Patent Publication Number: US-2018042735-A1

Title: System and method for spinal surgery utilizing a low-diameter sheathed portal shielding an oblique lateral approach through kambin&#39;s triangle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 14/875,460 filed Oct. 5, 2015, which claims the benefit of U.S. Provisional Application No. 62/059,892 filed Oct. 4, 2014, and the present application claims the benefit of U.S. Provisional Application No. 62/411,637 filed Oct. 23, 2016 and entitled “System for Spinal Fusion Surgery Utilizing a Low-Diameter Sheathed Portal Shielding an Oblique Lateral Approach”, and U.S. Provisional Patent Application No. 62/569,746 filed Oct. 9, 2017 and entitled “Neuromonitoring Dilation System,” which are all hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Degenerative spine conditions such as kyphosis, scoliosis, hyperlordosis, spondylolisthesis and others can lead to serious disease associated with the intervertebral disc. Related compression can cause pain, spinal instability, limited motion, and inflammation, which causes back pain. Conditions such as these are often treated by removing the disc, and fusing the two vertebrae on either side of the disc together into a single bony structure. 
     One primary aim of intervertebral fusion is to secure the vertebrae in place together, preventing them from moving relative to one another. The movement of one bony structure against another may lead to bone spurring which may impinge nerve structures and cause pain. Often, this creates a need for a surgeon to remove a part of the bone structure that impinges a nerve. This may occur via a laminectomy or facetectomy procedure to, for instance, decompress a nerve structure. 
     A problem associated with removing bony structures of the spine, however, is the reduction of the supportive bony tissue able to bear strain. By performing a procedure to fuse the bony structures of the spine together, in contrast, a much more stable solution may be provided. Some fusion procedures, however, notably trans-foraminal lumbar interbody fusion (TLIF) procedures, require a surgeon to remove bony tissue to access the interbody space for fusion bed creation and implant placement. While after fusion, such procedures can effectively treat pathology, the removal of bony supportive tissue elevates the risks to the patient if such a fusion fails. Therefore, significant problems remain to be solved in association with the widespread use of the methods and apparatuses associated with TLIFs. 
     Typical spinal fusion procedures begin with the steps associated with accessing the junction of at least two bodies of the spine generally separated by an interbody space. The access trajectory to the interbody space is of critical importance. Several problems derive from the typically known access trajectories associated with prior art methods of creating an access corridor to the interbody space. For instance, the surgeon&#39;s creation of a route through the soft and other tissue on or near the trajectory from the skin to the spine can cause damage to those and related tissues. 
     Typical spinal fusion procedures known in the art involve a discectomy step, intended to remove a diseased or inflamed disc between two vertebral bodies and prepare the disc space for fusion. A problem associated with the discectomy step, however, is that the tools and steps have heretofore not adequately been developed to accomplish an optimal discectomy via an oblique approach traversing the area of Kambin&#39;s Triangle through a tube of 10 millimeters or less. Following the discectomy step, typical spinal fusion procedures incorporate a decortication step. During the decortication, a surgeon scrapes or scratches the end plates of the vertebral bodies to prepare the fusion bed. Decortication provides access to the blood vessels that exist in the deep, cancellous bone, as well as access to the pluripotent stem cells that support the healing process. A problem associated with the decortication step, however, is that the tools and steps have heretofore not adequately been developed to accomplish an optimal decortication via an oblique approach traversing the area of Kambin&#39;s Triangle through a tube of 10 millimeters or less. Following the decortication step, a surgeon typically performs the step of deposition of bone graft material. Such bone graft material may include autograft, xenograft, allograft, and synthetic graft materials to promote fusion. The fusion process is further supported by biological factors present in the bone graft material. A problem associated with the deposition of bone graft material step, however, is that the tools and steps have heretofore not adequately been developed to accomplish an optimal deposition of bone graft material via an oblique approach traversing the area of Kambin&#39;s Triangle through a tube of 10 millimeters or less. 
     Common routes to access the junction and/or the associated interbody space include, for example, those established by an anterior approach during a Anterior Lumbar Interbody Fusion (“ALIF”) procedure, a posterior approach during a Posterior Lumbar Interbody Fusion (“PLIF”) procedure, a lateral approach during Lateral Lumbar Interbody Fusion (LLIF) procedure (also referred to as eXtreme Lateral Interbody Fusion “XLIF” or Direct Lateral Interbody Fusion “DLIF”) and a transforaminal approach during the previously-mentioned Trans-foraminal Interbody Fusion (“TLIF”) procedure. A variety of instruments and implants exist to facilitate fusion following these approaches. 
     The ALIF procedure generally approaches the spine through the front of the human body. This may require a surgeon to open the stomach with a relatively large incision (usually three to five inches), and may necessitate further cuts through soft tissue. In many cases, however, the rectus abdominus muscle and the peritoneum may be retracted to the side without further damage. A problem associated with this procedure is that the associated generally anterior path comes within the vicinity of the great vessels, which carries a risk of aortic vascular laceration and bleeding out. Once through these obstacles, one or more vertebral bodies and associated interbody spaces can then be accessed. 
     The PLIF procedure approaches the spine from behind, or posterior to, the vertebral bodies. In this case, another relatively large initial incision (usually three to six inches) is required. Once inside the patient&#39;s body, the surgeon strips the left and right lower back muscles off of the lamina and spinous processes at one or more vertebral levels. The lamina and spinous processes may then be removed—along with any other bone cutting that may be necessary—in order to visualize the nerves. A problem associated with this procedure is that after the nerves can be seen, the surgeon retracts them to one side, a step which carries a high incidence of nerve bruising or damage. Once the nerves are moved, the interbody space can be accessed. 
     The TLIF procedure, like PLIF, also begins generally posterior to the spine, but takes an off-center approach through the patient&#39;s body into the spine, rather than approaching the spine from a direct posterior angle. A problem associated with this procedure is that because of the TLIF approach angle, the surgeon is generally required to remove part of or the entire facet joint of the spine in order to visualize the vertebral bodies and interbody space and to remove the disc material. As a result of the removal of at least part of a facet, increased spinal instability can result. Accordingly, if the associated vertebral bodies do not fuse following the procedure, the patient will experience chronic instability as one side of the spine is supported by an intact facet joint while the other is not. Another problem is that in many cases, to accomplish a TLIF procedure, a surgeon must retract the dura to one side, increasing the likelihood of nerve damage. 
     The LLIF approach begins from a lateral position to the spine. The LLIF approach requires dissection of the oblique abdominal muscle structures and the psoas, posing risks to the patient. A problem associated with the LLIF procedure is that because this approach is performed trans-psoas, the psoas and the nerve structures therein are retracted for long periods of time, increasing to the risk of nerve damage. The resulting trauma to the psoas and sensory nerve structures may produce frequent, undesirable post-operative side effects. These effects include, but are not limited to, leg pain, numbness and foot drop. 
     In previously-known types of anterior Oblique Lateral Interbody Fusion surgery, also commonly referred to as the “OLIF” procedure and OLIF system offered by Medtronic (referred to herein as the “Anterior OLIF”), the surgeon utilizes an anterior oblique trajectory to the spine during surgery to avoid the psoas muscle. Further, the trajectory employed by the Anterior OLIF approach accesses the spine away from the peritoneum, which provides advantages over the ALIF approach. With the exception of the iliolumbar vein and possible transitional bifurcation of great vessels, the Anterior OLIF trajectory also avoids most vasculature. Previously-known Anterior OLIF approaches can also advantageously lower the risks of tissue damage to the paraspinal muscles, nerve impaction to the spinal cord, epidural scarring, perineural fibrosis, and iatrogenic trauma. As a result, there is less tissue damage, and injury to the psoas muscle and lumbar plexus is avoided. Because of this, there is a much lower risk of sciatica-related neuropathies, such as cruralgia. 
     An alternative procedure to the above approaches is known as the Oblique Lateral Lumbar Interbody Fusion approach (referred to herein as “OLLIF” or the “OLLIF procedure” or the “OLLIF approach”), where the surgeon approaches from a posterior oblique trajectory to avoid the great vessels and also to cause minimal tissue trauma. Despite the remarkable advantages of the OLLIF, many surgeons have been reluctant to adopt the technique due to the required passage through Kambin&#39;s Triangle, which may place one or more of the exiting nerves and/or nerve roots at risk. In spinal anatomy, Kambin&#39;s Triangle is known as a generally right triangle that is defined by the exiting nerve (forming the hypotenuse), the caudal vertebral body (forming the base) and the traversing nerve root (forming the height). As used herein, the term “Kambin&#39;s Triangle” more generally refers to the area generally bounded by the exiting nerve, the vertebral body and the traversing nerve root, though the structures forming the boundary may not truly resemble a triangle, and though the boundary may not form a closed, contiguous loop. 
     A major problem associated with OLLIF is the trajectory near the nerves forming the boundary of Kambin&#39;s Triangle. In previously-known OLLIF methods and systems, without protection against impacting the nerves of Kambin&#39;s Triangle, a high incidence of associated nerve bruising or other nerve trauma has been known. Prior art solutions utilizing the OLLIF approach have not yet solved the challenges associated with establishing a durable trajectory for passage of implantation and implants through a shielded approach with a sufficiently small diameter to enable passage through Kambin&#39;s Triangle, protecting such implantation and implants from harming the nerves associated with Kambin&#39;s Triangle. A related problem associated with the approach stems from the diminutive dimensions of Kambin&#39;s Triangle. Generally, the diameter of space available to create a path directly through Kambin&#39;s Triangle is 15 millimeters or less. Therefore, the optimal implants and instrumentation designed to traverse Kambin&#39;s Triangle and accomplish a successful fusion procedure with a sufficiently low diameter remain to be developed. There is a need for an implant design, and a corresponding design for a system of surgical instrumentation, to enable spinal fusion surgery with the placement of an implant customized to fit through Kambin&#39;s Triangle to enable the avoidance damage to the structures comprising or near Kambin&#39;s Triangle. 
     Unlike the TLIF procedure, in an OLLIF procedure bony structures (for instance, the bony structures comprising the facet joint) do not need to be removed, which maximizes spinal stability during healing post-procedure. As the pathway is relatively avascular and less innervated, previously-known OLLIF approaches lower the risk of complication during discectomy and end plate preparation. As many as 3 or more levels of fusion can be performed in this manner, through a small, 4 cm incision. Still, many surgeons prefer the more ubiquitous TLIF procedure, as it allows surgeons to avoid the less familiar and more clustered nerves associated with Kambin&#39;s Triangle. Therefore, a need remains to develop instrumentation and implants associated with an enhanced OLLIF procedure that more safely allows surgeons to traverse the anatomy near the trajectory associated with the OLLIF surgical approach. 
     An advantage of the OLLIF procedure over the LLIF procedures in particular is the comparatively lower amount of blood loss during surgery. Previously-known OLLIF approaches also tend to have a lower incidence of hernias and ileuses than LLIF. Unlike the LLIF approach, typical previously-known OLLIF procedures avoid the psoas muscle. As such, with previously-known OLLIF approaches, there is a reduced incidence of nerve trauma associated with nerves in or near the psoas compared to LLIF and other approaches that require a trans-psoas access. Still, many surgeons prefer the better known LLIF procedure, as it allows surgeons to avoid the less familiar and more clustered nerves associated with Kambin&#39;s Triangle. Further, the relatively smaller footprint of implants traditionally associated with OLLIF may lead to a higher risk of subsidence relative to the LLIF procedure. Therefore, a need remains to develop instrumentation and implants associated with an enhanced OLLIF procedure that more safely allows surgeons to traverse the anatomy near the trajectory associated with the OLLIF surgical approach. There is also a need to reduce the risk of subsidence associated with implants of a diameter that can safely travel through Kambin&#39;s Triangle. 
     Kambin&#39;s Triangle is known to be a safe portal for epidural injection needles as such needles have a small diameter. A problem with the approach associated with prior procedures is that the dimensions of Kambin&#39;s Triangle allow for an approach trajectory path that is too narrow for many standard surgical instruments. Despite being a potentially preferable approach to the spine, many surgeons are reluctant to utilize an approach near or through Kambin&#39;s Triangle to accomplish procedures related to the interbody space because instruments and/or implants are larger than those utilized during epidural injection-type procedures, and therefore pose an increased risk of contact nerves comprising or near to Kambin&#39;s Triangle. 
     Moreover, a substantially lateral passage through the ilium, such as that described in U.S. Pat. No. 8,790,406 to Smith (the “&#39;406 patent”) has yet to be perfected. More specifically, a direct lateral trajectory wide enough to access the L5-S1 interbody space for placement of an interbody cage, especially a monolithic, non-expandable cage, has led to a high incidence of intractable pain. A trajectory that traverses the ilium, but then travels above the Sacral Ala may lead to unintended deflection of instrumentation superiorly and possibly into the nerve root, causing damage to the nerves. Previously known trajectories near the Sacral Ala have failed to anticipate the need to incorporate sheathing into the surgical approach to shield structures external to the approach trajectory from the passage of instrumentation and/or implants prior to and/or during the traversal through the bone. A need therefore remains for an improved approach and cage design to enable spinal fusion at the lumbosacral (L5-S1) junction. 
     The geometries and anatomical structures close to the L5-S1 junction pose extreme and unique challenges related to surgical access. It is difficult, even for those skilled in the art, to comprehend the complex anatomy and multiple geometries of the sacrum, ilium and associated nerves at the L5-S1 junction. The plane of the endplate inferior to the L5-S1 disc space angles inferiorly in an anterior direction relative to the plane of the endplate superior to the L5-S1 disc space. Many fail to clearly comprehend that the structure of the sacrum partially surrounds the disc space in a lateral direction. Specifically, the Sacral Ala often extends superiorly relative to the L5-S1 inferior endplate laterally from the disc space. The Sacral Ala exists in a generally superior orientation lateral to the L5-S1 disc space relative to the lower endplate of the L5-S1 disc space. As such, at L5-S1, other approaches, including that described in U.S. Pat. No. 8,790,406 to Smith fail to appreciate and address of the location of the Sacral Ala relative to the lower ½ of Kambin&#39;s Triangle, which represents a safer “safe zone” for surgical approach (differing from the larger “safe zone” described in U.S. Pat. No. 8,790,406 to Smith) of surgical access. Moreover, other approaches including that described in U.S. Pat. No. 8,790,406 to Smith fail to incorporate steps to target the lower half of Kambin&#39;s Triangle at L5-51. As other approaches have failed to consider, the lower half of Kambin&#39;s Triangle is medial to the Sacral Ala. 
     The complex geometry of the sacrum, ilium and nerves near the L5-S1 junction is difficult to visualize and comprehend in two dimensions, which has contributed to the development of sub-optimal methods of surgical approach. Instead, other approaches (including that described in U.S. Pat. No. 8,790,406 to Smith) traverse through the ilium only to avoid penetrating the exterior of the Sacral Ala by traveling on a path located superior to the Sacral Ala. As such, this approach located superior to the Sacral Ala takes a path closer to, or in contact with, the nerve root exiting L5, thereby causing risk. 
