the present invention relates to systems and methods for accessing intervertebral space and inserting spine implants between vertebral bodies . those of skill in the art will recognize that the following description is merely illustrative of the principles of the invention , which may be applied in various ways to provide many different alternative embodiments . this description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts in the appended claims . the present invention provides access to the spine through the use of a postero - lateral approach . a minimally invasive dilation and / or access device employing such an approach would have significant advantages in spinal orthopaedic procedures over the lateral and anterior approaches . these advantages may include avoiding the need to turn the patient during surgery , less muscle retraction , less blood loss , less operating room time , minimized damage to the vascular system , organs , nerves and muscles , faster recovery , and an improved overall outcome for the patient . referring to fig1 , one embodiment of an arcuate cannula assembly 10 is shown . the assembly 10 comprises a targeting post 12 , a guide arm 14 , and a curved penetrating guide member 16 . an instrument support arm 20 holds the assembly and connects to an operating table ( not shown ). the assembly 10 may further comprise a series of graduated curved cannulas ( not shown in fig1 ), which are introduced sequentially over the guide member 16 to create access to a targeted portion of a spine . use of the arcuate cannula assembly 10 creates an access portal a to the intervertebral disc space or any element of the anterior spinal column through an arcuate path , from a postero - lateral approach . the access portal is an unimpeded passage through which surgical instruments , implants and other materials may be passed to complete a variety of intervertebral procedures . this arcuate postero - lateral approach may be advantageous in performing a number of procedures , including but not limited to : implantation of motion preservation devices , total disk replacement , implantation of interbody devices , discectomy , lateral plating with or without dynamic elements , vertebra fixation or graft compression using plates or staples , foraminotomy , decompression , annulotomy , nucleotomy , annulus or nucleus repair , vertebral body biopsy , vertebroplasty , height restoration of a collapsed vertebral body ( vertebral body augmentation ), implantation of a fusion cage with stabilization features , implantation of a fusion cage with teeth to hold endplates together , or implantation of a curved or straight staple across the disc space to provide compression on the cage and stabilization of the cage . referring to fig2 , a perspective view of the targeting post 12 is shown . the targeting post 12 comprises an elongate shaft 30 with a distal end 32 and a proximal end 34 . a rounded tip 36 is at the terminus of the distal end 32 . the proximal end 34 adjoins a rectangular connector block 38 which has a first side 42 and a second side 43 . adjoining the connector block 38 on the first side 42 is a support arm attachment post 44 . the attachment post 44 has a receiving slot 46 which extends transversely into the attachment post through an interface surface 45 . in the preferred embodiment the receiving slot 46 includes an internally threaded surface . a radial spline 48 encircles the receiving slot 46 on the interface surface 45 . adjoining the connector block 38 on the second side 43 is a rotation post 50 . extending distally from the rotation post 50 is an optional stop feature 52 . referring to fig3 , a perspective view of a support arm 20 is shown . the support arm 20 comprises a shaft 60 which attaches to the operating table via various linkages , pivots , or connections to allow multiple degrees of freedom to accommodate the positioning of the instrument to be held . a wide variety of differently - configured instrument support arms are well known in the art and the assembly 10 may be compatible with the instrument support arm of choice for the surgeon . a distal end 61 of the shaft 60 has a first side 62 and a second side 63 . extending transversely through the distal end 61 from the first side 62 to the second side 63 is a screw channel 66 . on the first side 62 , an interface surface 65 has a radial spline 64 which encircles the opening of the screw channel 66 . the radial spline 64 is configured to mate with the radial spline 48 on the targeting post 12 when the post is connected to the support arm 20 . extending through the channel 66 is a thumb screw 68 , and a shaft 70 protrudes from the channel 66 on the second side 63 . in the preferred embodiment , shaft 70 includes an externally threaded surface configured to interface with the threaded receiving slot 46 on the targeting post 12 . referring to fig4 , the targeting post 12 is introduced into the patient from a postero - lateral approach through a small incision on the patient &# 39 ; s back posterior to the targeted spine segment . the distal end 32 of the targeting post 