Abstract:
The present invention relates generally to medical devices, systems, and methods for use in surgery. In particular, the disclosed system and methods relate to an intervertebral spinal implant sized and dimensioned for the lumbar spine implantable via a posterior approach. The system includes an implant, instruments for delivering the implant.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a non-provisional patent application claiming the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/927,421, filed on Jan. 14, 2014, entitled “Oblique TLIF Implant and Related Methods,” and U.S. Provisional Patent Application Ser. No. 62/009,647 filed on Jun. 9, 2014, entitled “Oblique TLIF Implant and Related Methods,” the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth in their entirety herein. 
    
    
     FIELD 
     The present invention relates generally to spinal surgery and, more particularly, to a device for spinal fusion comprising a spinal fusion implant of non-bone construction to be introduced into any variety of spinal target sites. 
     BACKGROUND 
     Currently there nearly 500,000 spine fusion procedures performed each year in the United States. One of the causes of back pain and disability derives from the rupture or degeneration of one or more intervertebral discs in the spine. Surgical procedures are commonly performed to correct problems with displaced, damaged, or degenerated intervertebral discs due to trauma, disease, or aging. Generally, spinal fusion procedures involve removing some or the all of the diseased or damaged disc, and inserting one or more intervertebral implants into the resulting disc space. 
     Minimally invasive methods of performing spinal fusion have gained popularity in recent years due to the many benefits of the procedure which include diminished dissection of body tissue and lower blood loss during surgery resulting in reduced surgery time, lower post-operative pain and a quicker recovery for patients. Transforaminal lumbar interbody fusion (TLIF) procedures provide unilateral access to a desired target site. The TLIF technique involves approaching the spine in a similar manner as a posterior approach but more from the left or right of the spine through a midline incision in a patient&#39;s back. This procedure requires only one incision in the back of a patient and involves placing a fusion device into the intervertebral disc space. Introducing the intervertebral implant serves to restore the height (“disc height”) between adjacent vertebrae, which reduces if not eliminates neural impingement commonly associated with a damaged or diseased disc. 
     SUMMARY OF THE INVENTION 
     The spinal fusion implant of the present invention may be comprised of any suitable non-bone composition, including but not limited to polymer compositions (e.g. poly-ether-ether-ketone (PEEK) and/or poly-ether-ketone-ketone (PEKK)), ceramic, metal, or any combination of these materials. The spinal fusion implant of the present invention may be provided in any number of suitable shapes and sizes depending upon the particular surgical procedure or need. The spinal fusion implant may be dimensioned for use in any part of the spine (e.g. cervical, lumbar and/or thoracic) without departing from the scope of the present invention. The implant may be dimensioned, by way of example only, having a width ranging between 8 and 14 mm, a height ranging between 8 and 18 mm, and a length ranging between 25 and 45 mm. 
     According to one broad aspect of the present invention, the spinal fusion implant includes a top surface, a bottom surface, lateral sides, a proximal end, and a distal end. The spinal fusion implant of the present invention may be used to provide temporary or permanent fixation along an orthopedic target site. To do so, the spinal fusion implant may be introduced into a disc space while locked to a surgical insertion instrument and thereafter manipulated in the proper orientation and released. Once deposited in the disc space, the spinal fusion implant of the present invention effects fusion over time as the natural healing process integrates and binds the implant. 
     The spinal fusion implant of the present invention may be provided with any number of additional features for promoting fusion, such as one or more apertures extending between the top and bottom surfaces which allow a boney bridge to form through the spinal fusion implant. The spinal implant may also be preferably equipped with one or more lateral openings which facilitate visualization at the time of implantation and at subsequent clinical evaluations. 
     The spinal fusion implant may also be provided with any number of suitable anti-migration features to prevent the implant from migrating or moving from the disc space after implantation. Suitable anti-migration features may include, but are not necessarily limited to, angled teeth or ridges formed along the top and bottom surfaces of the implant and/or rod elements disposed within the distal and/or proximal ends. 
     The spinal fusion implant may be provided with one or more radiographic markers at the proximal and/or distal ends. These markers allow for a more detailed visualization of the implant during and after insertion (through radiography) and allow for a more accurate and effective placement of the implant. 
     The proximal end of the spinal fusion implant may be provided with a surface that is tapered (angled) to avoid dural impingement after implantation. Additionally, the tapered nature of the proximal surface can aid in overall fit of the spinal fusion implant within the intervertebral disc space. Significantly, the tapered proximal surface on the proximal end enables the spinal fusion implant to maximize contact with the posterior portion of the cortical ring of each adjacent vertebral body. 
