimally invasive percutaneous procedure without the need to first create a cut with a separate sharp edge such as a scalpel. A sharper tip formed by a distal end  16  with a more acute angle Θ (see  FIG. 3 b   ) will prevent the tip  24  from slipping off to the sides of the ligament. 
     Turning now to  FIGS. 4 a -4 e   , there is shown another variation of a dilator  10  according to the present invention wherein like reference numbers are used to describe like parts. Referring first to  FIG. 4 a   , the dilator  10  has an elongated body  12 , a proximal end  14  and a distal end  16 . The dilator  10  includes a pair of channels  18  shown in  FIGS. 4 b , 4 c , 4 d  and 4 e    that are oppositely located from each other and run parallel to the longitudinal axis of the dilator  10 . The distal end  20  of the channel  18  commences in the distal end  16  and the proximal end  22  of the channel  18  ends in the body  12  portion of the dilator  10 . When inserted in a patient and aligned with the adjacent spinous processes, the channels  18  are advantageous for distracting the spinous processes apart as well as for keeping the dilator  10  in position between the spinous processes while being inserted especially in a “kissing” condition of the spine where the posterior tips of adjacent spinous processes are in close proximity, touch or “kiss”. In one variation, the channels  18  are absent from the dilator  10 . The distal end  16  of the dilator  10  is a tapered portion where the diameter or cross-sectional area is less than the diameter or cross-sectional area of the body portion  12  and decreases toward the distal end  16 . In the embodiment shown in  FIGS. 4 a -4 e   , the distal end  16  portion has a pyramid shape formed by four substantially flat faces  28  that angle towards each other at the distal end  16  and form a tip  24  shown in  FIG. 4 d   . An end view of the distal end  16  is shown in  FIG. 4 d    illustrating the tip  24  of the pyramid. A cannulated variation of the dilator  10  is not shown but is within the scope of the present invention wherein the tip  24  would include an opening. When a cross-section of the distal end  16  is taken at a location distal to the channels  18  and perpendicular to the longitudinal axis of the dilator  10  as shown in  FIG. 4 e   , the cross-sectional area  26  of the distal end  16  is a quadrilateral and, in the variation shown in  FIG. 4 e   , the quadrilateral is a rhombus in which one of the diagonals  30  or the longest diagonal  30  is aligned with the channels  18  as opposed to the variation of  FIGS. 3 a -3 e    in which none of the diagonals are aligned with the channels  18 . It is the intersection of two faces  28  that align with one channel  18  and the intersection of opposite two faces  28  that align with the other channel  18 . In a variation in which no channels  18  are included, the difference between the dilator of  FIGS. 3 a -3 e    is in the shape of the quadrilateral. The pyramid-shaped dilator of  FIGS. 4 a -4 e    is generally employed as a first dilator  10  and may be cannulated for passing over a guide wire or if used as a subsequent dilator for passing over a previous dilator. The distal end  16  is positioned in the patient such that one of the diagonals or longest diagonal  30  is aligned along the cephalad-caudal direction when puncturing the supraspinous ligament or otherwise aligned substantially parallel to the fibrous strands of the ligament such that the intersection of faces  28  form an edge along which ligament is split. The pyramid-shaped dilator  10  shown in  FIGS. 4 a -4 e    in either the channeled or non-channeled variations, splits ligament more readily than either of the channeled or non-channeled variations of the dilator  10  of  FIGS. 3 a -3 e    where the intersections of faces  28  are not aligned with the channels  18  or does not have a diagonal  30  that is longer relative to the other diagonal  30  which can be aligned with the fibrous ligament strands for easier splitting. The variation of  FIGS. 4 a -4 e    can be used with or without a scalpel or other sharp edge, for example, to create a small opening in the ligament prior to insertion of the dilator  10  of  FIGS. 4 a -4 e    which then splits the ligament to create a larger opening as it is inserted. The intersection of faces  28  or diagonal  30 , when aligned substantially parallel to the ligament strands, assist in centering the location of the splitting on the ligament. It should be noted that a sharper tip, intersection or diagonal may be formed by a distal end  16  with a more acute angle Θ (see  FIG. 4 b   ) thereby, creating or pproaching a knife-like edge that can pierce the ligament without first using a sharp edge and therefore well suited for percutaneous procedures. 