12 is advanced antero - medially through the patient just lateral to the targeted intervertebral disc until the tip 36 reaches a desired reference location at the anterior lateral half or one third of the disc . the blunt shape of the tip 36 gently pushes tissues aside as the post 12 is advanced in . the post 12 may also be wired as an electrode during insertion , allowing for nerve monitoring or electromyography ( emg ) to avoid nerves as the post 12 advances through the tissues . of special concern is avoidance of the nerve roots exiting the spinal column as the psoas muscle adjacent to the spine is penetrated by the post 12 . the targeting post 12 is inserted so that it is coplanar with the superior endplate of the inferior vertebral body for the intervertebral level to be treated . preferably , the post 12 is aligned parallel with the sagittal plane of the patient , but other orientations are possible if necessary to avoid nerves or other obstacles . when the distal end 32 of the targeting post 12 has reached the reference location , the proximal end 30 is attached to the support arm 20 via the thumb screw 68 . the protruding screw shaft 70 is threaded into the receiving slot 46 . as the thumb screw 68 is threaded in , the radial splines 44 , 64 mesh , locking the targeting post 12 to the support arm 20 . once attachment is made between the targeting post 12 and the support arm 20 , the various degrees of freedom of the support arm 20 are locked down to provide sufficiently rigid instrument stabilization . in position adjacent to the spine , the targeting post 12 acts as a stabilizing and reference guide for subsequent cannulas , instruments and implants . referring to fig5 , the penetrating guide member 16 is shown . the guide member 16 is curved and may be arcuate ( i . e ., may extend along a fixed radius of curvature ). the guide member 16 has a proximal end 110 , and a distal end 112 with an insertion tip 113 . the insertion tip 113 may be rounded or optionally pointed , to penetrate muscles and fascia . two attachment recesses 114 at the proximal end facilitate attaching the guide member 16 to the guide arm 14 , and are also configured to connect to an instrument support arm . a narrow channel may optionally extend the length of the guide member 16 , sized to receive a wire for nerve monitoring or emg during dilation . referring to fig6 a , a perspective view of the guide arm 14 is shown . the guide arm 14 has a first side 80 and a second side 82 . at a proximal end is a pinned end 84 ; a latch end 86 is at the opposite distal end . the pinned end 84 has an attachment feature 88 which is shaped to rotatably attach to the rotation post 50 on the targeting post 12 . inserted into a horizontal slot 89 in the latch end 86 is a spring loaded guide member latch assembly 90 which is shaped to grip the penetrating guide member 16 . the guide member latch assembly 90 has a sliding latch bar 92 with a keyhole 94 and a tab 96 . on the first side 80 of the guide arm 14 , near the latch end 86 is a round guide member opening 100 . directly opposite it on the second side 82 may optionally be a smaller pinhole opening 102 . fig6 b is an enlarged view of the sliding latch bar 92 . keyhole 94 has a rounded lobe 95 disposed toward the tab 96 , and an ovoid lobe 97 opposite the tab 96 . the rounded lobe 95 is sized to fit around the proximal end 110 of the guide member 16 ( not shown ). the ovoid lobe 97 is sized to hold the attachment recesses 114 of the guide member 16 . the tab 96 may be grasped to move the sliding latch bar 92 within the horizontal slot 89 . a spring ( not shown ) is disposed in the horizontal slot 89 to provide resistance against the sliding latch bar 92 . fig7 is an enlarged view of the latch end 86 of the guide arm 14 , showing the guide member 16 latched in the latch assembly 90 . to latch the guide member 16 in the latch assembly 90 , first the sliding latch bar 92 is introduced into the horizontal slot 89 until the rounded lobe 95 of the keyhole 94 lines up with the guide member opening 100 . the proximal end 110 of the guide member 16 is inserted such that the attachment recesses 114 are adjacent to the lined up keyhole 94 and opening 100 . the sliding latch bar 92 is released , and the spring ( not shown ) pushes the sliding latch bar 92 distally until the ovoid lobe 97 of the keyhole 94 slides around the attachment recesses 114 of the guide member 16 . the force of the spring traps the guide member 16 in the latch assembly 90 , as the guide member is pinned between the ovoid lobe 97 and the latch end 86 of the guide bar 14 adjacent the guide member opening 100 . referring to fig8 , the support arm 20 , targeting post 12 , guide arm 14 and penetrating guide member 16 are shown , with the guide arm 14 and penetrating guide member 16 in a first position . the attachment feature 88 on the guide arm 14 is engaged with the rotation post 50 on the targeting post 12 . thus attached , the guide arm 14 can rotate about the axis of the rotation post 50 ; however the stop feature 52 on the rotation post 50 may prevent the guide arm 14 from rotating entirely about the rotation post 50 . the guide arm 14 is sized to match the radius of the curve of the penetrating guide member 16 , such that the arc centerpoint of the penetrating guide member 16 is coincident with the center of rotation , or axis of the rotation post 50 . the guide member latch 90 holds the penetrating guide member 16 as seen in fig7 . after the penetrating guide member 16 is attached to the guide arm 14 , the guide arm 14 is rotated so that the insertion tip 113 of the guide member 16 makes contact with the skin . at this point , the guide member 16 is lifted and an incision of approximately 1 - 5 cm is made into the skin and fascia . as shown in fig9 , the guide member 16 is then advanced into the incision via rotation of the guide arm 14 . the guide member penetrates the soft tissues and fascia of the patient , and is advanced antero - medially along an arcuate path until the insertion tip 113 is at the lateral margin of the targeted disc , at a target location . the target location is at a known position relative to the reference location provided by the distal end 32 of the targeting post 12 , as the guide bar 14 holds the guide member 16 in a fixed relationship as the guide bar 14 rotates about the rotation post 50 . at this point the guide arm and guide member are in a second position . the guide member 16 may have a rounded insertion tip , or a sharp , pointed insertion tip if necessary to penetrate the tissues . emg monitoring may be used to ensure safe passage of the guide member through the fascia . the optional pinhole opening 102 creates access for a wire to pass through the guide arm into the guide member 16 if it is desirable to connect an electrode to the guide member 16 for nerve monitoring . the stop feature 52 ( seen in fig2 ) stops rotation of the guide arm 14 and prevents the guide member 16 from extending past the margin of the disc and contacting the spinal cord . once the guide member 16 is correctly positioned adjacent the targeted location , the guide arm 14 is detached from the guide member 16 and the targeting post 12 . the guide member 16 is left in the patient to serve as a guide for one cannula or series of cannulas which are graduated in size , and which are inserted sequentially from smaller to larger to increase the cross - sectional area of the access portal to the area to be treated . referring to fig1 , a single cannula 18 is shown . the cannula 18 is curved and generally tubular in form , with a tubular support wall 128 which has an open distal end 122 and an open proximal end 124 . the distal end 122 is rounded so that tissues are pushed aside gently as the cannula is inserted through the patient . a bore 130 runs the length of the cannula 18 from the open distal end 122 to the open proximal end 123 , and provides access to the targeted spinal area for instrument insertion , and insertion and removal of interbody devices , arthroscopic devices , implants , bone graft materials , bone cement , and other materials and devices . a cross - sectional shape of the support wall 128 of the bore 130 is generally curved , and may specifically be round , oval , elliptical or another curved shape . the open proximal end 123 has a plurality of grip features 126 which allow the surgeon to grip the cannula . optionally , the cannula 18 may have attachment features to allow attachment of the cannula to the instrument support arm . the cannula 18 may optionally be substantially radiolucent , and can comprise biocompatible polymers , elastomers , ceramics , or aluminum or other metals . the curve of the cannula 18 may be arcuate , and may sweep through an angle of about 90 ° such that the open proximal and distal ends 124 , 122 are substantially perpendicular to each other . referring to fig1 , a series of graduated cannulas 15 , 17 , 18 are inserted one at a time over the proximal end 110 of the penetrating guide member 16 , and advanced antero - medially over the guide member 16 until the corresponding distal end reaches the distal end 112 of the guide member 16 . each cannula 17 , 18 is shorter in length and larger in cross - sectional area than the next smallest cannula , to allow the surgeon to grip each cannula as it is installed and removed . as each cannula 15 , 17 , 18 is inserted , the access portal through the soft tissues and fascia is increased in size , creating increased access to the targeted portion of the spine . the number of cannulas inserted is determined by the desired cross - sectional area of the opening to the spine ; in many instances two to five cannulas will be inserted . once all cannulas 15 , 17 , 18 are inserted around the penetrating guide member 16 , the guide member 16 and the inner cannulas 15 , 17 are removed , leaving the largest cannula 18 in the patient . this cannula may be attached via an attachment feature ( not shown ) to the support arm 20 , to provide additional stabilization for removal of the smaller cannulas , and for subsequent instrument insertion and procedures . in one embodiment of the invention , the largest cannula 18 may have a tooth portion ( not shown ) which extends longitudinally from the insertion end 122 . during insertion , the tooth portion is placed between the superior and inferior endplates of the intervertebral space , to assist in maintaining access to the space . fig1 is a postero - lateral view of a portion of a spine with a cannula inserted according to the procedure previously described . when in place in the patient , the bore 130 of the cannula 18 is an access portal through which surgical instruments , implants and other materials may be passed to complete a variety of intervertebral procedures . surgical instruments used in conjunction with the cannula 18 may have rigid , curved shafts or flexible shafts to navigate through the cannula 18 to the intervertebral space . the cannula 18 may be sized to accommodate passage of an interbody fusion implant ( not shown in fig1 ). another embodiment of the invention comprises a targeting post which is capable of cephalad - caudal adjustment . fig1 is a perspective view of an arcuate cannula assembly 210 which includes an adjustable targeting post 212 , a guide arm 214 and a penetrating guide member 16 . the adjustable targeting post 212 has a shaft 230 which has a distal end 232 and a proximal end 234 . proximally adjacent to the proximal end 234 of the shaft 230 is a connection portion 240 , which extends in a cephalad - caudal direction and comprises a guide arm connector 250 , a cephalad - caudal adjustment feature 238 , and a support arm attachment post 244 . the cephalad - caudal adjustment feature 238 can be adjusted to lengthen or shorten the cephalad - caudal length of the connection portion 240 . thus , after the targeting post is inserted into the patient , the length of the connection portion 240 can be adjusted as necessary to attain the necessary offset to adjust the resultant cephalad - caudal distance between the guide member 16 and the targeting post 212 . the adjustment allows the target location to vary along the cephalad - caudal direction such that the known position of the target location is offset relative to the reference location . cephalad - caudal offset of the guide arm 214 and the attached guide member 16 may be useful in avoidance of nerve structures and other objects during the dilation process . another application of the invention comprises a bilateral implementation of two arcuate cannula assemblies . in this embodiment , two assemblies 10 are used together , one on each lateral side of the spine . referring to fig1 , portions of two assemblies 10 , which comprise two targeting posts 12 , two penetrating guide members 16 , and two cannulas 18 , are shown adjacent to each lateral side of the spine . this embodiment permits enhanced access to the targeted area , since access may be attained from both lateral sides simultaneously . instruments , implants , or other materials may be pushed or pulled into the intervertebral space , or through the entire access pathway . another embodiment of the invention further comprises an interbody device . fig1 a is an anterior perspective view of a portion of a spine with a cannula 18 and an interbody device 300 which may be inserted through the arcuate cannula assembly previously disclosed . fig1 b is a perspective view of the interbody device 300 of fig1 a . the interbody device 300 has a generally rectangular box - like shape , and is slightly curved along its longitudinal axis . the interbody device 300 may optionally have a radius of curvature substantially the same as that of the cannula 18 . in any case , the bore 130 of the cannula 18 is sized to accommodate passage of the interbody device 300 . because use of the arcuate cannula assembly 10 allows improved access to the intervertebral space , the interbody device 300 may have a larger footprint than many other interbody devices , and can extend across most of the medial - lateral width of the intervertebral space , to provide for increased stability , increased bone in - growth , and improved fusion . a curved insertion tool and curved tamp ( not shown ) are used to insert and seat the interbody device 300 in the intervertebral space . in the alternative , a flexible insertion tool and / or a flexible tamp may be used . the arcuate postero - lateral approach described above may have many advantages for spinal procedures , particularly procedures involving anterior vertebral column elements . this approach may be used to insert motion preservation devices , such as total disc replacements . by accessing the disc space via an arcuate postero - lateral approach , the surgeon is able to spare the anterior longitudinal ligament as well as avoid complications with the great vessels . this approach also provides for revision options with virtually the same instrumentation and implant designs by accessing the disc space from the opposite lateral side as the first surgery . this approach also allows for total disc replacement ( tdr ) endplate retention features which are more desirable than anterior approach tdr features , such as endplate keels or teeth which are oriented in the frontal plane to resist the high shear loads seen in the lumber spine lordotic region . this approach may also be used for various intervertebral disc treatment or resection procedures such as annulotomy , nucleotomy , discectomy , annulus replacement , nucleus replacement , and decompression due to a bulging or extruded disc . during an annulotomy , the surgeon may provide an access portal in the manner described previously , and open and / or remove a portion or all of the disc annulus . during a nucleotomy , the surgeon may provide an access portal in the manner described previously , and open and / or resect a portion of the intervertebral disc nucleus . during a discectomy , the surgeon may remove a portion or the entire intervertebral disc through the access portal in order to accomplish decompression of the nerve roots , dura , or spinal cord . this procedure may be done as a conservative therapy to relieve patient symptoms or pain , or it may be done in preparation for total disc replacement or fusion . for annulus repair or replacement , the arcuate postero - lateral approach may facilitate a larger needle and avoidance of complicated vascular structure and may allow a pathway for a prosthetic annulus to be placed or formed in the intervertebral space . using a bilateral arcuate approach such as that depicted in fig1 could further facilitate the creation of bounding elements , such as a shield , guard , mold , or equivalent such that the annulus may be repaired , formed , inserted , created , or augmented . similar benefits are realized for a nucleus replacement procedure where all or a portion of the intervertebral nucleus is repaired or resected and replaced , created or augmented via various techniques . a prosthetic nucleus may be delivered via a passageway that is larger than that afforded by a transpedicular approach , and less complicated and less risky than an anterior approach , by using the arcuate postero - lateral approach described above . various intervertebral disc treatment methods have been postulated , such as using electrosurgical therapies . it is readily apparent to one of skill in the art how conducting these therapies via an arcuate postero - lateral approach may benefit the surgeon as well as improve clinical outcomes . the arcuate postero - lateral approach may also be utilized for additional vertebral body motion segment stabilization procedures such as interbody device insertion , lateral plating , anterior plating , lateral or anterior plating with dynamic stabilization or compression elements , deformity correction , and / or graft compression devices or procedures . the arcuate postero - lateral access portal such as that depicted in fig1 a may facilitate interbody fusion procedures by allowing a single surgical exposure or patient positioning to insert all required stabilization elements such as an interbody fusion device similar to that depicted in fig1 b , or posterior stabilization hardware such as pedicle screws , rods , hooks , and facet screws , among others . by approaching the intervertebral disc space with a tangential or almost straight medial - lateral trajectory right next to the vertebral body , the interbody device may more fully occupy the intervertebral space . this may result in a multitude of advantages such a leveraging the higher strength cortical regions on the vertebral body endplates , allowing more cross - sectional surface area or a larger footprint for improved stability , allowing more bone graft surface are to encourage better osteointegration , bony fusion , and 360 ° fusion . the interbody device may also comprise a lordotic angle which does not require over - distraction such as is the case with transforaminal lumbar interbody fusion ( tlif ) and posterior lumbar interbody fusion ( plif ) procedures . the arcuate postero - lateral approach may also be used for lateral plating procedures , in which the implanted plates may comprise fixed , dynamic , or compressive elements . this approach again allows a single patient positioning to conduct lateral plating as well as posterior stabilization hardware such as screws , hooks and rods . these plates may be used for local deformity correction or prevention procedures to treat local scoliosis , kyphosis , hyper - lordosis , or spondylolisthesis . additionally , the arcuate postero - lateral approach may allow for novel graft compression devices or procedures that enable the surgeon to apply improved local compressive forces between vertebral bodies or an interbody device . benefits of improved local compressive forces include improved bone graft incorporation , fusion , interbody device stability , as well as a potentially reduced risk of interbody device expulsion that is often the result of over - compressing the disc space and applying unintended moments via traditional pedicle screws and rods . such graft compression devices include lateral plates with compression features , vertebral body staples which cooperate with the superior and inferior vertebral bodies to apply compression , and integrated interbody device with arms that cooperate with the vertebral bodies to apply compression via screws , tapered surfaces , or the like . various central canal or foraminal decompression procedures may be performed with the arcuate postero - lateral approach described previously . decompression procedures are conducted to resect soft or hard tissues that may be impinging on neural elements such as the exiting nerve roots , dura , or spinal cord , resulting in various pathologies such as radiolopathy , myelopathy , pain , tingling , numbness , and loss of motor or sensory control . for example , anterior central canal decompression required due to a diseased intervertebral disc is often a difficult procedure . by using the disclosed arcuate postero - lateral approach , this decompression procedure allows for improved patient positioning , access , and patient outcomes . foraminal decompression procedures via an arcuate postero - lateral approach may also allow the surgeon an improved trajectory and passageway to decompress the foramen . procedures involving the vertebral body , such as vertebral body biopsy , vertebral body height restoration , and vertebroplasty may successfully utilize the arcuate postero - lateral approach . often patients who are experiencing symptoms associated with vertebral body disease , collapse , or fracture will undergo a biopsy of the vertebral body to assess the condition of the structure . osteoporotic patients , especially female geriatric patients , may experience vertebral body collapse or fracture . this is an extremely painful and debilitating condition which may be addressed via vertebroplasty through the disclosed arcuate postero - lateral approach . often , vertebroplasty , kyphoplasty or arcuplasty procedures are conducted via a transpedicular approach , to inject a hardenable compound such as pmma cement into the vertebral body to create an internal cast - like structure to stabilize the bony fragments or fractures . the arcuate postero - lateral approach has numerous advantages for such a procedure . it may allow for a larger access needle than a transpedicular approach and accordingly reduces pressure requirements for the viscous hardenable compounds . in addition , it will likely result in less post - operative pain due to not violating the pedicle , and it allows for a more preferable trajectory of the access needle . vertebroplasties conducted via a transpedicular approach often require a bilateral approach for sufficient vertebral body stabilization . by using the trajectory of the arcuate postero - lateral approach , the surgeon or radiologist may use a single needle and single approach for a complete fill , because the access needle can be advanced to the distal portions and gradually retracted during injection to accomplish a complete fill . vertebral body height restoration procedures have recently been disclosed in the art to address collapsed vertebral bodies . the arcuate postero - lateral approach may facilitate such vertebral height restoration procedures by removing the size limitation imposed by the transpedicular approach . additionally , the ability to access the lateral margins of the vertebral body may be beneficial in insertion of an implant to restore vertebral height and fix it in place via a hardenable compound , or conduct an internal vertebral body distraction and secure the vertebral body via a hardenable compound . the present invention may be embodied in other specific forms without departing from its spirit or essential characteristics . for example , above are described various alternative examples of systems for accessing intervertebral space . it is appreciated that various features of the above - described examples can be mixed and matched to form a variety of other alternatives . it is also appreciated that this system should not be limited creating access to the intervertebral space . this arcuate access system may be used to obtain access to any portion of the spine . as such , the described embodiments are to be considered in all respects only as illustrative and not restrictive . the scope of the invention is , therefore , indicated by the appended claims rather than by the foregoing description . all changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope .
a system for accessing a spine from a curved postero - lateral approach may include a curved cannula positioned along a curved path from an opening in the skin to a location proximate the spine . a guide member may be first inserted to establish the path between the tissues and fascia , and one or more intermediate cannulas may be temporarily inserted over the guide member to dilate the tissues prior to insertion of the main cannula . an interbody device may be implanted in an intervertebral space through the cannula . the system may include a guide bar removably coupled to a targeting post . the targeting post may be inserted adjacent the spine to provide a target , and the guide bar may be removably attached to the guide member , to guide it along the path to the target location . an external support arm may be secured to any other component of the system .