     The distal end of the spinal fusion implant may be provided with a conical (bullet-shaped) shape including a pair of first tapered (angled) surfaces and a pair of second tapered (angled) surfaces. The first tapered surfaces extend between the lateral surfaces and the distal end of the implant, and function to distract the vertebrae adjacent to the target intervertebral space during certain methods of insertion of the spinal fusion implant. The second tapered surfaces extend between the top and bottom surfaces and the distal end of the spinal fusion implant, and function to maximize contact with the anterior portion of the cortical ring of each adjacent vertebral body. Furthermore, the second tapered surfaces provide for a better fit with the contour of the vertebral body endplates, allowing for a more anterior positioning of the spinal fusion implant and thus advantageous utilization of the cortical rings of the vertebral bodies. The distal end of the spinal fusion implant may be at least partially asymmetrically curved about the longitudinal axis to approximate the anterior portion of the cortical ring of each adjacent vertebral body when the implant is placed in its desired final oblique position. 
     The spinal fusion implant may be provided with a variable height along at least a portion of the implant. In one embodiment, the variable height tapers in a direction oblique to both the length and width of the implant. The oblique taper imparts a greater height to the anterior aspect of the intervertebral disc space when the spinal fusion implant is positioned obliquely within the disc space. Imparting a greater height to the anterior aspect of the disc space restores the natural lordotic curvature of the lumbar spine. 
     The spinal fusion implant may be provided with at least one set of variable opposing corner rounds varying in their radii along the length of the implant to allow for more gradual lead-in at the distal end which facilitates easier and safer insertion of the spinal fusion implant. 
     The spinal fusion implant may be further provided with asymmetric convex top and bottom surfaces between first and second lateral sides to approximate the anatomical concavities of the inferior endplate of the superior vertebra and the superior endplate of the inferior vertebra. 
     The spinal fusion implant may be further provided with asymmetrically convex top and bottom surfaces along the length of the implant between proximal and distal ends of the implant. The asymmetric curvature enables the spinal fusion implant to even better match the anatomical concavities of the inferior endplate of the superior vertebra and the superior endplate of the inferior vertebra. 
     The spinal fusion implant may be introduced into a spinal target site through use of any of a variety of suitable surgical instruments having the capability to engage the implant. The spinal fusion implant is capable of being used in minimally invasive surgical procedures, needing only a relatively small operative corridor for insertion. 
     The spinal fusion implant may be inserted into the intervertebral space and rotated into final position. Once the implant has been positioned in its desired location within the intervertebral space, the user will then rotate the implant 90° such that the top and bottom surfaces face in a caudad/cephalad direction and the anti-migration features engage the vertebral bodies. Significantly, the direction of rotation is critical to ensure proper placement of the implant such that the edges of the proximal surface rest on the cortical ring of the vertebral bodies and the proximal surface does not protrude into the spinal canal. For example, if the spinal fusion implant approaches a patient&#39;s spine posteriorly from the right with the (longer) first lateral side facing caudally, then implant must be rotated in a counter-clockwise direction to achieve proper positioning. 