     Turning now to  FIGS. 5 a -5 e   , there is shown another variation of a dilator  10  according to the present invention wherein like reference numbers are used to describe like parts. Referring first to  FIG. 5 a   , the dilator  10  has an elongated body  12 , a proximal end  14  and a distal end  16 . The dilator  10  includes a pair of channels  18  shown in  FIGS. 5 b , 5 c , 5 d  and 5 e    that are oppositely located from each other and run parallel to the longitudinal axis of the dilator  10 . The distal end  20  of the channel  18  commences in the distal end  16  and the proximal end  22  of the channel  18  ends in the body  12  portion of the dilator  10 . When inserted in a patient and aligned with the adjacent spinous processes, the channels  18  are advantageous for distracting the spinous processes apart as well as for keeping the dilator  10  in position between the spinous processes while being inserted especially in a “kissing” condition of the spine where the posterior tips of adjacent spinous processes are in close proximity, touch or “kiss”. In one variation, the channels  18  are absent from the dilator  10 . The distal end  16  of the dilator  10  is a tapered portion where the diameter or cross-sectional area is less than the diameter or cross-sectional area of the body portion  12  and decreases toward the distal end  16 . In the embodiment shown in  FIGS. 5 a -5 e   , the distal end  16  portion has two curved faces  28  that angle towards each other at the distal end  16  and form a tip  24  shown in  FIG. 5 d   . An end view of the distal end  16  is shown in  FIG. 5 d    illustrating the tip  24 . A cannulated variation of the dilator  10  is not shown but is within the scope of the present invention wherein the tip  24  would include an opening. In yet another variation, the tip  24  includes an opening to a blade housing through which a blade may extend. The blade (not shown) may also be retractable. When a cross-section of the distal end  16  is taken at a location distal to the channels  18  and perpendicular to the longitudinal axis of the dilator  10  as shown in  FIG. 5 e   , the cross-sectional area  26  of the distal end  16  is comprised of an area bounded by two curved lines in which the length is aligned with the channels  18 . It is the intersections of two faces  28  that align with one channel  18 . In a variation in which no channels  18  are included, the length is aligned with the length of the ligament. The dilator  10  of  FIGS. 5 a -5 e    is generally employed as a first dilator  10  and may be cannulated for passing over a guide wire or if used as a subsequent dilator for passing over a previous dilator. The distal end  16  is positioned in the patient such that the length of the tip  24  is aligned along the cephalad-caudal direction when puncturing the supraspinous ligament or otherwise aligned substantially parallel to the fibrous strands of the ligament or to the ligament itself such that the intersections of faces  28  form an edge along which ligament is split. The variation of  FIGS. 5 a -5 e    can be used with or without a scalpel or other sharp edge, for example, to create a small opening in the ligament prior to insertion of the dilator  10  of  FIGS. 5 a -5 e    which then splits the ligament to create a larger opening as it is inserted. The intersection of faces  28  when aligned substantially parallel to the ligament strands, assist in centering the location of the splitting on the ligament. It should be noted that a sharper tip, intersection or diagonal may be formed by a distal end  16  with a more acute angle Θ (see  FIG. 5 b   ) thereby, creating or approaching a knife-like edge that can pierce the ligament without first using a sharp edge and therefore well suited for percutaneous procedures. 