     Previously known approaches travelling superior to the Sacral Ala travel closer to the L5 nerve root, which forms a boundary of Kambin&#39;s Triangle. Thus, such previously known approaches targeting the upper half of Kambin&#39;s Triangle place the L5 nerve root at risk. A problem associated with the methods associated with previously known approaches is that the instrumentation and implants following such methods often brush off and are forced in a superior direction by the exterior surface of the Sacral Ala, resulting in a dangerous and undesirable method of surgical approach leading to the risk of damaging or contacting the L5 nerve root. Therefore, a need exists for a different method and system to surgically approach the L5-S1 disc space to avoid damage to the L5 nerve root and to ensure patient safety. 
     BRIEF SUMMARY OF CERTAIN EMBODIMENTS OF THE INVENTION 
     The certain embodiments described herein are preferable, in many cases, to the other approaches presented above. The certain embodiments described involves accessing the interbody space or the vertebral bodies from a posterior-oblique lateral trajectory, it is not necessary to retract the dura as with the PLIF or TLIF approaches, which thereby lowers the risk of nerve damage relative to those approaches. 
     Embodiments of the present invention are directed toward improvements in the system and method for facilitating the fusion of two vertebral bodies utilizing an oblique lateral surgical trajectory. Certain embodiments of the invention accesses the interbody space through a posterior-oblique lateral trajectory, which lowers the risk of nerve damage compared to other approaches such as PLIF or TLIF. Certain embodiments disclose a method for a surgical approach into one or more interbody spaces between two vertebral bodies on a trajectory through Kambin&#39;s Triangle. 
     Certain embodiments of the invention include a method to open a pathway into a target area between two vertebral bodies of the spine using a series of one or more dilators. Certain embodiments also incorporate a sheath, which may or may not form part of the dilation mechanism. 
     Certain embodiments of the invention comprise a system for placing an implant between two vertebral bodies. Certain embodiments of the invention comprise a system for placing an expandable interbody cage between two vertebral bodies. Certain embodiments of the invention incorporate a series of implant components that are assembled between two vertebral bodies. In certain embodiments of the invention an implant is defined as an expandable interbody cage comprising multiple components, including monolithic components and/or components comprising multiple parts that are individually placed through a sheath into an interbody space prior to combining the components into a fully linked construct, partially linked construct or loose construct comprised of implant components merely making contact with one another within the interbody space. In certain embodiments of the invention, the term “expanding the implant” refers to merely placing multiple implant components adjacent to or near one another within an interbody space, where the multiple implant components may optionally comprise monolithic implant components or implant components having multiple parts. 
     Certain embodiments of the invention incorporate a series of instruments to deliver an expandable interbody cage. In certain embodiments of the system the series of instruments includes a sheath. In a certain embodiment, the sheath is configured to dimensions to define an approach portal through Kambin&#39;s Triangle while protecting the structures comprising portions of Kambin&#39;s Triangle from anything passed through the sheath. Certain embodiments include an expandable interbody cage that collapses to a transit configuration that is able to travel through a sheath. In certain embodiments, an expandable interbody cage is removably attached to a guiding implement. 
     In certain embodiments, an implant, such as an expandable interbody cage, is provided. Certain embodiments comprise an implant having a collapsed form of a diameter size small enough during transit to traverse through a sheath. In certain embodiments, a sheath has an internal diameter of 10 millimeters or less. In certain embodiments, the implant can expand upon or after placement between two vertebral bodies following successful navigation through a sheath. In certain embodiments, an implant has features that rotate. In certain embodiments, a transit configuration or a retracted configuration indicates a form of the implant that allows passage through a sheath. In certain embodiments, a deployed configuration or an expanded configuration indicates a form of the implant that supports the vertebral disc space. In certain embodiments, a user controls the degree to which an implant switches between a transit configuration and a deployed configuration. In certain embodiments, the implant is structurally durable enough to withstand the forces necessary to physically separate two vertebrae. In certain embodiments, the implant comprises titanium, polyetheretherketone (PEEK), carbon fiber, ceramic, or other materials commonly utilized within orthopedic implants, or combinations thereof. In certain embodiments, the implant is detachably connected to delivery instruments utilized to transit the implant through the sheath to a target point between two vertebral bodies. In certain embodiments, the connection of the implant to delivery instruments is made via threaded connection points. In certain embodiments, the implant can be collapsed after placement and subsequently removed through the sheath. Certain embodiments incorporate a method to deliver and a method to remove the implant via for an oblique lateral approach through Kambin&#39;s Triangle. Certain embodiments of the invention comprise a deployment tool. In certain embodiments, the present inventors intend for the deployment tool to facilitate the placement and expansion of the implant apparatus. Certain embodiments include positioning tools, which enable a surgeon to place one or more implant at a targeted point between vertebral bodies in a desired configuration. In certain embodiments, a deployment tool is incorporated within an inserter. In certain embodiments, a deployment tool places force upon the implant apparatus, which translates from an axial dimension to one or more vertical and/or horizontal dimensions by the mechanisms incorporated within the implant. In certain embodiments, the placement of force by the inserter transforms the implant from its generally horizontal transit configuration into a deployed configuration. 
     In certain embodiments, the delivery tools including the deployment tool and inserter are detachable, and can therefore be removed once the implant is in position and successfully deployed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . Illustration of an exemplary Kambin&#39;s Triangle. 
         FIG. 2A . A cross-sectional view of a first dilator in certain embodiments. 
         FIG. 2B . A cross-sectional view of a first dilator in certain embodiments. 
         FIG. 3A . A side view of a first dilator in certain embodiments. 
         FIG. 3B . A close up view of a first dilator distal end in certain embodiments. 
         FIG. 3C . A close up view of a first dilator distal end in certain embodiments. 
         FIG. 3D . A close up view of a first dilator distal end in certain embodiments. 
         FIG. 3E . A close up view of a first dilator distal end in certain embodiments. 
         FIG. 4 . A bottom view of first dilator in certain embodiments. 
         FIG. 5 . A perspective view of a first dilator in certain embodiments. 
         FIG. 6 . A perspective view of a first dilator in certain embodiments. 
         FIG. 7 . A perspective view of second dilator in certain embodiments. 
         FIG. 8 . A close-up view of a distal end of a second dilator in certain embodiments. 
         FIG. 9 . A close-up view of a proximal end of a second dilator in certain embodiments. 
         FIG. 10 . A perspective view of a sheath in certain embodiments. 
         FIG. 11 . A superior view of a sheath in certain embodiments. 
         FIG. 12 . A perspective view of a dilator assembly in certain embodiments. 
         FIG. 13 . A perspective view of a dilator assembly in certain embodiments. 
         FIG. 14 . A perspective view of an implant in certain embodiments. 
         FIG. 15 . A center link in certain embodiments. 
         FIG. 16 . A center link in certain embodiments. 
         FIG. 17 . A perspective view of an end link lower portion in certain embodiments. 
         FIG. 18 . A perspective view of an end link upper portion in certain embodiments. 
         FIG. 19 . A close-up view of a hinge between a center link and end link in certain embodiments. 
         FIG. 20 . A perspective view of two end links and a dowel assembly in certain embodiments. 
         FIG. 21 . A view of an underside of an end link in certain embodiments. 
         FIG. 22 . A view of two end links fitting into complementary positions in certain embodiments. 
         FIG. 23 . A perspective view of two end links and a dowel assembled in transit form in certain embodiments. 
         FIG. 24 . A perspective view of an internal rod and two end links in a deployed form in certain embodiments. 
         FIG. 25 . A view demonstrating the placement of an internal rod within the space between two mated end links in a deployed form, in certain embodiments. 
         FIG. 26 . A view of a hinge between a center link and an end link, in certain embodiments. 
         FIG. 27 . A perspective view of a dowel positioned into an assembly having two center links and four end links, in certain embodiments. 
         FIG. 28 . An assembly of an implant in a deployed form, in certain embodiments. 
         FIG. 29 . An implant in a transit form, in certain embodiments. 
         FIG. 30 . A perspective view of an implant in certain embodiments. 
         FIG. 31 . A deployment tool used with an implant in certain embodiments. 
         FIG. 32 . An implant in certain embodiments. 
         FIG. 33 . A diagram of steps used in the delivery of an implant in certain embodiments. 
         FIG. 34 . A side view of an implant in certain embodiments. 
         FIG. 35 . A cut away view of a sheath in position through Kambin&#39;s Triangle, demonstrating safe passage of implant in through a route in certain embodiments. 
         FIG. 36A . An implant in an interbody space in certain embodiments. 
         FIG. 36B . A top-down view of an implant in an interbody space in certain embodiments. 
         FIG. 37A . A close up view of a cutter assembly distal end in certain embodiments. 
         FIG. 37B . A close up view of a cutter assembly distal end in certain embodiments. 
         FIG. 37C . A close up view of a cutter assembly distal end in certain embodiments. 
         FIG. 37D . A close up view of a cutter assembly distal end in certain embodiments. 
         FIG. 37E . A close up view of a cutter assembly distal end in certain embodiments. 
         FIG. 37F . A side view of a cutter assembly with a cutter adjuster in a closed position in certain embodiments. 
         FIG. 37G . A side view of a cutter assembly with a cutter adjuster in an open position in certain embodiments. 
         FIG. 37H . A perspective view of a cutter adjuster in certain embodiments. 
         FIG. 37I . A perspective view of a first knob in certain embodiments. 
         FIG. 37J . A cross-sectional view of a knob of a cutter adjuster in certain embodiments, where a cross-section is taken from an exemplary knob, in certain embodiments. 
         FIG. 37K . A close-up view of a cutter assembly in certain embodiments. 
         FIG. 38A . A perspective view of a discectomy instrumentation in a retracted configuration in certain embodiments. 
         FIG. 38B . A perspective view of a discectomy instrumentation in an expanded configuration in certain embodiments. 
         FIG. 38C . A perspective view of a discectomy instrumentation in an expanded configuration in certain embodiments. 
         FIG. 39A . A perspective view of an access dilator assembly in certain embodiments. 
         FIG. 39B . A perspective view of a first dilator in certain embodiments. 
         FIG. 39C . A perspective view of a first dilator in certain embodiments. 
         FIG. 39D . A side view of a first dilator in certain embodiments. 
         FIG. 39E . A side cross-sectional view of a first dilator in certain embodiments. 
         FIG. 39F . A side view of a sheath in certain embodiments. 
         FIG. 39G . A side cross-sectional view of a sheath in certain embodiments. 
         FIG. 39H . A perspective view of a sheath in certain embodiments. 
         FIG. 39I . A side view of a first dilator in certain embodiments. 
         FIG. 39J . A close up sectional view of a first dilator in certain embodiments. 
         FIG. 39K . A close up sectional view of a first dilator in certain embodiments. 
         FIG. 40A . A perspective view of an implant in a retracted configuration in certain embodiments. 
         FIG. 40B . A view from a distal end of an implant in a retracted configuration in certain embodiments. 
         FIG. 40C . A view from a proximal end of an implant in a retracted configuration in certain embodiments. 
         FIG. 40D . A side view of an implant in a retracted configuration in certain embodiments. 
         FIG. 40E . A side view of an implant in a retracted configuration in certain embodiments. 
         FIG. 40F . A perspective view of an implant in a retracted configuration in certain embodiments. 
         FIG. 41A . A perspective view of an implant in an expanded configuration in certain embodiments. 
         FIG. 41B . A view from a distal end of an implant in an expanded configuration in certain embodiments. 
         FIG. 41C . A view from a proximal end of an implant in an expanded configuration in certain embodiments. 
         FIG. 41D . A side view of an implant in an expanded configuration in certain embodiments. 
         FIG. 41E . A side view of an implant in an expanded configuration in certain embodiments. 
         FIG. 41F . A perspective view of an implant in an expanded configuration in certain embodiments. 
         FIG. 42A . A perspective view of an implant in a retracted configuration in certain embodiments, with certain features shown. 
         FIG. 42B . A perspective view of an implant in a retracted configuration in certain embodiments, with certain features shown. 
         FIG. 43A . A perspective view of an implant in an expanded configuration in certain embodiments, with certain features shown. 
         FIG. 43B . A perspective view of an implant in an expanded configuration in certain embodiments, with certain features shown. 
         FIG. 44A . A front view of links in certain embodiments. 
         FIG. 44B . A perspective view of links in certain embodiments. 
         FIG. 44C . A side view of links in certain embodiments. 
         FIG. 44D . A perspective view of an implant in an expanded configuration in certain embodiments, with certain features shown. 
         FIG. 45A . A perspective view of a deployment instrument in certain embodiments. 
         FIG. 45B . A side view of a deployment instrument in certain embodiments. 
         FIG. 45C . A side view of a deployment instrument in certain embodiments. 
         FIG. 45D . A perspective view of a deployment instrument in certain embodiments. 
         FIG. 45E . A perspective view of a deployment instrument in certain embodiments. 
         FIG. 45F . An exploded view of an assembly including a delivery sheath, locking pin, locking pin lever, and a base tool block in certain embodiments. 
         FIG. 45G . A perspective view of an assembly including a delivery sheath, locking pin, locking pin lever, and a base tool block in certain embodiments. 
         FIG. 46 . A side view of a portion of a deployment instrument in certain embodiments. 
         FIG. 47 . A close-up view of a distal end of a deployment instrument in certain embodiments. 
         FIG. 48 . A perspective view of an implant with two or more wedges in certain embodiments. 
         FIG. 49A . A perspective view from a distal end of a central component in certain embodiments of the invention. 
         FIG. 49B . A perspective view from a proximal end of a central component in certain embodiments of the invention. 
         FIG. 49C . A side view of a central component and stem in certain embodiments of the invention. 
         FIG. 49D . A perspective view from a proximal end of a central component in certain embodiments of the invention. 
         FIG. 50A . A side view of a wedge in certain embodiments of the invention. 
         FIG. 50B . A cross sectional view of a wedge, indicated in  FIG. 50A , in certain embodiments of the invention 
         FIG. 50C . A perspective view of a wedge in certain embodiments of the invention. 
         FIG. 50D . A perspective view of a wedge in certain embodiments of the invention. 
         FIG. 50E . A cross-sectional view of a wedge in certain embodiments of the invention 
         FIG. 50F . A side view of a wedge in certain embodiments of the invention. 
         FIG. 51 . A perspective view of four wedges in certain embodiments. 
         FIG. 52 . A perspective view of an implant with two or more wedges in certain embodiments. 
         FIG. 53A . Exemplary step showing placement of a wedge through a working sheath in certain embodiments of the invention. 
         FIG. 53B . Exemplary step showing placement of a wedge through a working sheath in certain embodiments of the invention. 
         FIG. 53C . Exemplary step showing placement of a wedge through a working sheath in certain embodiments of the invention. 
         FIG. 53D . Exemplary step showing placement of a wedge through a working sheath in certain embodiments of the invention. 
         FIG. 54A . A top view of a dilator in certain embodiments of the invention. 
         FIG. 54B . A side view of a dilator in certain embodiments of the invention. 
         FIG. 54C . A perspective view from a proximal end of a dilator in certain embodiments of the invention. 
         FIG. 54D . A perspective view from a distal end of a dilator in certain embodiments of the invention. 
         FIG. 54E . A profile view of a proximal end of a dilator in certain embodiments of the invention. 
         FIG. 54F . A profile view of a distal end of a dilator in certain embodiments of the invention. 
         FIG. 55A . A perspective view of a sacrum, ilia and vertebrae where the route of a passage is through the ilium and the sacral ala to the L5-S1 interbody space, in certain embodiments. 
         FIG. 55B . A posterior sectional view of a portion of a sacrum and vertebrae, where the route of a passage is to the L5-S1 interbody space, in certain embodiments. 
         FIG. 55C . An oblique view of a sacrum and vertebrae, where the route of a passage is to the L5-S1 interbody space, in certain embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Descriptions of embodiments of the present invention disclosed herein are intended to serve as examples, and may not encompass all possible embodiments. One skilled in the art will recognize that variations to the embodiments disclosed herein may be made without compromising the essence of the invention. 
     Certain embodiments of the present invention are directed to a system and method for a surgical procedure to accomplish lumbar interbody fusion via a sheathed posterior oblique lateral approach. Certain embodiments of the invention incorporate one or more implants, which in varying embodiments may be expandable or non-expandable, insertable to a target point between two vertebral bodies through a low-diameter sheathed passage. Certain embodiments incorporate a variety of apparatuses including instrumentation and an expandable interbody cage insertable into a human body through a low-diameter, sheathed passage. For purposes related to the preferred embodiment of the invention, the term “low-diameter” refers to having an outer diameter equal to or less than 12 millimeters. The present inventors have recognized that a low-diameter sheath may safely pass on a trajectory through the area between the structures comprising the boundaries of Kambin&#39;s Triangle without causing permanent damage to the structures comprising the boundaries of Kambin&#39;s Triangle. 
     In certain embodiments, the implant is configured to expand following passage through a low-diameter sheathed passage and placement between vertebral bodies. In certain embodiments, the implant comprises an expandable interbody cage configured to a size and shape to fit through a low-diameter sheathed passage prior to expansion. 
     Certain embodiments are further directed towards a method of inserting instrumentation needed to prepare an interbody space (as used herein, the term “interbody space” is defined as the area generally between two vertebral bodies) for fusion and of inserting an implant into an interbody space through a sheathed passage that approaches the spine via a posterior oblique lateral trajectory. 
     Certain embodiments of the invention include instruments and steps associated with identifying an optimal trajectory to the interbody space. In a certain embodiment, these instruments include a radiopaque trajectory planning instrument, comprising of a thin elongated wire-like body of a length to span at least the distance between the interbody space and the incision point. In certain embodiments, the radiopaque trajectory planning instrument is visible on a radiographic image created with the instrument placed within the field of the image. Certain embodiments include a surgical skin marker that places a biocompatible solution on the patient&#39;s skin and serves to mark relevant anatomy viewed from the radiographic images. 
     Certain embodiments of the present invention include instruments and steps associated with neuromonitoring. The present inventors have recognized that neuromonitoring enables safe passage through Kambin&#39;s Triangle. The present inventors have also specifically recognized that steps associated with neuromonitoring allow surgeons to avoid an exiting nerve by enabling targeting of the lower half of the Kambin&#39;s Triangle associated with such exiting nerve. In certain embodiments the neuromonitoring probe is monopolar and unidirectional at the distal end. It will be appreciated that certain embodiments of a neuromonitoring probe have a distal end that electrically stimulates the nerves. In certain embodiments, a dilator assembly is adapted to receive a neuromonitoring probe, to allow the neuromonitoring probe to function while the neuromonitoring probe and dilator assembly work simultaneously to expand a passage. In certain embodiments, an access dilator assembly provided is adapted to incorporate a neuromonitoring probe into the dilator assembly. In certain embodiments, an access dilator assembly includes a slot on a proximal end configured to receive a flexible probe. In certain embodiments, a first dilator is configured to receive a neuromonitoring probe. 
     In certain embodiments, the system incorporates a guide wire. In varying embodiments, a guide wire is optionally referred to as a “Kirschner Wire” or “K-Wire.” In embodiments of the invention, a guide wire consists of a surgical instrument comprising a long member with an outer diameter of approximately 1 millimeter to 3 millimeters. In varying embodiments, a guide wire comprises stainless steel or nitinol. In certain embodiments, a guide wire has a sharp beveled tip. In certain embodiments, particularly where the guide wire is configured to pass through bone, the guide wire incorporates a drill tip. Certain embodiments of a guide wire incorporate a rounded blunt tip as to minimize tissue trauma. 
     Certain embodiments of the present invention incorporate instruments and steps associated with discectomy. Discectomy may be performed during a disc preparation step  1404  as shown in  FIG. 33 . In certain embodiments, discectomy instrumentation comprises instruments for the removal of vertebral disc material at a targeted interbody space. In certain embodiments, discectomy instrumentation is configured to pass through a sheathed passage. In certain embodiments, discectomy instrumentation is configured to pass through a low-diameter sheathed passage in a retracted state, then partially expand within a disc space, and then return to a retracted state for removal through a low-diameter sheathed passage. In certain embodiments, discectomy instrumentation includes, for example, a disc reamer having a cylindrical hollow body, capable of accessing and fitting into the interbody space and reaming disc tissue. In certain embodiments, discectomy instrumentation includes, for example, an elongate body with the distal end having a drill bit mechanism, allowing drilling through the disc material, capturing the disc material within the grooves of the drill bit, and extracting the disc material by removing the drill bit. In certain embodiments, an endoscope may be utilized in association with discectomy instrumentation for purposes associated with the inspection of the discectomy and endplate preparation prior to and following the insertion of discectomy instrumentation. 
     In certain embodiments, discectomy instrumentation includes, for example, loop cutters. Loop cutters include a flat, thin, ribbon of material deployable through an elongate tube. A loop cutter is accessible in a vertebral disc space to cut the disc tissue. In certain embodiments, discectomy instrumentation may take the form of embodiments disclosed within U.S. Pat. No. 7,500,977 B2, U.S. Patent Publication No. US 2007/0260270 A1, U.S. Patent Publication No. US 2008/0033466 A1, U.S. Pat. No. 7,632,274 B2, U.S. Patent Publication No. US 2007/0265652A1, U.S. Patent Publication No. US 2005/0149034 A1, U.S. Patent Publication No. US 2003/0191474 A1, U.S. Pat. No. 7,500,972 B2, and U.S. Pat. No. 7,588,574, which are incorporated herein by reference in their entirety. In certain embodiments, the loop cutters may take the form of embodiments described within the documents referred to within the preceding sentence. In certain embodiments, the loop cutters deploy at a substantially 45 degree angle. In certain embodiments, discectomy instrumentation including cutter assemblies are configured for an oblique lateral procedure and thereby differ from previously known loop cutters in the plane of deployment into the disc space. 
     Referring to  FIGS. 37A-K , in certain embodiments, a cutter assembly includes a cutter shaft, a cutter sheath, and a handle. In certain embodiments, a cutter shaft  1670  is attached to a cutter blade  1651 ,  1655 ,  1656 , where a cutter sheath  1653  is concentrically placed over the cutter shaft  1670 . The cutter sheath  1653 , cutter shaft  1670 , and handle  1669  components are preferably co-configured to enable the cutter blade and the cutter shaft  1663  to which it is attached be able to be “pushed-pulled” so as to retract the cutter blade into the cutter sheath and then extend the cutter blade from the distal end  1672  of the cutter sheath as needed. 
     Referring to  FIG. 37A , in certain embodiments, a cutter assembly deploys a cutter blade  1651  in a plane that is parallel to the plane that intersects the longitudinal axis  1652  of the instrument in order to cut in varying heights of the disc space. In certain embodiments, cutter assembly  1650  deploys a cutter blade  1651  in a plane that is parallel to the plane of the disc space. In certain embodiments, referring to  FIG. 37B , the act of sheathing the cutter blade into a protective sheath  1653  allows control of the effective radius  1654   a ,  1654   b  of the cutter blade  1655 . As seen in  FIG. 37B , in certain embodiments, the cutter blade is deployed generally laterally from a longitudinal axis  1652 . This change in radius can be determined from the user (proximal) end of the cutter assembly to match the varying concavities and heights between the vertebral endplates, using certain embodiments of a cutter adjuster as shown in  FIGS. 37F-J . In certain embodiments, the radius of a cutter blade is adjusted with a first knob  1659  and a second knob  1660 . It will be appreciated that in certain embodiments, a first knob  1659  and a second knob  1660  is placed on discectomy instrumentation disclosed in U.S. Pat. No. 7,500,977 B2, U.S. Patent Publication No. US 2007/0260270 A1, U.S. Patent Publication No. US 2008/0033466 A1, U.S. Pat. No. 7,632,274 B2, U.S. Patent Publication No. US 2007/0265652A1, U.S. Patent Publication No. US 2005/0149034 A1, U.S. Patent Publication No. US 2003/0191474 A1, U.S. Pat. No. 7,500,972 B2, and U.S. Pat. No. 7,588,574, which are incorporated herein by reference in their entirety. A first knob  1659  and second knob  1660  includes a primary slot  1661 ,  1662  that cuts from their center axis  1668  to the outer perimeter. The primary slot allows the first and second knob to slide over the cutter shaft and/or cutter sheath. A first knob  1659  further includes a connector  1663  having threads  1664  that allows a threaded connection with a threaded opening  1665  of a second knob  1660 . Referring to  FIG. 37I-J , the first knob  1659  has a second slot  1673  that cuts through the mid portion of the knob  1659 . It will be appreciated that a second knob  1660  includes a second slot in certain embodiments. Referring to  FIG. 37K , in certain embodiments, the second slot  1673  captures an end stop  1674 , which is connected to the cutter sheath  1653 . In certain embodiments, an end stop is a concentric collar attached to a cutter sheath and/or cutter shaft. In order to control the distance/radius and angle of the cutter blade that is exposed at a distal end  1672 , the first knob  1659  and second knob  1660  are rotated relative to each other along the threaded connection to create a distance between first and second knobs. In certain embodiments, a first surface  1666  of a second knob  1660  contacts the second surface  1667  located on a handle  1669 . In certain embodiments, a cutter shaft  1670  includes an end stop, while a second knob includes a second slot. In certain embodiments, a first knob and a second knob, both having a second slot, is positioned over a cutter assembly where a cutter sheath and cutter shaft have an end stop. 
     In certain embodiments, the cutter is adjusted using the following steps. A first knob and second knob are threaded together so the two knobs are fully engaged. The primary slot on both knobs should be radially aligned with each other. With the cutter sheath advanced distally, where a cutter blade is fully “sheathed” or housed in the sheath in its retracted state, the cutter adjuster is placed over the cutter sheath, end stop, and cutter shaft. With the cutter adjuster attached to the cutter assembly, the cutter adjuster assembly is pulled proximally until the cutter blade is deployed. The proximally located knob (e.g. second knob) is rotated relative to the distally located knob (e.g. first knob) to retract the cutter blade into the sheath. The knobs are turned until a preferred deployment position, for example, when the distance, radius, and angle of the cutter blade is appropriate, is set. In certain embodiments, the distance, radius, and angle of the cutter blade can be adjusted in situ by rotating the first and second knobs relative to each other. 
     In certain embodiments, the depth of the cutter blade and angle relative to the initial approach angle allows the user to prepare the desired footprint of the interbody space during steps related to discectomy. In certain embodiments, a cutter assembly  1650  as shown in  FIG. 37C-D  is used. In certain embodiments, a cutter blade  1656  has side walls  1657  that extend out and spread when the cutter blade  1656  is deployed. In certain embodiments, when the cutter  1656  is deployed, the side walls  1657  extend laterally beyond the outer wall  1658  of the sheath  1653 . 
     In certain embodiments, once the distal end of the cutter blade is in the desired location to debulk the disc space, the radius of the loop or distance of deployment may be set by the user. In certain embodiments, the cutter blade may be controllably rotated by a user using a handle affixed to the protective sheath at the proximal end to perform discectomy by removing material proximal to the superior and inferior endplates. Optionally, in certain embodiments, decortication of the superior and inferior endplates may be achieved by rotating the cutter blade to scrape the superior and inferior endplates. 
     In certain embodiments, discectomy instrumentation includes, for example, an endplate rasp. In certain embodiments, an endplate rasp has a spoon-shaped end, capable of accessing and fitting into the disc space, and decorticating the vertebral endplates. In certain embodiments, the discectomy instrumentation may take the form of embodiments described in U.S. Pat. No. 8,696,672 B2, U.S. Patent Publication No. 2011/0184420 A1, which are incorporated herein by reference in their entirety. In certain embodiments, the endplate rasp may take the form of embodiments described by the documents referred to within the preceding sentence. In certain embodiments, discectomy instrumentation includes, for example, disc material removal tools. Such disc material removal tools include, for example, surgical pituitaries capable of accessing and fitting into the disc space to remove disc material. In certain embodiments, the discectomy instrumentation may take the form of embodiments described in U.S. Pat. No. 8,052,613 B2, which is incorporated herein by reference in its entirety. In certain embodiments, the disc material removal tools may take the form of embodiments described by the document referred to within the previous sentence. 
     In certain embodiments, discectomy instrumentation includes, for example, an expandable discectomy tool  1700  as shown in  FIG. 38A-C . In certain embodiments, the expandable discectomy tool  1700  has a distal end  1701  and a proximal end  1702  and a plurality of center links  1703  and end links  1704 . In certain embodiments, the expandable discectomy tool  1700  is expanded in a similar manner to the expandable implant  1750  as exemplified and described, for example, in  FIGS. 40A-F  and  FIGS. 41A-F . The expandable discectomy tool  1700  includes cutting edges  1705  as seen in  FIG. 38B . Rotating the expandable discectomy tool  1700  in the vertebral disc space allows decortication of the upper and lower endplates. Referring to  FIG. 38C , in certain embodiments, an expandable discectomy tool  1700  includes a rasping surface  1706  on one or more center links  1703 . 
     Certain embodiments of the present invention include instruments and steps associated with trialing or inserting trial instruments. It will be appreciated that a trial allows evaluation and determination of a surgical area prior to placing an implant. In certain embodiments, a trialing instrument is capable of determining the measurement of interbody space height, while simultaneously distracting two vertebrae apart from each other. The trialing instrument, and the steps associated with placing the trials are performed through a sheath. In certain embodiments the trialing instrument resembles a pituitary, comprising an elongated portion of two slidably engaged semicircular extrusions. In certain embodiments, the semicircular extrusions are then connected to a handle in such a way that upon squeezing the handle, the superior semicircular extrusion slides over the inferior semicircular extrusion. In certain embodiments, trialing instrument is performed with an expandable cage similar to those shown in  FIGS. 14-32 , and similar to those shown in  FIGS. 40-44 . 
     In certain embodiments, instruments and steps are adapted to safely pass one or more non-expandable implants. In certain embodiments, instruments and steps are adapted to safely pass one or more expandable interbody implants and instrumentation through a sheath into an interbody space. In certain embodiments, non-expandable interbody cages and/or expandable interbody cages, and associated instruments, are passed through a low-diameter sheathed passage. In certain embodiments, a sheath is configured to follow a passage established through Kambin&#39;s Triangle. In certain embodiments, a sheath is configured to follow a passage from the skin into a L5-S1 interbody space by first passing through an ilium, a sacroiliac joint, a sacrum, and Kambin&#39;s Triangle. In certain embodiments, a sheath is configured to follow a passage from the skin into a L5-S1 interbody space by first passing through an ilium and a sacroiliac joint, but stopping prior to the sacrum, whereby an unsheathed passage is further created through the sacrum, through Kambin&#39;s Triangle and into an interbody space. In a certain embodiment, passage through Kambin&#39;s Triangle is accomplished by shielding Kambin&#39;s Triangle from physical impact associated with the passage of instrumentation and implants by a sheath. 
     In certain embodiments, in order to safely pass through Kambin&#39;s Triangle, dilation instruments and steps are adapted to widen or dilate the passage. In one example, the passage begins at an incision point in the skin and ends within an interbody space. As seen in  FIG. 1 , Kambin&#39;s Triangle  0104  is defined by a traversing nerve and/or superior articular process  0100 , the superior face  0103  of a vertebral body  0101 , and an exiting nerve root  0102  from the superior part of the neural foramen. In certain embodiments, dilation instruments comprise surgical grade aluminum with a Type III anodized coating and/or stainless steel. In certain embodiments, such instruments comprise radiolucent properties. In certain embodiments, an endoscope may be utilized in association with instrumentation for purposes associated with the inspection of the foramen and other structures near the passage prior to and following the insertion of dilation instrumentation. 
     In certain embodiments, dilation instruments incorporate a tapered distal end  1530 . In certain embodiments, a dilation instrument comprises a plurality of dilators. For example, a first dilator  1500  having a tapered distal end  1530  is slidably removable through a second dilator having a larger diameter. In a certain embodiment, a dilation instrument incorporates a neuromonitoring feature to allow for the detection of nerve structures located near the dilation instrument. In a certain embodiment, neuromonitoring is performed by sending an electrical current through the dilation instrument, which is measured at another point in a patient&#39;s body. In certain embodiments, the dilation instrument has a longitudinal hole to enable the dilation mechanism to slide over a prior placed neuromonitoring probe. In a certain embodiment, the series of dilators is configured such that the dimensions of the dilators can pass between the structures comprising the boundaries of Kambin&#39;s Triangle without contacting such structures while expanding the passage enough to accommodate the placement of a low-diameter sheath. 
     As seen in  FIGS. 2-6 , in certain embodiments, the dilation mechanism incorporates a first dilator  1500 . In certain embodiments, a first dilator comprises a tubular extrusion with a generally oblong shaped cross-section. In certain embodiments, the first dilator  1500  is 6 millimeters wide at its widest point and 260 millimeters in length, although other sizes can be considered. Referring to  FIG. 2A-B , in certain embodiments, the cross-sectional profile of the first dilator is circular, oval, or triangular. Referring to  FIG. 3-6 , in certain embodiment, a distal end  1520  of the first dilator  1500  comprises a bevel  1501  and rounded tip  1502 . In certain embodiments, the bevel  1501  and rounded tip  1502  minimizes the occurrence of tissue disruption during passage of the first dilator through Kambin&#39;s Triangle and proximal structures. In certain embodiments, a first dilator  1500  has a circular taper, and in certain embodiments, a first dilator  1500  has a bullet-shaped tip  1531  at the distal end. Generally, an exemplary tapered end  1530  as shown in  FIGS. 3B , D, and E allows for a gradual, atraumatic opening of tissue as the dilator progresses into the body. Referring to  FIG. 5  and  FIG. 6 , in certain embodiments, the distal area of a first dilator  1500  has a reference marking  1503  used to denote which side of first dilator  1500  should orient generally superiorly, and along an exiting nerve root  0102 . In certain embodiments, the proximal end  1521  of a first dilator  1500  incorporates one or more grooves  1504 . In certain embodiments, the grooves  1504  are oriented in a substantially orthogonal direction relative to the longitudinal axis  1522  of the first dilator  1500 . The grooves  1504  allow for improved user grip. 
     Referring to  FIGS. 2A-B  and  FIG. 6 , in certain embodiments, a through hole or cannula  1505  forms a contiguous channel through a first dilator  1500 . In certain embodiments, the cannula  1505  exists along a longitudinal axis  1522  of the first dilator  1500 . In certain embodiments, a cannula  1505  connects a proximal end  1521  and distal end  1520 . In certain embodiments, a cannula  1505  is offset (see  FIG. 2A ), or is centered (see  FIG. 2B ) on a first dilator. Referring to  FIG. 6 , in certain embodiments, a side aperture  1506  is located within grooves  1504 . A side aperture  1506  is further connected with a cannula  1505 . Referring to  FIG. 3 , in certain embodiments, one or more depth markers  1507  are located on an outer surface of the first dilator  1500 . In certain embodiments, a plurality of depth markers  1507  begin approximately 80 mm from a distal end, and ends 160 mm from a distal end, where the location of the depth marker is placed in 10 mm intervals. 
     In certain embodiments, neuromonitoring occurs while dilating a pathway to a target site. In certain embodiments, an access dilator assembly  1600  as shown in  FIGS. 39A-39K  allows nerve monitoring, soft tissue dilation, initial disc access and the delivery of a sheath. In certain embodiments, an access dilator assembly  1600  includes a first dilator, and a sheath. In certain embodiments, a first dilator, as seen in  FIGS. 39B-39E , I, is also referred to as a dilator shaft. It will be appreciated that a first dilator  1601  can be used with other instruments. In certain embodiments, neuromonitoring is performed as described in U.S. Provisional Patent Application No. 62/569,746 filed Oct. 9, 2017 and entitled “Neuromonitoring Dilation System,” which is hereby incorporated by reference in its entirety. 
     In certain embodiments, an access dilator assembly  1600  facilitates neuromonitoring by accommodating a standard disposable monopolar probe, such as a Cadwell 200 millimeter ball tip disposable monopolar probe, through a slot  1603  located on a proximal end  1604  of a first dilator  1601  as shown in  FIGS. 39B and 39E . A standard monopolar probe may further be pushed through the cannula  1609 , as seen in  FIG. 39E , towards the distal end  1605 . A standard disposable monopolar probe in such embodiments is delivered through the access dilator assembly  1600  prior to or during a surgical procedure. It will be appreciated that the cannula  1609  can also accommodate other instruments including guide wires or K-wires. In certain embodiments, a cannula is connected with an opening located generally near the first dilator proximal end  1604 , and extends towards a first dilator distal end  1605 . In certain embodiments, a cannula is connected with a tip aperture  1622  of a first dilator distal end  1605 . Referring now to FIG. J-K, in certain embodiments, the cannulation is a blind hole, where the cannulation  1609  extends from the proximal end, and ends at a stop  1623  located within a volume of a distal piece  1620  that is conductive. In certain embodiments, the cannulation  1609  extends from the proximal end and ends at a stop  1624  prior to crossing into a distal piece  1620  that is conductive. In certain embodiments, the purpose of a cannulation comprising a blind hole is to allow the stimulating tip of the disposable monopolar probe to make contact with a conductive tapered tip or distal piece, and by extension allow the conductive tapered tip to have stimulation capabilities. 
     In certain embodiments, the distal end of the standard disposable monopolar probe is intended to make contact with an electrically conductive distal end  1605  of the first dilator  1601 . In certain embodiments, the distal end  1605  of the first dilator  1601  includes a distal piece  1620  made of an electrically conductive material, such as stainless steel. In certain embodiments, the distal piece  1620  of the first dilator  1601  has a taper  1606 . A tapered profile  1606  facilitates entry into the disc space and dilation up to the diameter of the sheath  1602 . In certain embodiments, as seen in Figs. B-D, the distal piece  1620  of the first dilator  1601  includes a disc penetrator or flattened tip  1618 . Contacting the monopolar stimulating tip of a standard disposable monopolar probe with the distal piece  1620  allows electrical stimulation of the distal end, as to determine the proximity of the access dilator assembly  1600  to nerves, including for example, edges of Kambin&#39;s Triangle. 
     In certain embodiments, the distal end  1605  of the access dilator assembly  1600  is electrically conductive, while the shaft  1607  is electrically insulated. In certain embodiments, the distal piece  1620  is electrically conductive. In certain embodiments, the end of a disposable monopolar probe contacts the electrically conductive distal piece  1620 . The shaft  1607  has an insulating material in order to localize the electric current to the distal end  1605 . The insulating quality of the shaft  1607  further prevents shunting or shorting out of the neuromonitoring signal. In certain embodiments, the insulating material of the main shaft of the dilation mechanism comprises a non-conductive metal, such as aluminum, (e.g. type III anodized aluminum). In certain embodiments, the proximal end  1604  of the access dilator assembly  1600  features a quick connect feature  1608  as seen in  FIGS. 39B-D . The quick connect feature  1608  and a shaft  1607  is attached, for example, through a number of attachment mechanisms known, including, but not limited to threaded attachment, adhesive, and interference fit. It will be appreciated that a probe shaft  1607  and a distal piece  1620  are connected through a number of known attachment mechanisms. 
     In certain embodiments, distal end  1605  of the access dilator assembly  1600  is electrically insulated. In certain embodiments, the distal piece  1620  is electrically insulated. In such embodiment, the end of a disposable monopolar probe is exposed at the end of a distal piece  1620  through a tip aperture  1622  (shown in  FIG. 39E ). 
     Referring to  FIGS. 39B-D , I, in certain embodiments, the quick connect feature  1608  allows attachment of a standard surgical handle. Referring to  FIGS. 39B, 39C, 39E, and 39I , in certain embodiments, the quick connect feature  1608  and/or the shaft  1607  incorporates a slot  1603  designed to accommodate a standard disposable monopolar probe, while the standard surgical handle is attached. In certain embodiments, the slot  1603  facilitates the placement of the standard surgical handle on the quick connect feature  1608  with the monopolar probe in place by bending the standard disposable monopolar probe. Referring to  FIG. 39G , in certain embodiments, the first dilator  1601  is passed through the cannulation  1610  of the probe sheath  1602 . Certain embodiments of the sheath  1602  have an inner diameter  1619  of 9 mm. Referring to  FIGS. 39F-G , a proximal end  1613  of the sheath  1602  includes an impact collar  1614  further having a pin slot  1615 . The pin slot  1615  engages with the pin  1616  located on the first dilator  1601  (seen in  FIGS. 39B-D ). In certain embodiments, a sheath  1602  is assembled with a first dilator  1601  and inserted together into an interbody space. In certain embodiments, once the sheath creates a passage between an interbody space and the exterior of a patient, the first dilator  1601  is disengaged and removed. The impact collar  1614  of the sheath  1602  further contacts an impact collar  1617  located on the first dilator  1601  (seen in  FIGS. 39B-D ). Still referring to  FIG. 39G , in certain embodiments, the distal end of the  1612  of the sheath  1602  includes a sheath bevel  1611 . In certain embodiments, the bevel assists in positioning the sheath into interbody space. In certain embodiments, a sheath  1602  includes a handle  1621 , as shown in  FIG. 39H . 
     In certain embodiments, a first dilator has an outer surface lacking a pin  1616  and an impact collar  1614 , as shown in  FIG. 39I . In certain embodiments, a first dilator as shown in FIG.  39 I allows insertion into a proximal end of a dilator or a sheath. In certain embodiments, a first dilator includes a shaft  1607  and a distal piece  1620  that are both non-conductive. In certain embodiments, a shaft  1607  and a distal piece  1620  are a unitary piece. In certain embodiments, a shaft  1607  and a distal piece  1620 , and quick connect feature  1608  are non-conductive. In certain embodiments, a shaft  1607  and a distal piece  1620 , and quick connect feature  1608  are a unitary piece. In certain embodiments where a distal piece  1620  is non-conductive, the monopolar stimulating tip of a standard disposable monopolar probe is exposed through the distal end  1605 , through the distal piece  1620 . 
     In certain embodiments, a second dilator is slidably and removably placed over a first dilator. Referring to  FIGS. 7-9 , a second dilator  1508  has a cross-sectional profile similarly oblong to first dilator  1500 . In certain embodiments, the second dilator  1508  has an outer diameter with an 8 mm width at its widest point, and has a length of approximately 240 mm. In certain embodiments, the outer cross-sectional profile of the second dilator is circular, oval, or triangular in shape. In certain embodiments, the distal end  1523  of the second dilator  1508  incorporates a less steep inferior beveled surface  1509  than a first dilator  1500  bevel  1501  and a rounded tip  1510  to create an atraumatic tapered profile. In certain embodiments, the distal end of the second dilator minimizes tissue and nerve trauma during placement of dilation mechanisms. In certain embodiment, the distal end of second dilator  1508  incorporates a reference marking  1511  used to denote a side of second dilator  1508  that should face generally superior and tilted to match the angle of an exiting nerve root  0102 . Certain embodiments of second dilator  1508  comprise an oblong hole  1512  spanning the length of the instrument to match the outer oblong cross-section of first dilator  1500 . In certain embodiments, the proximal end  1524  of the second dilator  1508  comprises grooves  1513 . In certain embodiments, the second dilator  1508  incorporates depth markers. 
     As seen in  FIGS. 10-13 , in certain embodiments, a sheath  1514  covers a first dilator, and one or more second dilators. In certain embodiments, the sheath shields the pathway to the target area to protect surrounding nerves. In certain embodiments, the sheath shields external structures from being physically affected by the passage of instrumentation and/or one or more expandable or non-expandable interbody cages through the pathway. In a certain embodiment, the sheath is an elongate tube. In certain embodiments, the material of the sheath includes stainless steel, titanium, aluminum, and other metals, and in certain embodiments, it will be appreciated that other materials, including but not limited to plastics and polymers are used. In certain embodiments, a sheath of any size is used. In certain embodiments, the sheath has an external diameter ranging between 12 mm and 8 mm. In certain embodiments, the sheath has an internal diameter ranging between 10 mm and 6 mm. In certain embodiments, a sheath has an external diameter no greater than 12 mm. 
     Referring to  FIG. 12 , in certain embodiments, the sheath  1514  is slidable and removable relative to the first dilator  1500  and/or the second dilator  1508 . Referring to  FIG. 11 ,  FIG. 12 , and  FIG. 13 , in certain embodiments, the sheath  1514  has a shaft  1515  and an oval shaped protrusion or a handle  1519   a ,  1519   b . In certain embodiments, the length of the shaft  1515  is approximately 220 mm, with an outer diameter of 10.5 mm, although other sizes may also be used. Referring to  FIG. 11  and  FIG. 12 , the shaft  1515  includes a cannula  1516  connecting a proximal end  1526  and a distal end  1525 . In certain embodiments, the sheath cannula has a diameter of approximately 9 mm. In certain embodiments, the sheath  1514  distal end  1525  has a rounded tip  1517 . The rounded tip  1517  minimizes tissue damage and nerve disruption while passing through Kambin&#39;s Triangle and other tissues. In certain embodiments, the sheath  1514  includes a hydrophobic coating. Referring to  FIG. 11 , in certain embodiments, the proximal end  1526  of a sheath  1514  incorporates a hole or opening  1518 . In certain embodiments, a cannula  1516  is located between a distal end  1525  and proximal end  1526 , where the cannula  1516  is connected with opening  1518 . For example, the opening  1518  has a surface that tapers towards the cannula  1516 . Certain embodiments of the sheath  1514  have a handle, such as a T-shaped handle, at the proximal end. In certain embodiment, the proximal handle incorporates an oval-shaped protrusion  1519   a  perpendicular to the axis of circular shaft  1515  and located around the large hole  1518 . A second oval shaped protrusion  1519   b  is oriented 180 degrees from a first oval cross-sectioned protrusion  1519   a  with respect to the large hole  1518 . In a certain embodiment, oval shaped protrusions  1519   a  and  1519   b  improve grip. 
     In certain embodiments, an implant includes an expandable interbody cage  1000  is placed into the space between vertebral discs. Referring to  FIG. 14 , in certain embodiments, the expandable interbody cage  1000  includes two long structural elements or center links  1100 , and four short elements or end links  1200 . In certain alternative embodiments, an expandable interbody cage  1750  includes four center links that separate from each other during deployment as shown in  FIGS. 40A-F  and  FIGS. 41A-F . In certain embodiments, the arrangement of the structural elements allows a center link to contact a vertebral endplate when the expandable interbody cage  1000  is deployed. Referring to  FIG. 14 , in certain embodiments, a distal end  1226  end link  1200   c  is connected with a center link  1100  distal end, and a proximal end  1227  end link  1200   d  is connected with a center link  1100  proximal end. In certain embodiments, a center link and end link are hingeably connected. In certain embodiments, a first end link is hingeably connected with a second end link. In certain embodiments, pulling on a distal end towards the proximal end causes the center link to expand or extend in a direction away from a longitudinal axis  1228  of an implant or cage. In certain embodiments, a stem or an internal rod guides the proximal end  1227  end link  1200  and a distal end  1226  end link  1200 . Referring to  FIG. 14 , in certain embodiments, the end links  1200  are arranged in pairs that form load-bearing hinges. In alternative embodiments, the system may incorporate one or more non-expandable interbody implants each comprising a singular solid structure. In certain embodiments, an implant comprises an assemblable interbody cage  1850  comprising two or more wedges  1851 , as shown in  FIG. 48 . As used herein, the term “assemblable” means “able to be assembled during and/or following placement within an interbody space.” In certain embodiments, the material of the expandable interbody cage includes, but is not limited to titanium, polyetheretherketone (PEEK), carbon fiber, and/or stainless steel. 
     As seen in  FIG. 15 , certain embodiments of a center link  1100  have a lateral surface  1101 , a ridged surface  1102 , a hinge portion  1103 , and a hole  1104 . In certain embodiments, a ridged surface  1102  is shaped to engage one or more vertebral endplates. In certain embodiments, a ridged surface of a center link  1100  provides for increased purchase with one or more vertebral endplates. In certain embodiments, the purchase stabilizes an expandable interbody cage  1000  following deployment, preventing its within the interbody space. 
     As seen in  FIG. 16 , in certain embodiments, center link  1100  includes a first radius cutout  1105 , a second radius cutout  1106 , and an interior surface  1107 . As seen in  FIG. 14 , first radius cutout  1105  is shaped to mate with first convex surface  1215  and second convex surface  1217  of end link  1200 , as depicted in  FIG. 14 . Referring to  FIG. 16 , second radius cutout  1106  is shaped to mate with curvature of external protrusion  1201  and internal protrusion  1202 , for example, support surface  1219  of the external protrusion  1201  and support surface  1220  of internal protrusion  1202  as seen in  FIG. 21 . Referring to  FIG. 14  and  FIG. 16 , a cutout  1108 , also referred to as a groove, cuts into interior surface  1107  along its axial dimension allows slideable movement of an internal rod  1300  (seen in  FIG. 14  and  FIG. 24 ) in certain embodiments. In certain embodiments, an internal rod is referred to as a “stem.” 
     As seen in  FIG. 17 , in certain embodiments, an end link  1200  has an external protrusion  1201  and an internal protrusion  1202 . External protrusion  1201  incorporates an outer short lateral surface  1203  and a dowel passage  1204 . Internal protrusion  1202  has a dowel passage  1206 . An internal protrusion has a support surface  1220  having a rounded shape to promote an axial rotation around a pin inserted in an inner hinge passage  1206  without obstruction, in certain embodiments. 
     As seen in  FIG. 18 , in certain embodiments, an end link  1200  has a first protrusion or first knuckle  1207 , and a second protrusion or second knuckle  1208 . A first knuckle  1207  has a pinhole  1209 . Second knuckle has a  1208  has a lateral surface  1205  and pinhole  1210 . In certain embodiments, a first knuckle  1207  and second knuckle  1208  have a ridged surface  1211 . Still referring to  FIG. 18 , a gap  1225  is located between a first knuckle  1207  and second knuckle  1208 . 
     As seen in  FIG. 19 , in certain embodiments, an end link  1200  ridged surface  1211  is oriented obliquely to center link  1100  ridged surface  1102 , such that rotation of end link  1200  when an expandable interbody cage  1000  is in a deployed or expanded configuration, ridged surface  1211  and long ridged surface  1102  form a generally contiguous surface. In certain embodiments, when an expandable interbody cage  1000  is in a deployed configuration, the end link  1200  ridged surface  1211  is substantially planar with a center link  1100  ridged surface  1102 . 
     In certain embodiments, as seen in  FIG. 20 , each end link has a pinhole  1209  and pinhole  1210 . As seen in  FIG. 20 , a first end link  1200   a  is paired with a second end link  1200   b . In certain embodiments, a first end link and second end link are identical. In certain embodiments, one end link can be inverted and mated with another end link, where a dowel is placed through dowel openings  1214  of a first end link  1200   a  and second end link  1200   b . In certain embodiments, a proximal dowel  1301  is placed through a first end link  1200   a  and a second end link  1200   b . The first end link  1200   a  and the second end link  1200   b  are thus able to rotate around the dowel and relative to each other. 
     As depicted in  FIG. 21 , in certain embodiments, a first knuckle  1207  has a first convex surface  1215  and a first concave surface  1216 , and a second knuckle  1208  has a second convex surface  1217  and second concave surface  1218 . The outer surface of external protrusion  1201  has an external support surface  1219 . Internal protrusion  1202  has an internal support surface  1220 . The external support surface  1219  provides a load bearing surface area. In certain embodiments, the curvature of the first concave surface  1216 , internal support surface  1220 , second concave surface  1218 , and external support surface  1219  are substantially the same, allowing the surfaces  1216 ,  1218 ,  1219 ,  1220  of one end link to rotate relative to a the surfaces  1216 ,  1218 ,  1219 ,  1220  of another end link. In certain embodiments, contacts between first concave surface  1216  on a first end link  1200  and internal support surface  1220  on a second end link  1200 , and between second concave surface  1218  on a first end link  1200  and external large support surface  1219  on a second end link  1200  are load bearing. Thus, in certain embodiments, the present inventors have recognized that load is distributed among a first end link  1200   a  to a second end link  1200   b  when expandable interbody cage  1000  is in a deployed state. 
     As depicted in  FIG. 22 , in a certain embodiment of the invention, the bowed exterior surface of internal protrusion  1202  meets the bowed exterior of end link  1200  at an angle, forming an angled projection  1221 . A first end link has a wedge cut  1222  able to receive an angled projection  1221  of a second corresponding end link when mated, creating a tight fit between the first and second end link, as shown, for example, in  FIG. 22  and  FIG. 23 , creating a tight fit between a first end link  1200   a  and second end link  1200   b . In an embodiment, the position of angled projection  1221  and wedge cut  1222  halt rotation when a first end link  1200   a  and a second end link  1200   b  have rotated 180 degrees relative to each other. In alternative embodiments, the form factor of these elements may halt rotation at alternative positions, such as angles greater than 180 degrees. 
     As seen in  FIG. 23 , in certain embodiments, a first end link  1200   a  is shaped to mate with a second, inverted end link  1200   b . When mated in the configuration seen in  FIG. 23 , both subunits are in a reference position, which is referred to as zero degrees of rotation relative to each other. From this position, both subunits are able to rotate around a dowel, such as a proximal dowel  1301  seen in  FIG. 23 . In an embodiment, both subunits are able to rotate to a final position of 180 degrees relative to each other. 
     As seen in  FIG. 24 , in certain embodiments, the space between a first internal protrusion  1202   a  on a first end link  1200   a  and a first external protrusion  1201   a  on a first end link  1200   a  is of the corresponding shape and dimensions to mate with a second internal protrusion  1202   b  from a second, inverted end link  1200   b . The space between a first internal protrusion  1202   a  and a second internal protrusion  1202   b  is specifically dimensioned to accommodate an internal rod  1300 . End link  1200  further incorporates transit shelf  1223 . Transit shelf  1223  braces an end link  1200  against an internal rod  1300  when a first end link  1200   a  and a second end link  1200   b  are in transit position. In certain embodiments, internal rod  1300  spans the length of expandable interbody cage  1000 . 
     As seen in  FIG. 25 , in certain embodiments, end link  1200  further incorporates a cutout or a deploy shelf  1224 . Deploy shelf  1224  is a passage that is formed when a first end link  1200   a  and a second end link  1200   b  are mated in a deploy position. The form factor of a first end link  1200   a  and a second end link  1200   b  are such that a hole is formed when the two subunits are mated, allowing an internal rod  1300  to traverse. Curvature of a cutout or a deploy shelf  1224  is designed to accommodate internal rod  1300  while a first end link  1200   a  and a second end link  1200   b  are in a deployed state. 
     As seen in  FIG. 26 , in a certain embodiment, expandable interbody cage  1000  is assembled such that hinge portion  1103  is positioned between first protrusion  1207  and second protrusion  1208 , which positions outer pinhole  1209 , hole  1104 , and inner pinhole  1210  in alignment and allows a pin  1303  to be inserted through the entire width of the expandable interbody cage  1000 , forming a joint. This assembly allows end link  1200  and center link  1100  to rotate around pin  1303 . 
     As seen in  FIG. 27 , in a certain embodiment, a channel is formed by outer dowel passage  1204  and inner dowel passage  1206  when a second end link  1200  is inverted and mated with a first end link  1200 . The channel formed is of the appropriate dimensions to mate with a proximal dowel  1301  or a distal dowel  1302 . Proximal dowel  1301  and distal dowel  1302  each act as the pin of a hinge, allowing a first end link  1200  and a second end link  1200  to rotate around a proximal dowel  1301  or a distal dowel  1302  relative to each other. Proximal dowel  1301  and distal dowel  1302  further incorporate dowel perforation  1304 , which is of the corresponding dimensions to mate with internal rod  1300 . 
     In certain embodiments, as seen in  FIG. 28 , internal rod  1300  is fixedly attached to distal dowel  1302 . Internal rod  1300  spans the length of the expandable interbody cage  1000  and exits the proximal end through the channel formed between a first deploy shelf  1224   a  and a second deploy shelf  1224   b  when expandable interbody cage  1000  is in deployed configuration. At a position proximal to expandable interbody cage  1000 , internal rod  1300  removably engages transit rod  1305 . In the preferred embodiment, the removable engagement takes place via threaded surfaces. 
     When in transit form or retracted configuration, as seen in  FIG. 29 , varying embodiments of expandable interbody cage  1000  have a rounded profile when viewed from the axial dimension that is able to pass through a sheath  1514  or cannula of the corresponding dimensions. In certain embodiments, an expandable interbody cage  1000  includes an elongated form extending from a proximal end to a distal end. In certain embodiments, components of expandable interbody cage  1000  are sequentially stacked within the sheath  1514  prior to placement within the interbody space, as depicted in  FIGS. 48-53 . In certain embodiments, sequentially stacked components incorporate directionally tapered ends forming wedges that controllably slide against each other into different intended areas of the interbody space. In certain embodiments, a rounded profile is formed from long lateral surface  1101 , outer short lateral surface  1203  and inner short lateral surface  1205 . In certain embodiments, the rounded profile makes efficient use of structural material in the expandable interbody cage  1000  that enables fit through a narrow, rounded passage. In certain embodiments, the rounded profile also increases radial adjustability around the axis of the expandable interbody cage  1000 . In certain embodiments, the diameter of the rounded profile is 9 millimeters, enabling the expandable interbody cage  1000  to fit into a sheath  1514  having an inner diameter of approximately 9 mm. It will be appreciated that in varying embodiments, a diameter of the expandable interbody cage  1000  in transit mode or configuration is between 7 mm and 12 mm. In alternative embodiments, the axial profile of the expandable interbody cage  1000 , and correspondingly the sheath  1514 , is substantially oval, substantially rectangular, or substantially rectangular with rounded edges in shape, corresponding to the parameters of the generally oblong boundary of Kambin&#39;s Triangle. 
     In varying embodiments, expandable interbody cage  1000  is transformable from a transit mode into a deployed mode. As seen in  FIG. 30 , in certain embodiments, end links  1200  rotate around a proximal dowel  1301  or distal dowel  1302  during a shift between transit mode and deployed mode. End links  1200  slide along internal rod  1300  towards the center, decreasing overall length of expandable interbody cage  1000  and increasing the distance between center links  1100 . Compression of the expandable interbody cage  1000  in its axial direction translates to a force in a vertical dimension through the rotatable joints. This force in the vertical direction drives center links  1100  away from each other. Transit rod  1305  is removably engaged with internal rod  1300 , such as by threads. Internal rod  1300  is further engaged with distal plate  1306 . In certain embodiments, as described and shown for  FIGS. 40A-F  and  FIGS. 41A-F , an implant includes an expandable interbody cage  1750  that transforms from a transit or retracted configuration to a deployed or expanded configuration. 
     In certain embodiments, portions and features of an implant are able to rotate to transition between a transit configuration and a deployed configuration. In certain embodiments, an implant as described in the following references are used during the methods associated with a deliver apparatus step  1405 , and deploying a cage step  1406 : U.S. Pat. No. 8,034,109 to Zwirkoski and filed Feb. 24, 2006, U.S Patent Publication No. 2006/0265077 to Zwirkoski and published Nov. 23, 2006, and U.S. Patent Publication No. 2012/0016481 to Zwirkoski and published 2012 Jan. 19, all of which are incorporated herein by reference. It will be appreciated that in certain embodiments, portions or features of an implant or cage are rotated in order to deploy the implant or cage. 
     As seen in  FIG. 34 , in a certain embodiment, expandable interbody cage  1000  comprises transit length  1001  and transit height  1002  when in transit mode, and deploy height  1003  when in deployed mode. Dimensions of center links  1100  and links  1200  may vary as required for different distraction heights. In a certain embodiment, expandable interbody cage  1000  comprises a transit length  1001  of 35 millimeters, transit height of 9 millimeters, and a deploy height  1003  of 12 millimeters in deployed configuration. In alternative embodiments, the transit form may comprise 35 millimeters transit length  1001  and 13 millimeters deploy height  1003  in deployed form; 37 millimeters in transit length  1001  and 14 millimeters in deployed height  1003 ; or 37 millimeters in transit length  1001  and 15 millimeters in deployed height  1003 . These dimensions are not comprehensive of all possible embodiments, and are strictly meant to serve as example embodiments for clarity. 
     As seen in  FIG. 35 , in certain embodiments, expandable interbody cage  1000  in transit form is protected from neural and other soft tissue. Transit rod  1305  is used to advance expandable interbody cage  1000  over K-wire, through the sheath  1514 , and into an interbody space. In a certain embodiment, expandable interbody cage  1000  is safely advanced through a sheath  1514  placed between the structures comprising Kambin&#39;s Triangle in this way, without nerve impaction. As seen in  FIG. 36 , in a certain embodiment, expandable interbody cage  1000  positioned in an interbody space, once safely through Kambin&#39;s Triangle and deployed, distracts two vertebral bodies  0101 . Following distraction, transit rod  1305  is safely removable through the sheath  1514 . 
     As seen in  FIG. 31 , in certain embodiments, the system incorporates a deployment tool or instrument. In a certain embodiment, the inserter operates to deploy an expandable interbody cage  1000  by mechanically transforming said expandable interbody cage  1000  from an undeployed (or retracted configuration) to a deployed (or expanded) configuration. The inserter attaches to an expandable interbody cage  1000  in certain embodiments through a threaded end designed to threadably engage with the expandable interbody cage  1000  to hold it. In certain embodiments, the inserter is a deployment tool that incorporates or abuts a tubular protrusion  1307  to facilitate the transfer of force. In a certain embodiment, the deployment tool incorporates a substantially tubular protrusion of the appropriate dimensions to fit through a low-diameter sheathed passage. In a certain embodiment, the deployment tool consists of a substantially elongate shape of a diameter to fit through the sheath  1514 . In certain embodiments, the deployment tool applies force to transit rod  1305 . In the preferred embodiment, the deployment tool functions to apply force through a mechanism substantially similar to a pop rivet gun. In certain embodiments, force on a transit rod  1305  is translated to distal plate  1306 , and a compression force is generated between distal plate  1306  and tubular protrusion  1307 . In certain embodiments, within the expandable interbody cage  1000 , said compression force is translated through rotatable joints, and forces a change in configuration of the implant from transit configuration to deploy configuration. In certain embodiments, compressive force applies to expandable interbody cage  1000  as tubular protrusion  1307  pushes on proximal end links  1200 , while transit rod  1305  pulls on distal plate  1306 . 
     In certain embodiments, an inserter, such as a deployment tool  1800  shown in  FIGS. 45A-G  allows delivery of implant. In certain embodiments, a deployment tool  1800  includes a distal end  1801  and a proximal end  1802 . A delivery sheath  1803  located towards the distal end  1801  allows placement of an implant or cage in the surgical site. In certain embodiments, a proximal end  1802  includes a delivery assembly  1804 . In certain embodiments, a delivery assembly  1804  includes a retention block  1805  threadably attached to an adjustment bolt  1806 . In certain embodiments, a guide column  1807  is disposed between a first block  1808  and second block  1809 , where a retention block  1805  is slideably connected with the guide column  1807 . Referring to  FIG. 45D , in certain embodiments, a first block  1808  has a threaded opening  1814  that is threadably engaged with threads  1815  of an adjustment bolt  1806 . In certain embodiments, an adjustment bolt  1806  is further rotatably connected with the retention block  1805 . Rotation of the adjustment bolt  1806  allows slideable adjustment of the retention block  1805  along a guide column  1807 . The guide column  1807  is oriented in a direction that is generally parallel with an axis  1811 , which runs in a generally longitudinal direction. 
     Referring to  FIGS. 45D-E , in certain embodiments, a retention block  1805  includes a retention hole  1810  that retains a portion of the deployment tool  1800 . In certain embodiments, a retention hole  1810  accommodates for example, a stem knob  1812 . In certain embodiments, a stem knob  1812  is connected to a stem connector  1813 . The stem connector  1813  is passed through a delivery sheath  1803  and has an end located near a deployment tool distal end  1801 , for example, near a distal end of a delivery sheath  1803 . Referring to  FIG. 47 , in certain embodiments, the stem connector  1813  end  1818  includes a tip  1816  that threadably engages with a corresponding threaded opening located on an expandable interbody cage. In certain embodiments, a corresponding threaded opening includes opening  1817  shown in  FIG. 40C , where the opening  1817  is located on the stem  1763  as seen in  FIG. 42A . In certain embodiments, the threaded tip  1816  engages with distal plate  1306  as shown in  FIG. 30 . In certain embodiments, threaded tip  1816  engages with a dowel perforation  1304  as shown in  FIG. 27 , where a dowel perforation  1304  includes a threading. In certain embodiments, attachment of the stem connector  1813  tip  1816  to an expandable interbody cage is through a slot and hole connection. 
     In certain embodiments, a base tool block  1819  is connected to a delivery assembly  1804 . In certain embodiments, base tool block  1819  is further connected with a delivery sheath  1803 . In certain embodiments, a delivery assembly  1804  pivots about an axis  1811 , which is, for example, located about a longitudinal axis of a guide column  1807 . A base tool block  1819 , in certain embodiments, includes a retention element  1820  that captures a portion of the delivery assembly  1804 . In certain embodiments, as shown in  FIG. 45E , the retention element  1820  retains the guide column  1807  when the instrument is in a closed position. In certain embodiments, a spring-actuated pin  1821  located within or near a retention element  1820  further restricts movement of delivery assembly  1804  when the instrument is in a closed position. In a closed position, the delivery assembly  1804  restricts slideable movement of a stem knob  1812  and stem connector  1813 , until the stem knob  1812  and stem connector  1813  are further adjusted by moving the retention block  1805 . In certain embodiments, rotation of the adjustment bolt  1806  controls the location of the retention block  1805 , which retains the stem knob  1812 , thus controlling the location of the stem connector  1813  end  1818 . 
     In certain embodiments, referring to  FIGS. 45E and 46 , a base tool block  1819  is pivotably connected with a delivery assembly  1804 . In certain embodiments, a portion of a guide column  1807  is placed through a first opening  1822  of a base tool block  1819 . In certain embodiments, a stem connector  1813  is passed through a second opening  1823  of a base tool block  1819 . In certain embodiments, a delivery sheath  1803  is joined with the second opening  1823  of the body  1819 . 
     In certain embodiments, a locking pin  1825  is laid along the delivery sheath  1803 . Referring to  FIG. 47 , the tip  1826  of the locking pin  1825  is located at the distal end  1801  of the deployment tool  1800 . In certain embodiments, a delivery sheath  1803  has a slit  1827  oriented in its longitudinal direction that accommodates the locking pin  1825 . A locking pin  1825  is connected with a locking pin lever  1828 . In certain embodiments, a locking pin lever  1828  is further guided into the base tool block  1819  with a guiding pin. For example, as shown in  FIGS. 45B-C  and  46 , a connector  1829  is attached to the locking pin lever  1828 , where the connector  1829  passes through a base tool block  1819  third opening  1824 . In certain embodiments, a locking pin  1825  and/or a locking pin lever  1828  has a spring-actuated connection with, for example a base tool block  1819 , as seen in  FIG. 45F . Referring to  FIG. 45F-G , a spring  1833  is placed between a locking pin lever  1828  and a base tool block  1819 . A delivery sheath  1803  is attached to the locking pin lever  1828 , and the delivery sheath is further secured to the base tool block  1819  with a fastener  1834 . In certain embodiments, the locking pin  1825  engages with a pin cutout  1831  as shown, for example, in  FIGS. 40A and 40C . Engagement of the locking pin  1825  with a pin cutout  1831  allows rotation of the implant or cage around the longitudinal axis  1832  of the delivery sheath  1803 . Pulling the locking pin towards the proximal end of the delivery tool releases the locking pin engagement with the pin cutout  1831  of an implant or cage. In certain embodiments, a deployment tool  1800  has a handle  1830  to allow a user to hold the delivery tool. In certain embodiments, a handle  1830  is attached to a base tool block  1819 . In certain embodiments, as shown in  FIG. 47 , the distal end interior surface of a delivery sheath  1803  has a thread  1835 . In certain embodiments, delivery sheath  1803  thread  1835  allows attachment to an expandable interbody cage. In certain embodiments, the thread  1835  threadably engages with thread  1769  located on a proximal element  1756  of an expandable interbody cage, as seen in  FIG. 42A . In certain embodiments, rotation of the deployment tool about the longitudinal axis  1832  allows release of the implant from the deployment tool. In certain embodiments, the deployment tool delivery sheath  1803  is rotatable about the longitudinal axis  1832 , as shown in  FIG. 45E . 
     As seen in  FIG. 32 , in certain embodiments, expandable interbody cage  1000 , when in deployed configuration, provides structural support through end links  1200 . In certain embodiments, expandable interbody cage  1000  can be used to distract two vertebral bodies during transformation from a transit configuration to a deployed configuration after insertion into an interbody space, as depicted by  FIGS. 35 and 36A . In varying embodiments, ridges  1211 , as shown for example in  FIGS. 18-19  engage and create purchase with the surface of a vertebral end plate. In alternative embodiments, expandable interbody cage  1000  is oriented 90 degrees axially, such that the expansion of the expandable interbody cage  1000  occurs in a plane substantially parallel to the plane of the interbody space, as depicted by  FIG. 36B . 
     In certain embodiments, an implant such as an expandable interbody cage  1750  shown in  FIGS. 40A-F  and  FIGS. 41A-F  is used. Referring to  FIG. 40A  and  FIG. 41A , in certain embodiments, an expandable interbody cage  1750  has a proximal end  1751  and a distal end  1752 . Referring to  FIGS. 40D-E  and  FIGS. 41D-E , a plurality of links, including a center link  1753 , and a proximal end link and a distal end link are disposed between a proximal end  1751  and a distal end  1752 . In certain embodiments, pulling on a distal end towards the proximal end causes the center link to expand or extend in a direction away from a longitudinal axis  1770  (seen in  FIG. 42A ) of an implant or cage. In certain embodiments, a stem or an internal rod guides the proximal end  1751  end link  1754  and a distal end  1752  end link  1755 . In certain embodiments, the center link  1753 , the proximal link  1754 , and distal link  1755  are disposed between a proximal element  1756  and a distal element  1757 . When the expandable interbody cage  1750  is in an expanded configuration as shown in  FIG. 41A-F , the center link  1753  assumes a position that increases the effective volume that the expandable interbody cage occupies. In a retracted state as shown in  FIG. 40A-F , the outer diameter  1758  (shown in  FIG. 40D ) of the cage  1750  is sized to pass through a dilator of 9 mm, although it will be appreciated that the outer diameter  1758  can range from 3 mm to 15 mm in certain embodiments, and is of any size in certain embodiments. In certain embodiments, as shown in  FIGS. 40A-F , the expandable interbody cage in a retracted configuration is generally cylindrical in shape. In certain embodiments, the expanded or deployed configuration has a substantially square or rectangular shape. In certain embodiments, the expanded or deployed configuration has a width that is generally greater than its height. Referring to  FIG. 40F , the outer surface  1760 ,  1761 , and  1762  of the center link  1753 , proximal link  1754 , and distal link  1755  have curved surface, although other types of surfaces can be used in other embodiments. 
     Referring to  FIGS. 40D-E  and  FIGS. 41D , in certain embodiments, the distal element  1757  has a tip  1759 . In certain embodiments, the tip  1759  includes a feature that allows a gradual, atraumatic opening of tissue, including, but not limited to, for example, a frustoconical shape, a bullet-nose shape, and a tapered shape. Referring to  FIG. 42A , in certain embodiments, the distal element  1757  includes a tip  1759 , a first hinge element  1764 , and a stem  1763 . Referring to  FIG. 42B , the distal element  1757  first hinge element  1764  is hingeably connected to a distal link  1755  at a second hinge element  1766 . In certain embodiments, a hinge element  1764  and a hinge element  1766  include knuckles, which are retained by a pin. Referring to  FIG. 42A , in certain embodiments, a proximal element  1756  includes a hinge element  1765 . Referring to  FIG. 42B , the proximal element  1756  first hinge element  1765  is hingeably connected to a proximal link  1754  at a second hinge element  1767 . 
     Referring to  FIG. 42A , in certain embodiments, a proximal element  1756  includes thread  1769  and an opening  1768 . In certain embodiments, a stem  1763  of the distal element  1757  passes through the opening  1768  of proximal element  1756 . In certain embodiments, referring to  FIGS. 41F and 43A , the stem  1763  passes through opening  1768  when the expandable interbody cage is in an expanded configuration. In certain embodiments, a cross-sectional profile of a stem  1763  keys in with the opening  1768  having a similar cross-sectional profile, preventing rotation of the distal element  1757  about a longitudinal axis  1770 . 
     In certain embodiments, a distal link  1755  and center link  1753  are hingeably connected, for example, as shown in  FIG. 43B . Still referring to  FIG. 43B , in certain embodiments, a center link  1753  and a proximal link  1754  are hingeably connected. When in an expanded configuration, the distance between distal element  1757  and proximal element  1756  is decreased, which displaces the center link  1753  away from the stem  1763 . In certain embodiments, as shown in  FIGS. 41A-F , an expandable interbody cage  1750  includes a plurality of center links, distal links, and proximal links. 
     Referring to  FIGS. 44A and 44B , in certain embodiments, the links have a cutout  1771 ,  1771   a, b . It will be appreciated that a cutout has a shape to accommodate an internal rod or stem  1763 , guide wire, or other objects. In certain embodiments, the cutout is radial. In certain embodiments, as seen in  FIGS. 44A-D , a proximal link and/or distal link includes a notch  1772 . In certain embodiments, when an expandable interbody cage  1750  is in an expanded configuration, a notch  1772   a  surface of a first link  1773   a  meets with a notch  1772   b  surface of a second link  1773   b  as seen in  FIG. 44D . In certain embodiments, a notch  1772  is located on a first end  1775  of a proximal link or distal link, where the first end  1775  is connected with a distal element  1757  or proximal element  1756 . In certain embodiments, a second end  1776  of a proximal link or distal link is connected with a center link. In certain embodiments, a second end  1776  includes a second notch  1774  as seen in  FIG. 44C-D . In certain embodiments, a second notch  1774  surface meets with an upper or lower end plate when an expandable interbody cage  1750  is placed inside a disc space. 
     In certain embodiments, a trialing instrument includes a form as in an expandable interbody cage  1750  shown in  FIGS. 40A-F  and  FIGS. 41A-F . In certain embodiments, a trialing instrument with a similar mechanism as described for  FIGS. 40A-F  and  FIGS. 41A-F  allows a trial implant to be placed in the vertebral disc space as to determine the correct size implant. A trialing instrument is inserted into the disc space, and expanded or deployed to determine whether the particular size is appropriate. The trialing instrument can further be retracted or collapsed and removed. 
     In certain embodiments, an implant includes an assemblable interbody cage  1850  comprising two or more wedges  1851 , as shown in  FIG. 48 . In certain embodiments, two or more wedges  1851  are placed into position by being guided by a central component  1852 . Referring to  FIG. 48 , assemblable interbody cage  1850  includes a form following a longitudinal axis  1849 . In certain embodiments, an assemblable interbody cage  1850  includes a distal end  1847  and a proximal end  1848 . Referring to  FIG. 49A-C , a central component  1852  has a proximal end  1853  and a distal end  1854 . A distal end  1854  has a tip  1855 , where in certain embodiments, a tip includes a feature for a gradual, atraumatic opening of tissue. In certain embodiments, the feature includes, but is not limited to, for example, a frustoconical shape, a bullet-nose shape, and a taper. In certain embodiments, a central component  1852  includes a plurality of rails  1856 . In certain embodiments, rails  1856  are positioned in a radially outward direction from the central component central stem  1857 . A slot  1859  is formed in a space between the rails  1856 . In certain embodiments, a slot  1859  has an opening  1860  connected with a proximal end of the central component. A rail  1856  further includes a retaining ledge  1861  in certain embodiments. In certain embodiments, a central component  1852  has a diameter  1862  that is adapted for use in an OLLIF approach. In certain embodiments, the diameter  1862  is approximately 9 mm, although it will be appreciated that the outer diameter can ranges from 3 mm to 15 mm in certain embodiments, and is of any size in certain embodiments. A stem  1866  attached to the central component central stem  1857 . In certain embodiments, a central component includes an attachment hole  1858  located on a central component proximal end  1853 , where a stem  1866  can attach to the central component. In certain embodiments, attachment of a central component to a stem is through a threaded connection. 
     Certain embodiments of the invention include two or more wedges  1851 . Referring to  FIG. 50A-D , a wedge  1851  has a proximal end  1864  and a distal end  1863  and oriented along a generally longitudinal axis  1874 . In certain embodiments, a distal end  1863  has a ramped surface  1871 , where a ramped surface helps to position a wedge into the disc space. A wedge  1851  has a rail cutout  1865  that accommodates an outer shape of a rail  1856 . A wedge  1851  further includes a keyed element  1873  on the interior portion  1868 , where the keyed element  1873  runs substantially along a longitudinal axis  1874 . The keyed element  1873  further includes a stem cutout  1867  in certain embodiments. Referring to  FIG. 50E-F , in certain embodiments, a wedge  1877  has a distal end  1863 , a proximal end  1864 , an interior portion  1868 , and an exterior portion  1869 . It will be appreciated that in certain embodiments, the exterior portion of a wedge is available in a number of different shapes, included having a rounded surface or a planar surface. In certain embodiments, a wedge  1877  has a keyed element  1878  that is rounded. It is contemplated that in certain embodiments, a keyed element  1878  of a wedge  1877  fits through a slot  1880  of a central component  1879  shown in  FIG. 49D . 
     Referring to  FIG. 51  showing a distal end perspective view of a plurality of wedges  1851 , when properly assembled, a cavity  1872  is created among the wedge  1851  pieces. Referring to  FIG. 52 , a plurality of wedges  1851  are placed around a central component  1852 , such that a central component  1852  is disposed in a cavity  1872  shown in  FIG. 51 . In certain embodiments, the keyed element  1873  of a wedge  1851  is placed within a slot  1859  of the central component  1852 . Referring to  FIGS. 49B and 52 , the retaining ledge  1861  of the rail  1856  constricts the keyed element  1873  of a wedge  1851  to a movement that is generally along a longitudinal axis. 
     In certain embodiments, wedges  1851  are sequentially delivered to a vertebral disc space. Referring to  FIGS. 53A-D , the wedges are placed through a working sheath. An exemplary view through a working sheath boundary  1875 , where the implant is viewed from the proximal side, is shown in  FIG. 53A-D . Referring to  FIG. 53A , a first wedge  1851   a  is placed through the sheath boundary  1875 , and positioned so that the keyed element  1873  fits between a first rail  1856   a  and a second rail  1856   b . Referring to  FIGS. 50B and 53A , a wedge has a surface profile  1870  located on an exterior portion  1869 . Referring to  FIG. 53A , the surface profile  1870  has a form matching that of a working sheath boundary  1875 . Furthermore, still referring to  FIG. 53A , the stem  1866  has an edge that engages with a stem cutout  1867  located on the wedge  1851   a . Initially, the central component, which is attached to a stem, is passed through a working sheath  1876 . Once the central component is in position, wedges are sequentially placed through the working sheath. As the wedge  1851   a  is passed through a working sheath  1876 , the stem cutout  1867  and the surface profile  1870  help to guide the wedge  1851   a  along the stem and the working sheath. The wedge is pushed out of the working sheath, until the wedge reaches the appropriate quadrant of a central component  1852 . The wedge is further pushed until it is engaged with the central component. Referring to  FIGS. 53A-D , once a first wedge  1851   a  is positioned into a central component  1852 , the sheath is repositioned in order to insert the other wedges  1851   b, c, d.    
     In certain embodiments, the stem  1866  has a non-circular profile. In certain embodiments, a stem  1866  has a square cross section. In certain embodiments, the stem  1866  generally has a non-circular profile to allow guidance of a wedge through the working sheath. In certain embodiments, a stem includes a cross section with other shapes. It will be appreciated that in certain embodiments, a central component has two or more slots, allowing it to accommodate two or more wedges. In certain embodiments, a central component holds two wedges, and in certain embodiments, a central component holds three wedges. In certain embodiments, a central component includes a central channel allowing delivery of graft material through the channel. In certain embodiments, the central component and wedge are made of a material suitable for orthopedic surgery, including, but not limited to titanium, polyetheretherketone (PEEK), carbon fiber, ceramic, stainless steel or other materials commonly utilized within orthopedic implants, or combinations thereof. 
     In certain embodiments, the assemblable interbody cage  1850  comprises two or more wedges  1851 , as shown in  FIG. 48 . In certain embodiments, two or more wedges  1851  are placed into position by being guided by a central component  1852 . Referring to  FIG. 49A-C , a central component  1852  has a proximal end  1853  and a distal end  1854 . A distal end  1854  has a tip  1855 , where in certain embodiments, a tip includes a feature for a gradual, atraumatic opening of tissue. In certain embodiments, the feature includes, but is not limited to, for example, a frustoconical shape, a bullet-nose shape, and a taper. In certain embodiments, a slot  1859  meets with a portion of the tip, and acts as a stop to prevent movement of a wedge. In certain embodiments, a central component  1852  includes a plurality of rails  1856 . In certain embodiments, rails  1856  are positioned in a radially outward direction from the central component central stem  1857 . A slot  1859  is formed in a space between the rails  1856 . In certain embodiments, a slot  1859  has an opening  1860  connected with a proximal end of the central component. A rail  1856  further includes a retaining ledge  1861  in certain embodiments. In certain embodiments, a central component  1852  has a diameter  1862  that is adapted for use in an OLLIF approach. In certain embodiments, the diameter  1862  is approximately 9 mm, although it will be appreciated that the diameter can ranges from 3 mm to 15 mm in certain embodiments, and is of any size in certain embodiments. A stem  1866  attached to the central component central stem  1857 . In certain embodiments, a central component includes an attachment hole  1858  located on a central component proximal end  1853 , where a stem  1866  can attach to the central component. In certain embodiments, attachment of a central component to a stem is through a threaded connection. 
     Certain embodiments of the invention include two or more wedges  1851 . Referring to  FIG. 50A-D , a wedge  1851  has a proximal end  1864  and a distal end  1863  and oriented along a generally longitudinal axis  1874 . In certain embodiments, a distal end  1863  has a ramped surface  1871 , where a ramped surface helps to wedge a wedge into the disc space. A wedge  1851  has a rail cutout  1865  that accommodates an outer shape of a rail  1856 . A wedge  1851  further includes a keyed element  1873  on the interior portion  1868  of the wedge  1851 , where the keyed element  1873  runs substantially along a longitudinal axis  1874 . The keyed element  1873  further includes a stem cutout  1867  in certain embodiments. Referring to  FIG. 50E-F , in certain embodiments, a wedge  1877  has a distal end  1863 , a proximal end  1864 , an interior portion  1868 , and an exterior portion  1869 . It will be appreciated that in certain embodiments, the exterior portion of a wedge is available in a number of different shapes, included having a rounded surface or a planar surface. In certain embodiments, a wedge  1877  has a keyed element  1878  that is rounded. It is contemplated that in certain embodiments, a keyed element  1878  of a wedge  1877  fits through a track  1880  of a central component  1879  shown in  FIG. 49D . 
     Referring to  FIG. 51  showing a distal end perspective view of a plurality of wedges  1851 , when properly assembled, a cavity  1872  is created among the wedge  1851  pieces. Referring to  FIG. 52 , a plurality of wedges  1851  are placed around a central component  1852 , such that a central component  1852  is disposed between a cavity  1872  as shown in  FIG. 51 . In certain embodiments, the keyed element  1873  of a wedge  1851  is placed within a slot  1859  of the central component  1852 . Referring to  FIGS. 49B and 52 , the retaining ledge  1861  of the rail  1856  constricts the keyed element  1873  of a wedge  1851  to a movement that is generally along a longitudinal axis. 
     In certain embodiments, the two or more wedges  1851  are sequentially delivered to a vertebral disc space. Referring to  FIGS. 53A-D , the wedges are placed through a working sheath. An exemplary view through a working sheath boundary  1875 , where the implant is viewed from the proximal side is shown in  FIG. 53A-D . Referring to  FIG. 53A , a first wedge  1851   a  is placed through the sheath boundary  1875 , and positioned so that the keyed element  1873  fits between a first track  1856   a  and a second track  1856   b . Referring to  FIGS. 50B and 53A , a wedge has a curved surface  1870  located on an exterior portion  1869 . Referring to  FIG. 53A , the curved surface  1870  has a curvature that matches the curved surface of the working sheath boundary  1875 . Furthermore, still referring to  FIG. 53A , the stem  1866  has an edge that engages with a stem cutout  1867  located on the wedge  1851   a . Initially, the central component, which is attached to a stem, is passed through a working sheath  1876 . Once the central component is in position, one or more wedges are sequentially placed through the working sheath. As the wedge  1851   a  is passed through a working sheath  1876 , the stem cutout  1867  and the curved surface  1870  of the wedge  1851   a  glide along the stem and the working sheath. The wedge is pushed out of the working sheath, until the wedge reaches the appropriate quadrant of a central component  1852 . The wedge is further pushed until it is engaged with the central component. Referring to  FIGS. 53A-D , once a first wedge  1851   a  is positioned into a central component  1852 , the sheath is repositioned in order to insert the other wedges  1851   b, c, d.    
     In certain embodiments, the stem  1866  has a non-circular profile. In certain embodiments, a stem  1866  has a square cross section. In certain embodiments, the stem  1866  generally has a non-circular profile to allow guidance of a wedge through the working sheath. In certain embodiments, a stem includes a cross section with other shapes. It will be appreciated that in certain embodiments, a central component has two or more tracks, allowing it to accommodate two or more wedges. In certain embodiments, a central component holds two wedges, and in certain embodiments, a central component holds three wedges. In certain embodiments, a central component includes a central channel allowing delivery of graft material through the channel. In certain embodiments, the central component and wedge are made of a material suitable for orthopedic surgery, including, but not limited to titanium, polyetheretherketone (PEEK), carbon fiber, ceramic, stainless steel or other materials commonly utilized within orthopedic implants, or combinations thereof. 
     The following paragraphs describe a preferred method of use of certain embodiments of the invention. One skilled in the art will recognize the variability in these steps based on factors such as surgeon preference and patient anatomy. 
     In certain embodiments, the method of use for the embodiments described herein are performed as shown in the flowchart of  FIG. 33 . In certain embodiments, the method includes identification of the route of entry step  1400 . In certain embodiments, during the identification step  1400  the most appropriate route of entry is identified. In certain embodiments, a surgeon identifies the end point of the surgical approach by identifying the interbody space between the two vertebral bodies to be fused. One skilled in the art will appreciate the variability inherent in this step, depending on the intended target. This step will generally involve identifying the target point within an interbody space or on a vertebral body and determining the appropriate incision site. This step is often executed with the aid of imaging technology, such as Computerized Tomography (CT) scanning and/or biplanar fluoroscopy. In certain embodiments, an endoscope may be utilized in association with instrumentation for purposes associated with the inspection of the foramen and other structures near the passage prior to and following the insertion of instrumentation during the identification of the route of entry step  1400 . 
     In certain embodiments, in order to accomplish the identification of the route of entry step  1400 , the surgical team must first accomplish the positioning and confirming step. To do so, the patient to be subjected to the surgery utilizing the system described herein is first positioned on an operating table in a generally prone position. Typically, bi-planar C-Arm system is used for intra-operative fluoroscopic monitoring, and is used to confirm that the positioning of the patient&#39;s spine best resembles the neutral position, such that the unique anatomy and pathologies of the patient allow for a neutral position. As one skilled in the art would recognize, the term “neutral position” refers to a position that exhibits the three natural curves present in a healthy spine from a lateral view, wherein the cervical (neck) region of the spine (C1-C7) is bent inward, the thoracic (upper back) region (T1-T12) bends outward, and the lumbar (lower back) region (L1-L5) bends inward. In a substantially neutral position, the patient&#39;s spine will ideally show equal spacing between pedicles on an anterior-posterior fluoroscopic view, and superimposed pedicles on a lateral fluoroscopic view. Thus, in association with the positioning and confirming step, a surgeon will confirm that the patient is an appropriate candidate for fusion utilizing an OLLIF approach or determine an adequate explanation for why an OLLIF approach is inappropriate based on the patient&#39;s unique anatomy. 
     In certain embodiments, in association with the identification of the route of entry step  1400 , the person performing the procedure performs a locating step. To perform the locating step, in an anterior-posterior view, the person performing the procedure locates the center of the disc via fluoroscopy in the vertical and horizontal planes. The surgeon or an assistant designates the midline and transverse plane by placing a radiopaque trajectory planning instrument over the skin while utilizing fluoroscopy. The person performing the surgery then engages in a step to mark a patient&#39;s skin to target the center of the disc. In certain embodiments the marks may include, for example, writing on a patient&#39;s skin. On a lateral plane, the radiopaque trajectory planning instrument determines the targeted disc&#39;s inclination angle. Following this, the person performing the surgery performs marking, whereby a skin marker is used to draw a line following the disc inclination angle (referred to as the “disc inclination line”) along the side of the patient towards the patient&#39;s posterior midline. In certain embodiments, the disc inclination line may indicate a trajectory that passes through the ilium, the sacrum, both or neither. On a lateral view, the person performing the surgery locates the center of the disc by repositioning the radiopaque trajectory planning instrument and drawing a second line along its trajectory on the skin&#39;s surface. Ideally, this second line will travel perpendicular to and intersect the disc inclination line. The person performing the procedure then engages in measuring to create a first depth measurement made along the disc inclination line from the dorsal skin to the center of the disc. The distance determined from this first measurement should then be applied from the midline marker laterally along the transverse plane distal from the center of the disc where a mark is made parallel to the midline. The intersection of this mark and the disc inclination line indicates the point of incision, or route of entry. 
     In certain embodiments of the invention, a passage  0106  is used to access the L5-S1 vertebral disc space. In certain embodiments, a passage  0106  traverses through both the sacrum  0108  and the ilium  0107 , as depicted by  FIG. 55A . In such embodiments, the passage  0106  through the ilium  0107  follows an oblique lateral route into the L5-S1 interbody space. In certain embodiments, the passage  0106  is located more posterior than the direct lateral route into the L5-S1 interbody space. The present inventors recognize that in certain embodiments, the passage  0106  along an oblique lateral trajectory is preferable to a direct lateral trajectory for accessing an L5-S1 vertebral disc space, as previously described direct lateral trajectories that use a monolithic, non-expandable cages are typically inferior, as the trajectory and type of implant used can lead to damage and intractable pain. In certain embodiments, a sheath that follows the passage  0106  has an outer diameter of no greater than 12 millimeters. Unlike the previously known direct lateral passage that passes solely through the ilium, certain embodiments use an oblique lateral passage  0106 , as depicted in  FIG. 55A-C , particularly using a sheath  0105  having an outer diameter of less than 12 millimeters, which leads to less pain for the patient following surgery. In certain embodiments, the present inventors have recognized that a passage  0106  that passes through both the ilium  0107  and through the sacral ala  0110 , using a sheath  0105  having an outer diameter of less than 12 millimeters, leads to a reduction in pain for the patient following surgery. The present inventors have also recognized that a less desirable trajectory that is located above or through a portion of a sacral ala may lead to unintended deflection of instrumentation, including deflection caused by contact of instrumentation with the external surface of the sacral ala, superiorly and possibly into the L5 nerve root. Therefore, in certain embodiments of the invention, the passage passes through bone, and particularly through the sacral ala and ilium. The present inventors have recognized that in certain embodiments, a passage created to access the L5-S1 level using this approach traverses both the ilium and sacral ala, as the passage through bone enables the surgeon to avoid a trajectory that undesirably comes near or into contact with one or more nerves forming the boundaries of Kambin&#39;s Triangle. 
     In a certain embodiment, the sheath  0105  follows a passage  0106  through the ilium  0107 . The sheath is angled such that it passes from the skin through both a posterior and superior quadrant of the ilium  0107  and the sacral ala  0110 , and into the disc space  0112  adjacent and inferior to the L5 vertebral body  0109 . Referring to  FIG. 55A-C , it will be appreciated that the plane of the S1 superior endplate  0114 , which is inferior to the L5-S1 disc space, angles inferiorly in an anterior direction relative to the plane of the endplate located superior to the L5-S1 disc space. For example, as shown in  FIG. 55B-C , an approximate location of an S1 superior endplate  0114  is marked. Still referring to  FIG. 55C , the approximate location of an edge  0113  of a L5 inferior endplate is marked. Referring to  FIG. 55A-C , an anterior edge  0115  of the S1 superior endplate  0114  is angled inferiorly from a posterior edge of the endplate  0114 . Previously described trajectories are located above or through a portion of a sacral ala, which may lead to unintended deflection of instrumentation in a superior direction, and possibly into the L5 nerve root. On the other hand, in certain embodiments, a passage  0106  passes through the sacral ala  0110  and forms an access opening  0116  within the L5-S1 disc space  0112 . Once inside the bone, the passage  0106  is passed through the bone structures of the sacrum  0108  and ilium  0107  until the passage  0106  reaches the L5-S1 disc space  0112 . Certain embodiments of the invention, as shown in  FIG. 55A-C  include a passage  0106  that is substantially lateral and generally stays within bone until it reaches a portion of the L5-S1 disc space  0112 . In certain embodiments, the passage  0106  avoids potential damage to the L5 exiting nerve root  0111 . 
     In certain embodiments, the identifying the route of entry step  1400  defines a path through both the ilium  0107  and the sacral ala  0110 . In an embodiment, the identifying the route of entry step  1400  may involve tapping, drilling or otherwise passing a wire through both the ilium  0107  and the sacral ala  0110 . In such embodiment, the guide wire may incorporate a drill trip configured to drill through both the ilium  0109  and the sacral ala  0110 . In such embodiment, the present inventors intend for the surgeon to utilize a guide wire to define a path into the lower half of Kambin&#39;s Triangle, or the half of Kambin&#39;s Triangle located farthest away from the L5 nerve root, after passing through both the ilium  0109  and the sacral ala  0110 . In such embodiment, the widen the passage  1403  step may include the utilization of drilling and/or boring instruments to drill and/or bore through the ilium and the sacrum. In certain embodiments, the passage  0106  traverses through at least part of the area within Kambin&#39;s Triangle  0104 . 
     In certain embodiments, the method of use for the embodiments described herein includes an insert needle  1401  step. One skilled in the art will appreciate the variability inherent in this step, depending on the intended target. In association with this step, prior to making an incision, local anesthetics may be used at the point of incision. Generally, in association with this step, a 9-12 mm incision is made at the point of incision. In the method associated with the preferred embodiment, a surgeon will insert a neuromonitoring probe, for example, a unidirectional, monopolar neuromonitoring probe, through the incision to target an interbody space through Kambin&#39;s Triangle. During the insert needle  1401  step, in the preferred embodiment, a surgeon should pass between the structures comprising Kambin&#39;s Triangle  1402 . In an embodiment, the neuromonitoring probe has a slot either on the lateral surface, or centered within that spans the length of the probe to hold a slidably and removably engaged trephine needle, also known as Kirschner Wire or K-Wire. In certain embodiments, neuromonitoring is performed with the instrument described for  FIGS. 39A-I . Using the neuromonitoring probe, an exiting nerve root  0102 , which forms the hypotenuse of Kambin&#39;s Triangle is mapped and identified. Surgeon should ensure that the neuromonitoring probe trajectory passes through Kambin&#39;s Triangle. Kambin&#39;s Triangle is an area that may be conceptualized as substantially a right triangle that is defined by the exiting nerve—which forms the hypotenuse—the superior endplate of the caudal vertebral body  0101 —which forms the base—and the traversing nerve  0102 —which forms the height. Those skilled in the art recognize that Kambin&#39;s Triangle may not form the precise shape of a triangle. Such mapping and identification takes place via electrical stimulation of the associated nerve structures. One skilled in the art will recognize this standard surgical practice as Triggered EMG. The surgeon determines nerve depolarization, for example, at a minimum level of 3 mA, to establish safe distance from the nerves associated with Kambin&#39;s Triangle. Anterior-posterior and lateral fluoroscopic imaging is viewed to confirm that the neuromonitoring probe is placed through Kambin&#39;s Triangle and touching the substantially lateral aspect of the targeted interbody space. Once safe placement and safe trajectory is confirmed, more specifically by confirmation of the trajectory through the “Safe Zone” of Kambin&#39;s Triangle, variably defined as the “lower half of Kambin&#39;s Triangle” or the “half of the area between the structures forming the boundary of Kambin&#39;s Triangle farthest away from the exiting nerve root,” the trephine needle is then be placed into the annulus of the targeted disc via the previously described slot. The neuromonitoring probe is then removed leaving the trephine needle to maintain and identify the safe trajectory though Kambin&#39;s Triangle to the interbody space. 
     In certain embodiments, the neuromonitoring probe is incorporated into the first dilator, generally through use of a neuromonitoring instrument, as depicted in  FIGS. 39A-I . In certain embodiments, the user places a standard disposable monopolar probe within a sheath or a first dilator, until the distal end of the monopolar probe makes contact with the stainless steel distal end of the first dilator. The user then bends the shaft of the standard disposable monopolar probe at an angle of approximately 30 degrees within the slot of the first dilator. The user then attaches a quarter-inch square quick connect palm handle to the quick connect feature of the first dilator. The user then slides the sheath onto the body of the first dilator and engages the pin features to accomplish a fully assembled state. The user then delivers the fully assembled first dilator into the body at the previously determined trajectory. As the user delivers the fully assembled neuromonitoring instrument to the targeted interbody space, the user views fluoroscopic images to determine when the distal tip of the first dilator contacts the annulus of the targeted interbody space. In certain embodiments, the user then stimulates the standard disposable monopolar probe to thereby stimulate the stainless steel distal end of the first dilator. The user then monitors the neuromonitoring threshold, and if the threshold is satisfactory, the user then impacts the palm handle at the proximal end of the first dilator with a mallet to dock the distal end, including, for example, the flattened tip and the conical tip into the disc space. The user impacts the handle until the opening of the sheath is fully docked within the disc space, as observable by viewing fluoroscopic imaging. The user then rotates the first dilator to disengage the pins from the sheath impact collar. The user then removes the first dilator and the standard disposable monopolar probe leaving only the sheath in place. 
     Still referring to  FIG. 33 , the method of use for the embodiments described herein includes a step  1403  to widen the passage. This step encompasses the insertion of one or more cannulas over a trephine needle placed into the body in the previous step in sequential order, creating a wider channel. First, in the method associated with certain embodiments, an initial dilator instrument, also referred to as a dilator or a first dilator is inserted over a trephine needle to widen an opening. The first dilator  1500  is passed over a trephine needle with initial reference marking  1503  facing parallel to the direction of an exiting nerve root  0102 , as determined from previous nerve mapping and anatomical knowledge. In varying embodiments, where the trephine needle and/or the first dilator is incorporated into the first dilator, all or part of the step  1403  to widen the passage and the insert needle step  1401  may be combined. It will be appreciated that in certain embodiments, an initial dilator instrument, also referred to as a first dilator, has features to widen the path without requiring a trephine needle. 
     In certain embodiments, the first dilator, once positioned safely through Kambin&#39;s Triangle, is rotated 90 degrees along a trephine needle. This rotation effectively displaces a traversing nerve root  0102  away from the trajectory of the approach into the interbody space. Referring to  FIGS. 54A-F , in certain embodiments, a dilator  1900  has a substantially elongate form. A dilator  1900  includes a proximal end  1901  and a distal end  1902 . Referring to  FIGS. 54A-B , in certain embodiments, a dilator  1900  has a first dimension  1903  that is greater than a second dimension  1904 . A profile of a dilator shaft  1907  has a shape that is generally elliptical, as shown in  FIGS. 54E-F . In certain embodiments, a dilator  1900  includes a cannula  1905  connecting the proximal end  1901  and a distal end  1902 . In certain embodiments, the distal end has a narrowed tip  1906 . Generally, the overall shape of the dilator  1900  allows positioning the dilator into Kambin&#39;s Triangle, and rotating it to displace a nerve root. In certain embodiments, the narrowed tip includes a taper that allows penetration into a vertebral disc. In certain embodiments, a side wall  1908  of a narrowed tip  1906  has a curvature that facilitates turning the dilator while the tip is in the disc space. In certain embodiments, a dilator  1900  can be used as a first dilator or an initial dilator during the approach as described herein. It will be appreciated that certain embodiments of a dilator  1900  include a reference marking  1909 . A reference marking includes, for example, a radiopaque marker, a radiolucent marker, a protrusion, a divot, or other physical feature that allows a surgeon to observe the orientation of an instrument. 
     In certain embodiments, the second dilator  1508  is then advanced over the first dilator  1500  of dilator  1900  through Kambin&#39;s Triangle to the substantially lateral aspect of the disc. In an embodiment, the second dilator  1508  is advanced over the first dilator  1500  with initial reference marking  1503  facing toward an exiting nerve root  0102  of Kambin&#39;s Triangle. 
     In certain embodiments, a third and optionally a fourth dilator may be used in addition to further expand the path of approach to an interbody space, preceding the placement of the final dilator instrument or sheath  1514 . In certain embodiments a sheath that has a profile that is substantially similar to the profile of a dilator shaft  1907 , for example, an elliptical profile. 
     In certain embodiments, a sheath  1514  is positioned over the first dilator  1500  or a second dilator  1508 . An impactor device  1528  is optionally used to seat a sheath  1514  into an interbody space. In certain embodiments, an impactor device  1528  includes a through opening  1529  that accommodates, for example, a guide wire. An impactor device  1528 , in certain embodiments, is shown in  FIG. 12 . 
     In a certain embodiment, once sheath  1514  is placed and anchored between vertebral endplates, a safe passage is established through a patient&#39;s superficial soft tissue, between the structures comprising Kambin&#39;s Triangle, and into an interbody space. In varying embodiments, the K-Wire, first dilator  1500 , and second dilator  1508  if previously placed are removed, leaving only sheath  1514  in place. 
     In certain embodiments, the disc is prepared for a placement of an implant. During a disc preparation step  1404 , steps associated with a discectomy and annulotomy are performed. In certain embodiments, discectomy instrumentation is used in steps related to discectomy and annulotomy. In an embodiment, the person performing the surgery removes interbody disc material using discectomy instrumentation to cut through the nucleus of a disc. Subsequently, the person performing the surgery then utilizes the discectomy instrumentation to remove the disc material through the sheath  1514 . In certain embodiments, the discectomy instrumentation also prepares the superior and inferior endplates of an interbody space, causing bleeding of such endplates. In certain embodiments, an endoscope may be utilized in association with discectomy instrumentation for purposes associated with the visual inspection of the discectomy and endplate preparation prior to and following the insertion of discectomy instrumentation. 
     In certain embodiments, an implant trialing step is optionally performed after removing disc material. In certain embodiments, trialing determines the appropriate size of expandable interbody cage  1000  to be placed into the interbody space. A trialing instrument is placed through the sheath  1514  and into an interbody space. In certain embodiments, trialing instrument is performed with an expandable cage similar to those shown in  FIGS. 14-32 , and similar to those shown in  FIGS. 40-44 . In certain embodiments, a delivery tool or instrument is used to deliver a trial instrument to the disc space. In certain embodiments, a deliver tool or instrument described for  FIGS. 45A-E  is used. Certain embodiments of a trialing instrument incorporates a handle, which, when squeezed, distracts an interbody space. Once the desired amount of distraction is achieved, the person performing the procedures engages in selecting an expandable interbody cage  1000  with appropriate height dimensions in its deployed configuration to match the distraction achieved with a trialing instrument. 
     In certain embodiments, following the trialing step, the person performing the surgery performs the step to inserting an implant or cage. During the insert cage step or deliver apparatus step  1405 , one or more than one implant is placed into the interbody space by passing through a sheath. In certain embodiments of the deliver apparatus step  1405 , a non-expandable cage or implant is inserted into an interbody space by passing through a sheath. In certain embodiments, during the insert cage step or deliver apparatus step  1405 , an expandable interbody cage  1000  is placed into an interbody space by passing through a sheath. The person performing the surgery then utilizes a deployment tool to transform the implant from transit configuration or a retracted configuration to a deployed configuration or an expanded configuration. Once the expandable interbody cage  1000  is placed and expanded, the person performing the procedure then may confirm or verify  1407  appropriate placement utilizing with fluoroscopic imaging. Following confirmation of expandable interbody cage placement location, any remaining instrumentation including the sheath  1514  may be removed  1408 . The person performing the procedure may then engage in the standard surgical close of the passageway. 
     In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. 
     The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 
     Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” “has”, “having,” “includes”, “including,” “contains”, “containing” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a”, “has . . . a”, “includes . . . a”, “contains . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. For the purposes of illustration related to example embodiments disclosed herein, “distal” is defined as the direction away from the surgeon, and “proximal” is defined as the direction toward the surgeon. The terms “a” and “an” are defined as one or more unless explicitly stated otherwise herein. The terms “substantially”, “essentially”, “approximately”, “about” or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art. The terms “coupled” and “linked” as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is “configured” in a certain way is configured in at least that way, but may also be configured in ways that are not listed. Also, the sequence of steps in a flow diagram or elements in the claims, even when preceded by a letter does not imply or require that sequence.