     A single spinal fusion implant may be provided and inserted into an intervertebral disc space and positioned obliquely across the disc space such that the proximal and distal ends are on opposite sides of the midline of the intervertebral space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a perspective view of an example of a spinal fusion implant according to one embodiment of the present invention; 
         FIG. 2  is a second perspective view of the spinal fusion implant of  FIG. 1 ; 
         FIG. 3  is a top view of the spinal fusion implant of  FIG. 1 ; 
         FIG. 4  is a bottom view of the spinal fusion implant of  FIG. 1 ; 
         FIG. 5  is a first side view of the spinal fusion implant of  FIG. 1 ; 
         FIG. 6  is a second side view of the spinal fusion implant of  FIG. 1 ; 
         FIG. 7  is a plan view of a proximal end of the spinal fusion implant of  FIG. 1 ; 
         FIG. 8  is a detailed perspective view the proximal end of the spinal fusion implant of  FIG. 1 ; 
         FIG. 9  is a detailed cross-section view of the proximal end of the spinal fusion implant of  FIG. 1 ; 
         FIG. 10  is a plan view of a distal end of the spinal fusion implant of  FIG. 1 ; 
         FIG. 11  is a cross-section view of the proximal end of the spinal fusion implant of  FIG. 1  taken along the line  1 - 1 ; 
         FIG. 12  is a cross-section view of the distal end of the spinal fusion implant of  FIG. 1  taken along the line  2 - 2 ; 
         FIG. 13  is a perspective view of an insertion instrument according to one embodiment of the present invention; 
         FIG. 14  is an exploded perspective view of the insertion instrument of  FIG. 13 ; 
         FIG. 15  is a detailed perspective view of the distal end of the insertion instrument of  FIG. 13 ; 
         FIG. 16  is a perspective view of the insertion instrument of  FIG. 13  coupled to the spinal fusion implant of  FIG. 1 ; 
         FIG. 17  is a perspective view of an implant trial instrument according to one embodiment of the present invention; 
         FIG. 18  is a side view of the distal head of the trial instrument of  FIG. 17  in a first orientation; 
         FIG. 19  is a side view of the distal head of the trial instrument of  FIG. 17  in a second orientation; 
         FIG. 20  is a side view of the distal head of the trial instrument of  FIG. 17  in a third orientation; 
         FIG. 21  A is a top plan view of an example of a spinal fusion implant inserted into an intervertebral disc space but not placed in a desired oblique configuration; 
         FIG. 21  B is an example lateral x-ray indicating the position of radiographic markers of the spinal fusion implant placed in the configuration shown in  FIG. 21  A; 
         FIG. 22  A is a top plan view of an example of a spinal fusion implant inserted into an intervertebral disc space placed in a desired oblique configuration; and 
         FIG. 22  B is an example of a lateral x-ray indicating the position of radiographic markers of the spinal fusion implant in the desired oblique configuration as shown  FIG. 22  A. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The spinal fusion implant, system, and methods disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination. 
       FIGS. 1-2  illustrate a spinal fusion implant  10  according to a first broad aspect of the present invention. The spinal fusion implant  10  may be constructed of any suitable non-bone composition, including but not limited to polymer compositions (e. g. poly-ether-ether-ketone (PEEK) and/or poly-ether-ketone-ketone (PEKK)), ceramic, metal and/or any combination of polymer compositions, ceramic and metal. The spinal fusion implant may be provided with a surface coating (for example, titanium plasma spray) to encourage bone growth onto endplate contacting surfaces. The spinal fusion implant  10  of the present invention may be provided in any number of shapes and sizes depending upon the particular surgical procedure or need. By way of example only, the spinal fusion implant  10  may have a width ranging between 8 and 14 mm, a height ranging between 6 and 18 mm, and a length ranging between 20 and 45 mm. 
     The spinal fusion implant  10  of the present invention includes a top surface  12 , a bottom surface  14 , first and second lateral sides  16 ,  18 , a proximal (posterior) end  20  and a distal (anterior) end  22 . The spinal fusion implant  10  of the present invention may be used to provide temporary or permanent fixation within an orthopedic target site. To do so, the spinal fusion implant  10  may be introduced into a disc space while locked to a surgical insertion instrument and thereafter employed in the proper orientation and released, as explained in further detail below. Once deposited in the disc space, the spinal fusion implant  10  of the present invention effects spinal fusion over time as the natural healing process integrates and binds the implant. 
       FIGS. 3-4  illustrate the top and bottom surfaces  12 ,  14 , respectively, of the spinal fusion implant  10 . The top and bottom surfaces  12 ,  14  are configured to engage the vertebral bodies adjoining the target disc space. Accordingly, the top and bottom surfaces  12 ,  14  each preferably include a plurality of anti-migration features designed to increase the friction between the spinal fusion implant  10  and the adjacent contacting surfaces of the vertebral bodies. Such anti-migration features may include ridges (or teeth)  24  provided along the top surface  12  and/or bottom surface  14 . The friction prohibits migration of the implant  10  after insertion into the intervertebral space and during the propagation of natural bony fusion. It should be appreciated by one skilled in the art that such ridges (or teeth)  24  can be oriented in a particular direction which will stabilize the implant in several degrees of rotation during placement. 
     The spinal fusion implant  10  of the present invention may also be provided with one or more radiographic markers to allow for visual determination of proper implant placement. The radiographic markers may be manufactured from any of a variety of suitable radiopaque materials, including but not limited to a metal, ceramic, and/or polymer material, preferably having radiopaque characteristics. The radiographic markers may be provided in any size or shape suitable to facilitate effective and accurate visualization of implant placement. 