     Turning now to  FIGS. 6 a -6 d   , there is shown another variation of a dilator  10  according to the present invention wherein like reference numbers are used to describe like parts. Referring first to  FIG. 6 a   , the dilator  10  has an elongated body  12 , a proximal end  14  and a distal end  16 . The dilator  10  includes a pair of channels  18  shown in  FIGS. 6 b , 6 c  and 6 d    that are oppositely located from each other and run parallel to the longitudinal axis of the dilator  10 . The distal end  20  of the channel  18  commences in the distal end  16  and the proximal end  22  of the channel  18  ends in the body  12  portion of the dilator  10 . When inserted in a patient and aligned with the adjacent spinous processes, the channels  18  are advantageous for distracting the spinous processes apart as well as for keeping the dilator  10  in position between the spinous processes while being inserted especially in a “kissing” condition of the spine where the posterior tips of adjacent spinous processes are in close proximity, touch or “kiss”. In one variation, the channels  18  are absent from the dilator  10 . The distal end  16  of the dilator  10  is a tapered portion where the diameter or cross-sectional area is less than the diameter or cross-sectional area of the body portion  12  and decreases toward the distal end  16 . In the embodiment shown in  FIGS. 6 a -6 d   , the distal end  16  portion has a surface  28 , that may also be curved that angles toward the distal end  16  and forms an opening  32  at tip  24  shown in  FIGS. 6 c  and 6 d   . An end view of the distal end  16  is shown in  FIG. 6 d    illustrating the opening  32  that forms distal end of the cannulation or bore  34  running along at least part of the length of the dilator  10 . Because of the central bore  34  is sized to received therein a smaller dilator  10  such as any of the dilators described above in  FIGS. 1-5 , the dilator  10  of  FIGS. 6 a -6 d    is generally employed as a second dilator  10  or dilator  10  subsequent for passing over a previous dilator. The distal end  16  is positioned over a previous dilator  10  in the patient such that the channels  18  are aligned generally perpendicular to the cephalad-caudal direction when puncturing the supraspinous ligament or otherwise aligned substantially perpendicular to the fibrous strands of the ligament or to the ligament itself. When inserted, the cannula of  FIGS. 6 a -6 d    continues to distract the spinous processes as they ride in the channels  18  with the channels  18  helping with maintaining the proper orientation of the dilators  10  between the spinous processes. In one variation, the channels  18  are ramped or angled towards the distal end to improve upon the distraction action provided by the dilator. 
     Turning now to  FIGS. 7 a -7 d   , there is shown another variation of a dilator  10  according to the present invention wherein like reference numbers are used to describe like parts. Referring first to  FIG. 7 a   , the dilator  10  has an elongated body  12 , a proximal end  14  and a distal end  16 . The dilator  10  includes a pair of channels  18  shown in  FIGS. 7 b , 7 c  and 7 d    that are oppositely located from each other and run parallel to the longitudinal axis of the dilator  10 . The distal end  20  of the channel  18  commences in the distal end  16  and the proximal end  22  of the channel  18  ends in the body  12  portion of the dilator  10 . In one variation, the channel  18  includes a flat base between two sidewalls. When inserted in a patient and aligned with the adjacent spinous processes, the channels  18  are advantageous for distracting the spinous processes apart as well as for keeping the dilator  10  in position between the spinous processes while being inserted especially in a “kissing” condition of the spine where the posterior tips of adjacent spinous process are in close proximity, touch or “kiss”. In one variation, the channels  18  are absent from the dilator  10 . The distal end  16  of the dilator  10  is a tapered portion where the diameter or cross-sectional area is less than the diameter or cross-sectional area of the body portion  12  and decreases toward the distal end  16 . In the embodiment shown in  FIGS. 7 a -7 d   , the distal end  16  portion has a surface  28  that may also be curved that angles toward the distal end  16  and forms an opening  32  at tip  24  shown in  FIGS. 7 c  and 7 d   . An end view of the distal end  16  is shown in  FIG. 7 d    illustrating the opening  32  that forms distal end of the cannulation or bore  34  running along at least part of the length of the dilator  10 . Because of the central bore  34  is sized to received therein a smaller dilator  10  such as any of the dilators described above in  FIGS. 1-5 , the dilator  10  of  FIGS. 7 a -7 d    is generally employed as a second dilator  10  or dilator  10  subsequent for passing over a previous dilator. The distal end  16  is positioned over a previous dilator  10  in the patient such that the channels  18  are aligned generally perpendicular to the cephalad-caudal direction when puncturing the supraspinous ligament or otherwise aligned substantially perpendicular to the fibrous strands of the ligament or to the ligament itself. The dilator of  FIGS. 7 a -7 d    further includes a pair of oppositely located flats  36  that are aligned with the channels  18 . At least part of the channel  18  is formed in the flats  36  and in one variation, the flat  36  is substantially parallel to the flat base of the channel  18 . The flats  36  create a lower profile for the dilator  10  which is advantageous for insertion between closely spaced spinous processes. When inserted, the cannula of  FIGS. 7 a -7 d    continues to distract the spinous processes as they ride in the channels  18  with the channels  18  helping with maintaining the proper orientation of the dilators  10  between the spinous processes. 