     The spinal fusion implant  10  include radiographic markers in the form of elongated cylinders extending generally perpendicularly through the implant  10  between the top and bottom surfaces  12 ,  14 . Alternatively, radiographic markers may include a shorter element which extends only partially from either the top surface  12  or the bottom surface  14  (that is, does not extend through the entire height of the implant  10 ). As a further alternative, radiographic markers may extend at least partially (but not fully) toward either or both of top and bottom surfaces  12 ,  14  (that is, radiographic markers may be disposed completely within the body of the implant  10 ). 
     As best appreciated in  FIGS. 7 and 10 , the proximal end  20  of the spinal fusion implant  10  may be provided with at least one radiographic marker  26  positioned extending at least partially through the implant between top and bottom surfaces  12 ,  14  near the intersection of second lateral side  18  and proximal surface  40 . Radiographic marker  26  may preferably indicate the position of the posterior-most portion of the implant  10 . Spinal fusion implant  10  may include at least one radiographic marker positioned at the intersection of the second lateral side  18  and the distal end  22 . Preferably, there are two markers positioned at this intersection, a first radiographic marker  28  extending at least partially through the implant from the top surface  12  and radiographic marker  30  extending at least partially through the implant from bottom surface  14  to identify the tallest points of the spinal fusion implant  10 . The distal end  22  may be provided with radiographic marker  32  comprising a unitary element fully extending between the top and bottom surfaces  12 ,  14  at or near the intersection of distal end  22  and first lateral side  16 . Radiographic marker  32  may preferably indicate the position of the anterior-most portion of the spinal fusion implant as well as an indication of the anterior height of the implant. As will be shown in greater detail below, the orientation of radiographic markers  26 ,  28 ,  30 ,  32  provide an indication of the oblique placement of the spinal fusion implant  10  and the direction of rotation that is needed to bring the implant  10  into the desired positioning. 
     The spinal fusion implant  10  includes a large aperture  34  extending between top and bottom surfaces  12 ,  14 .  FIGS. 1-4  illustrate aperture  34  extending in a vertical fashion between the top and bottom surfaces  12 ,  14 . The aperture  34  may be provided in any number of suitable shapes, including but not limited to generally circular, generally triangular and/or generally oblong (as shown by example in  FIGS. 3 and 4 ). This single aperture  34  is an additional feature for promoting fusion between the upper and lower vertebral bodies which allow a boney bridge to form through the spinal fusion implant  10 . 
     According to another further aspect of the present invention, this fusion may be facilitated or augmented by including osteoinductive material(s) within the aperture  34  and/or adjacent to the spinal fusion implant  10 . Such osteoinductive materials may be introduced before, during, or after insertion of the spinal fusion implant  10  of the present invention, and may include (but are not necessarily limited to) autologous bone harvested from the patient receiving the spinal fusion implant  10 , bone allograft, bone xenograft, any number of non-bone implants (e.g. ceramic, metallic, polymer), bone morphogenic protein, and bio-resorbable compositions, including but not limited to any of a variety of poly (D, L-lactide-co-glycolide) based polymers, such as those disclosed in U.S. Pat. No. 6,013,853. 
       FIGS. 5-6  depict the spinal fusion implant  10  from side views. First and second lateral sides  16 ,  18  are generally parallel to one another (shown best in  FIGS. 3-4 ). The spinal fusion implant  10  may be further provided with one or more lateral apertures  36  extending generally perpendicularly therethrough from one lateral side  16  to the other  18 . Lateral apertures  36  function to provide visualization at the time of implantation and at subsequent clinical evaluations. Lateral apertures  36  may be provided in any of a variety of suitable shapes, including but not limited to generally circular, generally triangular, generally rectangular, and/or generally oblong (shown by example in  FIG. 5-6 ), or any combination thereof. Although the spinal fusion implant  10  herein includes a pair of lateral apertures  36 , the spinal fusion implant  10  may include any number of lateral apertures  36  as desired. 
     Based on the generally radiolucent nature of the implant  10 , the lateral apertures  36  provide the ability to visualize the interior of the implant  10  during X-ray and/or other suitable imaging techniques which are undertaken from the lateral (or “side”) perspective of the implant  10 . If fusion has taken place, the lateral apertures  36  will provide a method for the surgeon to make follow up assessments as to the degree of fusion without any visual interference from the spinal fusion implant  10 . Further, the lateral apertures  36  will provide an avenue for cellular migration to the exterior of the spinal fusion implant  10 . Thus the spinal fusion implant  10  will serve as additional scaffolding for bone fusion on the exterior of the spinal fusion implant  10 . 
     The spinal fusion implant  10  further includes slots  38  extending from proximal surface  40  along first and second lateral sides  16 ,  18 . Slots  38  are sized and dimensioned to interface with distal insertion tangs  122  on insertion instrument  100  to provide steerability and torsional support during insertion and insert-and-rotate maneuvers as will be described in greater detail below. 
       FIG. 7  illustrates the proximal end  20  of the spinal fusion implant  10  of the present invention. The proximal end  20  has a proximal surface  40  that is tapered (angled) from the first lateral surface  16  to the second lateral surface  18 . This angular surface provides an advantage by allowing an oblique positioning of the spinal fusion implant  10  within the intervertebral space, without protruding into the spinal canal to avoid dural impingement after insertion. Additionally, the tapered nature of the proximal surface  40  can aid in overall fit of the spinal fusion implant  10  within the vertebral disc space. Significantly, the tapered proximal surface  40  on the proximal end  20  enables the spinal fusion implant  10  to maximize contact with the posterior portion of the cortical ring of each adjacent vertebral body. 
     The proximal end  20  may include a proximal engagement recess  46  which extends inwardly in a generally perpendicular fashion relative to the proximal end  20 . Although shown as having a generally semi-circular cross-section, it will be appreciated that the proximal engagement recess  46  may be provided having any number of suitable shapes or cross-sections, including but not limited to circular or triangular. Furthermore, the proximal engagement recess  46  may extend fully or at least partially along the length of the proximal surface  40 . Proximal engagement recess  46  is dimensioned to receive and engage with an insertion tool (described below) for inserting the spinal fusion implant  10  into the intervertebral space. 
     According to the embodiment shown (by way of example only in  FIGS. 7-9 ), the proximal engagement recess  46  is comprised of a keyed insertion slot  48 , a locking recess (undercut)  50 , and a locking wall  54 . Keyed insertion slot  48  is complementary in shape to the rotational lock  128  on the inner shaft  108 . When the rotational lock  128  is inserted into the keyed insertion slot  48 , it falls into the locking recess  50  (or undercut) after it rotates) and abuts locking wall  52 , thereby locking the spinal fusion implant  10  with insertion instrument  100  and preventing the implant  10  and insertion instrument  100  from moving relative to one another. 
       FIG. 10  illustrates the distal end  22  of the spinal fusion implant  10  of the present invention. The distal end  22  has a conical (bullet-shaped) distal nose including a pair of first tapered (angled) surfaces  54  and a pair of second tapered (angled) surfaces  56 . First tapered surfaces  54  extend between lateral surfaces  16 ,  18  and the distal end  22 . First tapered surface  54  extending from second lateral surface/side  18  is generously curved between distal nose and lateral side  18  and functions to distract the vertebrae adjacent to the target intervertebral space during insertion of the spinal fusion implant  10 . According to the embodiment shown, the distal end  22  is asymmetrically positioned relative to the longitudinal axis of the implant. Specifically, the distal end is preferentially curved (curved surface  44 ) towards the second lateral side  18 . The asymmetric distal end  22  facilitates insertion while providing maximal surface area of the spinal fusion implant  10 . The asymmetric distal end  22  provides a gradual lead-in taper which protects nervous tissue in the spinal canal when placing the spinal fusion implant  10  into the disc space on its side and utilizing the insert-and-rotate technique. Once implanted and rotated, the asymmetrical distal end  22  provides increased structural support by approximating the anatomical shape of the anterior portion of the cortical ring of each adjacent vertebral body. 
     The top and bottom surfaces  12 ,  14  may be angled or tapered from distal (anterior end)  22  to proximal (posterior) end  20 . According to an example embodiment, in which the implant  10  has a variable height tapering in a direction oblique to the length and width of the implant, as measured by the distance between the top and bottom surfaces  12 ,  14 . Because the variable height of the implant tapers in a direction oblique to the length of the implant, the height of the implant at the distal end  22  is greater than the height of the proximal end  20 . Because the direction in which the height of the implant tapers is also oblique to the width of the implant, the height of the first lateral side  16  differs from the height of the second lateral side  18  along at least a portion of the length of the implant. The practical result of this tapering along a direction oblique to the length and width of the implant is that when the spinal fusion implant  10  is inserted obliquely within the disc space, the effective height correction occurs generally parallel to the sagittal plane (i.e. anterior to posterior). This provides for optimal restoration of the natural lordotic curvature of the lumbar spine. By way of example only, the oblique tapering of the implant  10  height may occur at an angle measuring from 5 to 15 degrees. 
     The spinal fusion implant  10  preferably has variable rounds on opposing corners of the implant  10  when looking at the implant along its longitudinal axis. These opposing-corner variable rounds (shown here as Rounds A and B) vary in their radius along the length of the implant. In the embodiment shown in  FIGS. 11-12 , Rounds A-B have a larger radius at the distal end  22  and a smaller radius at the proximal end  20 . Smaller rounds (shown here as Rounds C and D) have constant and equal radii along the length of the spinal fusion implant. The radii at Rounds A and B are preferably approximately equal at any given cross-section along the entire length of the spinal fusion implant  10 . These opposing-corner variable rounds allow for more gradual lead-in at the distal end  22  which facilitates insertion of the spinal fusion implant  10  during insert and rotate maneuvers. 
     The spinal fusion implant may be further provided with asymmetric convex top and bottom surfaces between first and second lateral sides to approximate the anatomical concavities of the inferior endplate of the superior vertebra and the superior endplate of the inferior vertebra. According to one embodiment, the radius of curvature between first and second lateral sides  16 ,  18  is preferably smaller for the top surface  12  than the bottom surface  14  the curvature of the top surface  12  will be different at every cross-section along the length of the spinal fusion implant  10  than the curvature of the bottom surface  14 . It is well-known that vertebral body endplates generally have some degree of concavity, however the concavity of adjacent endplates within an intervertebral disc space are rarely identical. The degree of convexity of the top and bottom surfaces  12 ,  14  between first and second lateral sides  16 ,  18  is not identical to account for the asymmetrical concavity of the inferior endplate of the superior vertebral body and the superior endplate of the inferior body. According to a preferred embodiment, the top surface  12  has a larger degree of convexity between first and second lateral sides  16 ,  18  than bottom surface  14 . 
     It can be appreciated by one skilled in the art that the top and bottom surfaces  12 ,  14  may be configured in any number of suitable shapes to better match the natural contours of the vertebral end plates. For example, top and bottom surfaces  12 ,  14  may be generally planar, generally concave, and/or generally convex. According to one or more preferred embodiments, the top surface  12  of the spinal fusion implant  10  is convex to approximate the concave of the inferior endplates of the superior vertebral body and the bottom surface  14  of the spinal fusion implant is concave to approximate the concave surface of the superior endplates of the inferior vertebral body. The degree of convexity of the top and bottom surfaces  12 ,  14  along the length of the implant between proximal and distal ends  20 ,  22  is not identical to account for the asymmetrical concavity of the inferior endplate of the superior vertebral body and the superior endplate of the inferior vertebral body. According to one or more preferred embodiments, the top surface  12  has a greater amount of convexity than bottom surface  14  along the length of the implant. 
     The spinal fusion implant  10  may be introduced into a spinal target site through use of any of a variety of suitable surgical instruments having the capability to engage the implant. As described in  FIGS. 13-16 , the present invention includes an insertion instrument  100  for implanting the spinal fusion implant  10 . According to a broad aspect, the insertion instrument includes a proximal connection region  102 , a thumbwheel  104 , an outer shaft  106 , an inner shaft  108 , and a distal insertion region  110 . 
     The proximal connection region  102  is sized and dimensioned for attaching and/or detaching a handle (not shown). The thumbwheel  104  contains an inner aperture  112  for housing an interior spring (not shown) and a lock  114 . Lock  114  resides at least partially within thumbwheel  104  and includes an aperture  116  and is rotatable between locked and unlocked positions via thumbwheel  104  as will be described in greater detail below. Outer shaft  106  includes a proximal end  118  extending distally from the thumbwheel  104 , a central elongate bore  120  carrying the inner shaft  108  therethrough, and distal insertion tangs  122 . Distal insertion tangs  122  engage with the slots  38  of the spinal fusion implant  10  via insertion slides  124  as will be explained in greater detail below. The inner shaft  108  includes a distal region  128  which terminates in a rotational locking mechanism. As shown in  FIG. 15 , according to one embodiment, rotational lock  128  includes a half-moon shaped key that is sized and dimensioned to fit into keyed insertion slot  48 . 
     The insertion instrument  100  is attached to the spinal fusion implant  10  by aligning the distal insertion tangs  122  with slots  38  on first and second lateral sides  16 ,  18 . The implant may be attached to the positioning insertion slides  1240  into the slots  38  until the distal insertion tangs  122  are fully inserted within the spinal fusion implant  10  and keyed rotational lock  128  is inserted into keyed insertion slot  48  on the proximal end  20  of the spinal fusion implant  10 . Pushing the spring-loaded thumbwheel  104  slightly and spinning it to the right moves the lock from the unlocked position to the locked position. As the thumbwheel  104  moves, the rotational lock rotates 180 degrees and it falls into the locking recess  50  (or undercut) and abuts locking wall  52 . Thus, the rotational lock  128  prevents the implant  10  from moving in an axial direction while the distal insertion tangs  122  prevent translation in the medial/lateral and cranial/caudal directions thereby locking the spinal fusion implant  10  with insertion instrument  100  and preventing the implant  10  and insertion instrument  100  from moving relative to one another. 
     The spinal fusion implant  10  may be introduced into a spinal target site having first been prepared through the use of one or more trial instruments having the capability to size the spinal target site. As described in  FIGS. 17-20 , the present invention a trial instrument for selecting the proper size of spinal fusion implant  10  and determining the correct position of the spinal fusion implant  10  under fluoroscopy prior to insertion. The trial instrument  200  comprises a connector portion  202 , a shaft portion  204 , and a trial head  206 . The trial head  206  has a size and shape of the spinal fusion implant  10  to be used. The trial head  200  has a series of windows  208 ,  210 ,  212  formed within lateral sides  214 . Each window  208 ,  210 ,  212  preferably extends generally perpendicularly from one lateral side  214  of the trial head  206  to the other. 
     The trial instrument  200  may be formed of any material that prevents the passage of x-rays therethrough (e.g. titanium). Since the windows extend completely through the trial head  206 , the x-rays are able to pass through and both the size and shape of the windows  208 ,  210 ,  212  are discernable under fluoroscopy. In the example shown here, the trial head  206  is provided with three windows, however any number may be used. By way of example, the trial head  206  has a proximal shaped window  208 , a distal shaped window  210 , and a central shaped window  212 . The shaped windows  208 ,  210 ,  212  are arranged linearly along the axis of the trial inserter  20 . The central window  212  is shown as having a generally rectangular shape, however other shapes are possible. The central window  212  comprises an aperture having a longitudinal axis that is perpendicular to the longitudinal axis of the trial inserter  200 . The proximal and distal shaped windows are positioned on either side (proximal side and distal side, respectively) of the central window  212 . By way of example, the proximal shaped window  208  has a generally triangular shape and is arranged to “point” in a proximal direction. The distal shaped window  210  also has a generally triangular shape and is arranged to “point” in a distal direction. The proximal and distal shaped windows  208 ,  210  comprise apertures having co-planar non-parallel longitudinal axes. 
     When viewed from the correct side, the axes of the proximal and distal shaped windows  208 ,  210  are divergent from one another and the central axis. This allows for rapid visual determination of rotational positioning of the trial head  206 , and also for immediate instruction on how to correct improper positioning. The parallax distortion of the fluoroscopic image is minimal when the trial head  206  is centered within the image and therefore the x-rays travel within a directly parallel manner. Because metal prevents the passage of x-rays, a window will appear smaller if it is not directly aligned with the direction of the x-rays. As a result, one window (e.g. the central shaped window  212 ) is an indicator of directly parallel alignment), one window (e.g. the distal shaped window  208 ) is an indicator of “hyper-obliqueness” and one window (e.g. the proximal shaped window  210 ) is an indicator of hypo-obliqueness.” 
     Because the user immediately knows whether the trial instrument  200  is “hyper-oblique” or “hypo-oblique”, he/she also immediately knows the direction to move his/her hand to get the correct placement of the trial head  206 , thereby decreasing the need for trial inserter repositioning and localizing x-rays thus reducing x-ray fluoroscopic exposure for user and patient. By way of example, when a trial instrument  200  is inserted directly oblique (proper position), the proximal and distal shaped windows  208 ,  210  will appear the same size under fluoroscopy. However when one of the proximal and distal shaped windows  208 ,  210  is larger than the other, the surgeon/user knows that the trial is not correctly positioned. Pivoting the trial head  206  in the direction of the larger window, the desired position may be achieved.  FIG. 19  is an example of a “hyper-oblique” trial head  206 . In this instance, the distal shaped window  210  is larger than the proximal shaped window  208 , indicating suboptimal positioning of the trial  200 . Pivoting the trial  200  in the direction of the distal shaped window  210  will bring the trial head  206  into ideal trial positioning.  FIG. 20  is an example of a “hypo-oblique” trial head  206 . In this instance, the proximal shaped window  208  is larger than the distal shaped window  210 , also indicating suboptimal positioning in the trial  200 . Pivoting the trial  200  in the direction of the proximal shaped window  208  will bring the trial head  206  into ideal trial positioning. 
     According to a broad aspect of the present invention, the spinal fusion implant  10  is capable of being used in minimally invasive surgical procedures, needing only a relatively small operative corridor for insertion. By way of example only, the spinal fusion implant  10  will now be described in relation to a transforaminal lumbar interbody fusion (TLIF) technique, in which the intervertebral disc space is approached from a postero-lateral direction, however it should be understood that the spinal fusion implant  10  is capable of use in a variety of surgical procedures not described herein. After creation of this operative corridor and preparing the disc space (using techniques commonly known and used in the art), a trial inserter (e.g. the trial inserter of  FIGS. 17-20 ) may be used to select the proper size of the spinal fusion implant  10 . 
     The spinal fusion implant  10  is mated to an insertion device (e.g. insertion instrument  100 ) and advanced through the operative corridor toward the target intervertebral space. The spinal fusion implant  10  may be oriented with the lateral sides  16 ,  18  facing in a caudad/cephalad direction, for example with the first lateral side  16  facing a caudad (inferior) direction and the second lateral side  18  facing a cephalad (superior) direction. When the distal end  22  of the implant  10  reaches the intervertebral disc space, each of the pair of first tapered surfaces  54  will come into contact with one of the adjacent vertebral bodies. As the implant  10  is advanced into the intervertebral disc space, the pair of first tapered surfaces  54  will serve to distract the vertebral bodies, allowing the implant to fully enter the intervertebral space. 
     Since the first and second lateral sides  16 ,  18  are preferably provided with generally smooth surfaces, the spinal fusion implant  10  should advance with relative ease into the disc space once the adjacent vertebral bodies have been distracted. Once the implant  10  has been positioned in its desired location, the user will then rotate the implant 90° such that the top and bottom surfaces  12 ,  14  face in a caudad/cephalad direction and the anti-migration features  24  engage the vertebral bodies. Significantly, the direction of rotation is critical to ensure proper placement of the implant  10  such that the edges of the proximal surface  40  rest on the cortical ring of the vertebral bodies, that the proximal surface  40  does not protrude into the spinal canal, and the implant  10  tapers in the appropriate direction (e.g. anterior to posterior rather than posterior to anterior). For example, if the spinal fusion implant  10  approaches a patient&#39;s spine posteriorly from the right with the (longer) first lateral side  16  facing caudally, then implant  10  must be rotated in a counter-clockwise direction to achieve proper positioning. Similarly, if the spinal fusion implant  10  approaches a patient&#39;s spine posteriorly from the left side with the (longer) first lateral side  16  facing caudally, then implant  10  must be rotated in a clockwise direction to achieve proper positioning. According to one embodiment the implant may include one or more markings or other indicia to help facilitate the proper positioning. According to one embodiment (not shown), for example, the first lateral side  16  may be marked with “lateral” to indicate that it should face to the exterior of the disc space, and the second lateral side  17  may be marked with “medial” to indicate that it should face the interior of the disc space when the implant  10  is rotated into position. Once the spinal fusion implant  10  has been rotated into position, the insertion instrument  100  may be detached and removed from the operative corridor. 
     In accordance with the present invention, the user is provided with one or more methods to aid in verifying the desired positioning of the spinal fusion implant  10  using lateral fluoroscopy to localize internal visualization markers and verify movement of the spinal fusion implant  10  within the disc space.  FIG. 21  A depicts a spinal fusion implant  10  positioned in the intervertebral disc space, however not in the desired oblique alignment. As illustrated in  FIG. 21  B, radiographic markers  28 ,  30  will appear spaced apart from radiographic marker  32  on lateral fluoroscopy.  FIG. 22  A depicts a spinal fusion implant  10  positioned in the intervertebral disc space, in the desired oblique alignment. As illustrated in  FIG. 22  B, radiographic markers  32  will appear to align with radiographic marker  32  on lateral fluoroscopy. 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined herein.