     An entry point is selected on the patient&#39;s skin to obtain access to the targeted surgical site, and an incision of appropriate length (block  100  of  FIG. 9 ) is made through the dermal layers of a patient&#39;s body at the entry point. The length and depth of the incision may be larger depending on whether the clinician is using an open, mini-open, or minimally invasive, percutaneous approach. If a guide wire is used, the tip of the guide wire is then positioned within the incision and guided toward the spine using a cannulated T-handled trocar. If a ligament such as the supraspinous or interspinous ligament is to be punctured with a sharp edge other than with the dilator, the sharp edge or scalpel is used to create a small cut in the ligament. One of the first dilators, such as any one of the dilators  10  described above in reference to  FIGS. 1-5 , is then inserted (over the guidewire if one is used) into the incision (block  102  of  FIG. 9 ) and into the cut in the ligament (if the ligament is pre-cut with a scalpel or other sharp edge). The first dilator is properly oriented (such that diagonal or edges are aligned with ligamentous strands as described above) and further inserted (block  104  of  FIG. 9 ) to spread apart body tissue and/or pierce and/or split and/or cut the ligament. After the first dilator is inserted a second dilator, such as any one of the dilators  10  described above in reference to  FIGS. 6-7 , is then passed over the proximal end  14  of the first dilator and further passed over the first dilator into the incision to further spread apart tissue and/or split the ligament (block  106  of  FIG. 9 ). Any number of additional dilators, that are preferably cannulated for passing over the one or more previous dilators, are then inserted. At block  108  of  FIG. 9 , a dilator with a channel  18  is oriented such that one of the adjacent spinous processes is positioned inside the channel  18  and in one variation, the other of the adjacent spinous processes is tracked inside the oppositely located channel  18 . Such placement of the dilator with respect to the spinous processes stabilizes the dilator with respect to the spine. Advancement of the dilator relative to the adjacent spinous processes, ramps the adjacent spinous processes first at the tip of the distal portion and then inside the channel  18  if one is employed to distract the adjacent spinous processes. Subsequent dilators placed over the previous dilator may further distract the spinous processes. In one variation, the channels  18  themselves may be flat or further ramped to further distract the adjacent spinous processes. After the desired amount of dilation with dilators is achieved, a cannula  40  of the type shown in  FIGS. 8 a -8 b    is passed over the last dilator  10  (block  110  of  FIG. 9 ) such that the dilators  10  are received in the cannula bore  42 . The cannula  40  may further include oppositely located channels  44  for receiving the adjacent spinous processes, stabilizing the spinous processes with respect to the dilator and for further distraction of the adjacent spinous processes. The channels  44  are formed by four wings  46  extending outwardly from the surface. At block  112  of  FIG. 9 , with the cannula  40  in place, the dilators  10  inside the cannula bore  42  are removed leaving an open cannula bore  42  through which surgery can be performed or an implant be inserted. 
     All publications mentioned anywhere herein are incorporated herein by reference as part of the detailed description of the present invention to disclose and describe the methods and/or materials in connection with which the publications are cited or in connection with the present invention. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. 
     The preceding merely illustrates the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope.