Patent Publication Number: US-2023147669-A1

Title: Screwless interbody device for spinal surgery

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority and benefits under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 63/014,542 filed on Apr. 23, 2020, which is incorporated herein in its entirety by reference. 
    
    
     FIELD 
     The current disclosure is generally directed to interbody devices inserted between adjacent vertebrae to stabilize the intervertebral space. The interbody devices also facilitate fusion of adjacent vertebrae. Friction elements extend from a superior endplate and an inferior endplate of the interbody devices to engage the adjacent vertebrae to prevent unintended or inadvertent movement of the interbody devices. 
     SUMMARY 
     Anterior cervical discectomy and fusions (ACDF) were developed in the 1950&#39;s and are a common surgery performed by neurosurgeons and orthopedic spine surgeons. The surgeries are utilized to alleviate compression of the spinal cord or the exiting nerve roots which can cause myelopathy and radiculopathy respectively. The surgeries are also utilized for cases where the cervical spine is unstable due to ligamentous injury, fracture, or degenerative disease. 
     A similar surgery called an anterior lumbar instrumentation and fusion (ALIF) is used for similar conditions in a lumbar spine. The main differences in the lumbar spine are that the spinal elements are much larger requiring larger instrumentation and larger implants. Another key difference is that the spinal cord is typically not present in the lumbar spine ending at approximately the L1 level and therefore myelopathy is not typically an issue with the lumbar spine. 
     Current surgical technique calls for the removal of disc material from an anterior approach either through the neck or through the abdomen. This portion of the surgery alleviates the compression but creates instability and would allow for one vertebral body to collapse on another if no spacer were to be placed between the vertebral bodies. 
     Initially, when these techniques were developed, a bone graft was used as a spacer. The bone graft could be either autologous (harvested from the patient) or from a bone donor. 
     As the ACDF and ALIF techniques were further developed, surgeons began fixing a plate over the surface of the bone implant with anchoring screws in the vertebral body above and the vertebral body below. This is the most common technique today. However, harvesting autologous bone graft from the patient is painful and use of donor graft material is expensive. Additionally, accurately orienting the screws for the plate is difficult and time consuming, increasing the risk and time required to perform the ACDF or ALIF surgery. 
     Later, and referring now to  FIG.  1   , artificial cages  12 A came into use to replace the bone graft used in ACDF and ALIF surgeries. The cages can be manufactured from either plastic or metal with the typical plastic cage consisting of PEEK and the typical metal cage consisting of titanium. The cages are frequently designed to allow either autologous or donor bone to be packed within them to allow for boney fusion to occur through the spacer. Although the cage eliminates the need to harvest bone from the patient, after the artificial cage is inserted into the disc space  6  between adjacent vertebrae  2 , a plate  8  is still used and is fixed to the adjacent vertebrae. Thus, ACDF and ALIF surgeries performed with cages still suffer from the problems and inefficiencies required to properly orient and insert screws  10  into the vertebral body  2 A above and the vertebral body  2 B below the cage. 
     More recently, further development of the ACDF and ALIF techniques resulted in the introduction of a stand-alone cage that does not require the use of a plate. The cage is anchored to one or more of the adjacent vertebrae by screws  14 . Examples of cages  12 B for ACDF surgeries are illustrated in  FIG.  2    and cages  12 C used in ALIF surgery are shown in  FIG.  3   . In many cases the cages  12 B,  12 C are fixated to the upper and lower vertebral bodies in exactly the same way as a plate. The cages use two or more screws  14  oriented at an angle to the cage to fix the cage to the adjacent vertebral bodies. At least one screw  14  is anchored into the vertebral body cranially (or upwardly) and at least one screw anchored into the vertebral body caudally (or downwardly). Alternative fixation devices other than screws are less commonly used, such as blades or shims  16  (shown in  FIGS.  2 E- 2 G ) which can be projected into the vertebral body cranially and caudally. 
     Typically placing the screws  14  or shims  16  of the cage  12 B,  12 C is the most difficult and time consuming portion of the ACDF and ALIF procedures due to the angle which must be achieved to set the screws or shims. It is difficult to orient the instruments used to set the screws or shims at the required angle within the surgical space. Additionally, the instruments needed to place the screws create a risk of injury to critical structures in the neck or abdomen of the patient. Moreover, if a screw or shim is not properly oriented, or if a screw is over tightened, the vertebral body may be damaged. Further, apertures formed in the vertebrae for the screws can weaken the vertebrae, especially when the vertebrae is already damaged or weakened due to disease. 
     All of the cages  12 A,  12 B, and  12 C of the related art shown in  FIGS.  1 - 3    require some form of fixation. The fixation may be a plate to hold the cage within the disc space or screws to fix the cage to the upper and lower vertebrae. Without the fixation, the cages will move unintentionally resulting in unsuccessful fusion of the vertebrae. 
     An alternative to spinal fusion provided by ACDF and ALIF procedures is a disc replacement procedure. Referring now to  FIG.  4   , disc replacement devices  18  are shown. The disc replacement device is an alternative to a spacer made from bone and to a cage  12 . The disc replacement devices are configured to move and articulate to preserve some motion between adjacent vertebrae. Accordingly, disc replacement devices are not suitable for use in a spinal fusion procedure, such as ACDF and ALIF procedures. 
     Disc replacement devices  18  typically include a superior or upper endplate  20 A flexibly joined to an inferior or lower endplate  20 B by a medial element  22 . The medial element  22  may comprise a plastic or a disc of polyethylene. The cranial (upper) and caudal (lower) endplates may include micro porous areas to allow for bone to grow. 
     Some disc replacement devices  18  are designed to be fixated initially with simple friction. The endplates  20 A,  20 B may include a keel  24  or teeth  26  to engage one or more of the adjacent vertebral bodies. These friction fit disc replacement devices  18  are noted to be easy to place because no anchor screws are required. Disc replacement devices  18  have shown good fixation without significant complications due to migration out of the intended location. 
     Currently there is no cage available for use in ACDF and ALIF procedures which combines the ease of a friction fit device with a simple spacer which is not designed to act as a disc replacement. Accordingly, there is an unmet need for an interbody device for use in fusion procedures that frictionally engages an upper and a lower vertebral body. 
     It is one aspect of the present disclosure to provide an interbody device to frictionally engage an upper vertebral body and a lower vertebral body and which facilitates fusion of the upper and lower vertebral bodies. 
     In embodiments, a superior endplate of the interbody device includes a friction element. 
     In embodiments, an inferior endplate of the interbody device includes a friction element. 
     In embodiments, the friction element extends at least a portion of a length of an endplate between an anterior surface and a posterior surface of the interbody device. 
     Optionally, the friction element is oriented approximately parallel to a median plane of the interbody device. Alternatively, in another embodiment, a friction element is oriented at an oblique angle to the median plane. 
     In some embodiments, the friction element comprises a keel projecting from an endplate of the interbody device. 
     In another embodiment, the friction element comprises a tooth projecting from an endplate of the interbody device. 
     Optionally, the friction element comprises a plurality of teeth arranged in a row. 
     In embodiments, the teeth have free ends that are pointed. 
     In embodiments, free ends of at least some of the teeth are oriented toward an anterior surface of the interbody device. 
     Optionally, for any of the embodiments, a free end of at least one tooth is oriented toward a posterior surface of the interbody device. 
     Optionally, the superior endplate includes two rows of teeth. 
     In embodiments, a first row of teeth of the superior endplate extends along a first axis and the second row of teeth of the superior endplate extends along a second axis. In some embodiments, the first axis is approximately parallel to the second axis. Alternatively, in another embodiment, the first axis is oriented at an oblique angle to the second axis. 
     Optionally, the inferior endplate includes two rows of teeth. 
     In some embodiments, a third row of teeth of the inferior endplate extends along a third axis and the fourth row of teeth of the inferior endplate extends along a fourth axis. In embodiments, the third axis is approximately parallel to the fourth axis. Alternatively, in another embodiment, the first axis is oriented at an oblique angle to the second axis. 
     In some embodiments, the first axis is oriented at an oblique angle to the third axis. 
     In embodiments, the superior endplate includes a friction element that is offset from a friction element of the inferior endplate. Optionally, the superior friction element extends along a first axis and the inferior friction elements extends along a second axis. In some embodiments, the first axis is approximately parallel to the second axis. Alternatively, in another embodiment, the first axis is oriented at an oblique angle to the second axis. 
     Optionally, in any of the embodiments, the superior endplate may be generally planar. 
     Alternatively, in another embodiment, the superior endplate includes a dome the projects upwardly (or cranially). 
     Optionally, in any of the embodiments, the inferior endplate may be generally planar. 
     Alternatively, in another embodiment, the inferior endplate includes a dome the projects downwardly (or caudally). 
     In embodiments, one or more of the superior endplate and the inferior endplate comprise a micro porous material for bone ingrowth and/or adherence. 
     In some embodiments, the interbody device comprises a plastic. 
     In embodiments, the plastic is a thermoplastic polymer. 
     Optionally, the plastic is a polyether ether ketone (PEEK). 
     Additionally, or alternatively, in other embodiments, the interbody device comprises a metal. 
     In some embodiments, the metal is titanium or a titanium alloy. 
     In embodiments, the interbody device is produced by an additive manufacturing process. 
     Optionally, for any of the embodiments, the interbody device may be produced by a 3-D printing process. 
     In embodiments, the interbody device is treated to produce micro- and nanoscale surface roughness. 
     In some embodiments, the treatment of the interbody device includes acid etching to promote the micro- and nanoscale surface roughness. 
     Additionally, or alternatively, the interbody device is heat treated to promote the micro- and nanoscale surface roughness. 
     Optionally, for any of the embodiments, the interbody device may be rigid. 
     In embodiments, the interbody device is of a one-piece construction. 
     One aspect of the present disclosure is an interbody device for a spinal fusion procedure which frictionally engages an upper vertebral body and a lower vertebral body. The interbody device comprises: (1) a superior endplate opposite to an inferior endplate; (2) a superior friction element extending from the superior endplate; (3) an inferior friction element extending from the inferior endplate; and (4) an anterior surface including an engagement feature. 
     In some embodiments, the superior friction element extends generally linearly. The friction element may extend approximately parallel to a median plane bisecting the interbody device. Alternatively, the friction element is oriented at an oblique angle to the median plane. 
     Optionally, two superior friction elements extend from the superior endplate. In some embodiments, at least one inferior friction element extends from the inferior endplate. 
     In some embodiments, a first one of the superior friction elements extends along a first axis and a second one of the superior friction elements extends along a second axis. Optionally, the first axis is approximately parallel to the second axis. Alternatively, in another embodiment, the first axis is oriented at an oblique angle to the second axis. 
     Optionally, an inferior friction element extends along a third axis. In some embodiments, the third axis is oriented at an oblique angle to at least one of the first axis and the second axis. 
     Additionally, the inferior friction element can optionally be offset from the superior friction elements. In embodiments, the inferior friction element is offset medially from one or more of the superior friction elements. 
     In some embodiments, the friction elements comprise a keel extending from the endplates. 
     Optionally, the keel includes projections extending from lateral surfaces of the keel. 
     In some embodiments, the projections are movable to a first position to facilitate movement of the interbody device in a posterior direction. 
     Additionally, or alternatively, the projections can be movable to a second position to limit movement of the interbody device in an anterior direction. 
     For any of the embodiments, the friction elements may optionally comprise a plurality of teeth arranged in a row. 
     Optionally, the teeth include a free end that is pointed. 
     Additionally, or alternatively, one or more of the endplates of the interbody device can include a dome. 
     In embodiments, one or more of the superior endplate and the inferior endplate comprise a micro porous material for bone ingrowth. 
     Optionally, the interbody device comprises a PEEK. 
     In embodiments, the interbody device comprises a metal. 
     In some embodiments, the interbody device is rigid. 
     In embodiments, the interbody device is produced by an additive manufacturing process. 
     In embodiments, the interbody device is configured to frictionally engage the upper vertebral body and the lower vertebral body without the use of a screw and without the use of a plate. 
     It is another aspect of the present disclosure to provide a method of performing a spinal fusion procedure. The method comprises: (1) removing disc material from a disc space between an upper vertebral body and a lower vertebral body; (2) positioning an interbody device in the disc space, the interbody device generally including: (a) a superior endplate with a superior friction element to engage the upper vertebral body; and (b) an inferior endplate opposite to the superior endplate, the inferior endplate including an inferior friction element to engage the lower vertebral body. The interbody device is retained in a predetermined position within the disc space by frictional engagement of the friction elements with the vertebral bodies. 
     In embodiments, no screws or plates are used to retain the interbody device in the predetermined position. Optionally, the interbody device has no moving parts. 
     For any of the embodiments, the friction elements optionally comprise one of a keel and a plurality of teeth arranged in one or more rows. 
     Optionally, the method further comprises forming a slot in the upper vertebral body to receive the superior friction element. 
     In some embodiments, positioning the interbody device in the disc space includes aligning a friction element extending from the superior endplate with the slot in the upper vertebral body. 
     Additionally, or alternatively, the method can include forming a slot in the lower vertebral body to receive the inferior friction element. 
     In embodiments, the slot in the upper vertebral body is formed such that it is positioned offset medially from the slot in the lower vertebral body. 
     Optionally, the slot in one or more of the upper vertebral body and the lower vertebral body is formed by a cutting instrument. 
     In some embodiments, positioning the interbody device in the disc space includes aligning a friction element extending from the inferior endplate with the slot in the lower vertebral body. 
     Optionally, the method includes placing bone graft material in the disc space. 
     In any of the embodiments, the method optionally includes packing bone graft material in the interbody device. 
     Alternatively, in another embodiment, no bone graft or bone graft substitute is positioned within the disc space with the interbody device. 
     In embodiments, no screws are positioned in the upper vertebral body or the lower vertebral body during the spinal fusion procedure. 
     In some embodiments, the interbody device includes a dome extending from one or more of the superior endplate and the inferior endplate. 
     In embodiments, positioning the interbody device in the disc space includes aligning a dome extending from the superior endplate with a recess in the upper vertebral body. 
     Additionally, or alternatively, positioning the interbody device in the disc space may include aligning a dome extending from the inferior endplate with a recess in the lower vertebral body. 
     Another aspect of the present disclosure is a method of forming an interbody device for a spinal fusion procedure which is configured to engage one or more of an upper vertebral body and a lower vertebral body. The method comprises: (1) forming the interbody device with a superior endplate opposite to an inferior endplate; (2) forming a superior friction element extending from the superior endplate; and (3) forming an inferior friction element extending from the inferior endplate. 
     Optionally, the method can include forming an engagement feature on an anterior surface of the interbody device. 
     In some embodiments, the friction elements include one or more of a bump, a ridge, a tooth, and a projection. 
     In embodiments, the superior friction is formed to extend generally alone an axis. In some embodiments, the axis extends approximately parallel to a median plane bisecting the interbody device. Alternatively, the axis extends at an oblique angle to the median plane. 
     Optionally, forming the friction elements comprises forming two superior friction elements extending from the superior endplate and forming at least one inferior friction element that extends from the inferior endplate. 
     In embodiments, forming the superior and inferior friction elements comprises offsetting the inferior friction element medially from the superior friction elements. In this manner, the superior and inferior friction elements are not co-planar. 
     In some embodiments, forming the friction elements comprises forming a keel that extends from the endplates. 
     Optionally, the keel is formed with projections extending from lateral surfaces of the keel. 
     In embodiments, the projections are movable to a first position to facilitate movement of the interbody device in a posterior direction. 
     Additionally, or alternatively, the projections are formed to be movable to a second position to limit movement of the interbody device in an anterior direction. 
     For any of the embodiments, forming the friction elements optionally comprises forming a plurality of teeth arranged in a row. 
     Optionally, the teeth are formed with a free end that is pointed. 
     Additionally, or alternatively, the method includes forming a dome that projects from one or more of the endplates of the interbody device. 
     The method can also include forming a micro porous structure for bone ingrowth in one or more of the superior endplate and the inferior endplate. 
     In some embodiments, the interbody device comprises a plastic. 
     In embodiments, the plastic is a thermoplastic polymer. 
     Optionally, the interbody device comprises a PEEK. 
     In embodiments, the interbody device comprises a metal. 
     In some embodiments, the metal is titanium or a titanium alloy. 
     In any of the embodiments, the interbody device optionally is rigid. 
     Optionally, the method includes casting at least a portion of the interbody device. 
     Additionally, or alternatively, the method includes forging a portion of the interbody device. 
     In embodiments, the method includes forming, or finishing, a portion of the interbody device by an additive manufacturing process. 
     Optionally, the method includes forming micro- or nano-scale roughness by a 3-D additive process on a portion of the interbody device that has been cast and/or forged. 
     For any of the embodiments, the interbody device is optionally produced by a 3-D printing process. 
     In embodiments, the interbody device is of a one-piece construction. For example, the interbody device may be formed to have no moving parts. 
     In embodiments, the interbody device is configured to frictionally engage the upper vertebral body and the lower vertebral body without the use of a screw and without the use of a plate. 
     Optionally, the interbody device is formed with an interior cavity. 
     In embodiments, the method includes treating the interbody device to produce micro- and nanoscale surface roughness. 
     In some embodiments, the treatment of the interbody device includes acid etching to promote the micro- and nanoscale surface roughness. 
     Additionally, or alternatively, the method includes heat treating the interbody device to promote the micro- and nanoscale surface roughness. 
     One aspect of the present disclosure is an interbody device as substantially described herein. 
     Another aspect of the present disclosure is a method of using an interbody device as substantially described herein in a surgical procedure. 
     Still another aspect of the present disclosure is a method of forming an interbody device as substantially described herein. 
     Another aspect of the present disclosure is an interbody device for use in a spinal fusion procedure to frictionally engage an upper vertebral body and a lower vertebral body, comprising: (1) a superior endplate opposite to an inferior endplate; (2) a first lateral wall opposite to a second lateral wall; (3) an anterior wall opposite to a posterior wall, the anterior wall including an engagement feature; (4) a first friction means extending from the superior endplate and oriented at an oblique angle to a median plane that extends through the superior and inferior endplates and bisects the interbody device; and (5) a second friction means extending from the inferior endplate. 
     In embodiments, the friction means extend along a predetermined length. 
     In embodiments, the friction means have a predetermined height. 
     In embodiments, the friction means are keels. 
     In embodiments, the friction means each comprise a plurality of teeth that extend in a row. 
     One aspect of the present disclosure is an interbody device configured for use in a spinal fusion procedure to frictionally engage an upper vertebral body and a lower vertebral body, comprising: (1) a superior endplate opposite to an inferior endplate; (2) a first superior friction element extending from the superior endplate; (3) a first inferior friction element extending from the inferior endplate; and (4) an anterior wall including an engagement feature. 
     In embodiments, the first superior friction element is oriented at an oblique angle to a median plane that extends through the superior and inferior endplates and bisects the interbody device. 
     In embodiments, the interbody device further comprises a second superior friction element extending from the superior endplate. 
     In embodiments, the first superior friction element is on a first side of the median plane and the second superior friction element is on a second side of the median plane. 
     In embodiments, the second superior friction element is oriented at an oblique angle to the median plane and to the first superior friction element. 
     In embodiments, the first superior friction element is oriented at a first angle relative to the median plane and the second superior friction element is oriented at a second angle relative to the median plane. 
     In embodiments, the second angle is a positive angle. 
     In embodiments, the first angle is a negative angle that is of approximately equal magnitude to the second angle. 
     In embodiments, the interbody device further comprises a third superior friction element extending from the superior endplate. 
     In embodiments, the interbody device includes one or more of the previous embodiments and further comprises a second inferior friction element extending from the inferior endplate. 
     In embodiments, the second inferior friction element is on the first side of the median plane and the first inferior friction element is on the second side of the median plane. 
     In embodiments, the first inferior friction element is oriented at a third angle relative to the median plane. 
     In embodiments, the third angle is approximately equal to the first angle such that the first inferior friction element is approximately parallel to the first superior friction element. 
     In embodiments, the second inferior friction element is oriented at a fourth angle relative to the median plane. 
     In embodiments, the fourth angle is approximately equal to the second angle such that the second inferior friction element is approximately parallel to the second superior friction element. 
     In embodiments, the interbody device further comprises a third inferior friction element extending from the inferior endplate. 
     In embodiments, the interbody device includes one or more of the previous embodiments and one or more of the friction elements each comprise a plurality of teeth. 
     In embodiments, the plurality of teeth are arranged in a row. 
     In embodiments, the row of teeth is substantially linear. 
     In embodiment, the row of teeth extends along an arcuate path. 
     Alternatively, in embodiments, the plurality of teeth are not arranged in a row. 
     In embodiments, the teeth include: (a) a first side extending away from one of the superior and inferior endplates, the first side being proximate to the anterior wall; (b) a long side that is oriented at an oblique angle to the first side; and (c) a free end defined by an intersection of the long side with the first side. 
     In embodiments, the first side is approximately planar and the long side is approximately planar. 
     In embodiments, the interbody device includes one or more of the previous embodiments and further comprises a posterior wall positioned opposite to the anterior wall. 
     In embodiments, the posterior wall has a first height that is less than a second height of the anterior wall. 
     In embodiments, the superior endplate extends along a curved path between the anterior wall and the posterior wall such that the superior endplate is convex between the anterior and superior walls. 
     Additionally, or alternatively, in embodiments, the inferior endplate extends along a curved path between the anterior wall and the posterior wall such that the inferior endplate is convex between the anterior and superior walls. 
     In embodiments, the interbody device includes one or more of the previous embodiments and one or more of the superior endplate and the inferior endplate comprise a micro porous material for bone ingrowth. 
     In embodiments, the interbody device comprises a PEEK and includes one or more of the previous embodiments. 
     In embodiments, the interbody device comprises a metal and includes one or more of the previous embodiments. 
     In embodiments, the interbody device is rigid and includes one or more of the previous embodiments. 
     In embodiments, the interbody device is produced by an additive manufacturing process and includes one or more of the previous embodiments. 
     In embodiments, the interbody device includes one or more of the previous embodiments and the interbody device is configured to frictionally engage the upper vertebral body and the lower vertebral body without the use of a secondary anchor, such as a screw, a pin, a plate, and a rod. 
     In embodiments, the interbody device does not include any movable parts, such as deployable anchors or projections. 
     In embodiments, the interbody device includes one or more of the previous embodiments and the interbody device is static. 
     In embodiments, the interbody device includes one or more of the previous embodiments and further comprises one or more of: (a) a first lateral plate opposite to a second lateral plate; and (b) an interior cavity within the interbody device. 
     In embodiments, an aperture formed through one or more of the first and second lateral plates extends to the interior cavity. 
     In embodiments, the interbody device includes one or more of the previous embodiments and further comprises an interior cavity within the interbody device. 
     In embodiments, an orifice formed through one or more of the superior endplate and the inferior endplate extends to the interior cavity. 
     In embodiments, the interbody device includes one or more of the previous embodiments and the engagement feature includes an aperture that extends to an interior cavity within the interbody device. 
     Yet another aspect of the present disclosure is a method of performing a spinal fusion procedure, comprising: (1) removing disc material from a disc space between an upper vertebral body and a lower vertebral body; and (2) positioning an interbody device according to the present disclosure in the disc space such that a first superior friction element of the interbody device is oriented to engage the upper vertebral body, and a first inferior friction element is oriented to engage the lower vertebral body. In this manner, the interbody device is retained in a predetermined position within the disc space by frictional engagement of the friction elements with the vertebral bodies. 
     In embodiments, the interbody device includes: (a) a superior endplate opposite to an inferior endplate; (b) the first superior friction element extending from the superior endplate; (c) the first inferior friction element extending from the inferior endplate; and (d) an anterior wall including an engagement feature. 
     In embodiments, no screws or plates are used to retain the interbody device in the predetermined position. 
     In embodiments, the method further comprises one or more of: (a) forming a slot in the upper vertebral body to receive the superior friction element; and (b) forming a slot in the lower vertebral body to receive the inferior friction element. 
     In embodiments, the friction elements comprise one of a keel and a plurality of teeth. 
     In embodiments, the plurality of teeth are arranged in one or more rows. 
     In embodiment, the interbody device frictionally engages the upper vertebral body and the lower vertebral body without the use of a secondary anchor, such as a screw, a pin, a plate, and a rod. 
     It is another aspect of the present disclosure to provide a method of forming an interbody device configured for use in a spinal fusion procedure which is configured to engage one or more of an upper vertebral body and a lower vertebral body, comprising: (1) forming the interbody device with a superior endplate opposite to an inferior endplate; (2) forming a first friction element extending from the superior endplate; (3) forming a second friction element extending from the superior endplate; (4) forming a third friction element extending from the inferior endplate; (5) forming a fourth friction element extending from the inferior endplate; (6) forming an anterior wall extending between the superior and inferior endplates; and (7) forming an engagement feature that includes an aperture extending through the anterior wall to an interior cavity within the interbody device. 
     In embodiments, the first friction element extends at a first oblique angle to a median plane extending through the endplates and bisecting the interbody device. 
     In embodiments, the second friction element extends at a second oblique angle to the median plane. 
     In embodiments, the third friction element extends at a third oblique angle to the median plane. 
     In embodiments, the third friction element is approximately parallel to the first friction element. 
     In embodiments, the fourth friction element extends at a fourth oblique angle to the median plane. 
     In embodiments, the fourth friction element is approximately parallel to the second friction element. 
     In embodiments, the method includes one or more of the previous embodiments and further comprises forming a fifth friction element extending from one of the superior and inferior endplates. 
     In embodiments, the method includes one or more of the previous embodiments and further comprises forming a sixth friction element extending from one of the superior and inferior endplates. 
     In embodiments, one or more of the friction elements each comprise a plurality of teeth. 
     In embodiments, the teeth of the friction elements are arranged in a row. 
     Alternatively, in embodiment, the teeth of at least one of the friction elements are not arranged in a row. 
     In embodiments, the method includes forming the interbody device by an additive manufacturing process and the interbody devices includes one or more of the previous embodiments. 
     In embodiments, the method includes one or more of the previous embodiments and the interbody device is static. 
     In embodiments, the interbody device is formed of one or more of a PEEK and a metal. 
     In embodiments, the method includes one or more of the previous embodiments and further comprises treating the interbody device to produce micro- or nanoscale surface roughness. 
     In embodiments, the method includes acid etching the interbody device to promote the micro- or nanoscale surface roughness. 
     In embodiments, the method includes one or more of the previous embodiments and further comprises heat treating the interbody device to promote micro- or nanoscale surface roughness. 
     In embodiments, the method includes one or more of the previous embodiments and one or more of the superior endplate and the inferior endplate comprise a micro porous material for bone ingrowth. 
     In embodiments, the method includes one or more of the previous embodiments and further comprises finishing one or more exterior surface of the interbody device by shot blasting. 
     The Summary is neither intended nor should it be construed as being representative of the full extent and scope of the present disclosure. The present disclosure is set forth in various levels of detail in the Summary as well as in the attached drawings and the Detailed Description and no limitation as to the scope of the present disclosure is intended by either the inclusion or non-inclusion of elements, components, etc. in this Summary. Additional aspects of the present disclosure will become more clear from the Detailed Description, particularly when taken together with the drawings. 
     The phrases “at least one,” “one or more,” and “and/or,” as used herein, are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C,” “at least one of A, B, or C,” “one or more of A, B, and C,” “one or more of A, B, or C,” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. 
     The term “a” or “an” entity, as used herein, refers to one or more of that entity. As such, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. 
     Unless otherwise indicated, all numbers expressing quantities, dimensions, conditions, ratios, ranges, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about” or “approximately”. Accordingly, unless otherwise indicated, all numbers expressing quantities, dimensions, conditions, ratios, ranges, angles, relationships and so forth used in the specification and claims may be increased or decreased by approximately 10% to achieve satisfactory results. Additionally, where the meaning of the terms “about” or “approximately” as used herein would not otherwise be apparent to one of ordinary skill in the art, the terms “about” and “approximately” should be interpreted as meaning within plus or minus 10% of the stated value. 
     The term “parallel” means two objects are oriented at an angle within plus or minus 0° to 5° unless otherwise indicated. Similarly, the term “perpendicular” means two objects are oriented at angle of from 85° to 95° unless otherwise indicated. 
     All ranges described herein may be reduced to any sub-range or portion of the range, or to any value within the range without deviating from the invention. For example, the range “5 to 55” includes, but is not limited to, the sub-ranges “5 to 20” as well as “17 to 54.” 
     The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Accordingly, the terms “including,” “comprising,” or “having” and variations thereof can be used interchangeably herein. 
     It shall be understood that the term “means” as used herein shall be given its broadest possible interpretation in accordance with 35 U.S.C., Section 112(f). Accordingly, a claim incorporating the term “means” shall cover all structures, materials, or acts set forth herein, and all of the equivalents thereof. Further, the structures, materials, or acts and the equivalents thereof shall include all those described in the Summary, Brief Description of the Drawings, Detailed Description, Abstract, and Claims themselves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the disclosed system and together with the general description of the disclosure given above and the detailed description of the drawings given below, serve to explain the principles of the disclosed system(s) and device(s). 
         FIG.  1    illustrates a plate fixed to vertebrae of a patient; 
         FIGS.  2 A,  2 B,  2 C and  2 D  are views of cages related to the present disclosure and which are used in ACDF procedures and which include screws to anchor the cages; 
         FIGS.  2 E,  2 F and  2 G  are views of cages for ACDF procedures that have shims to anchor the cages; 
         FIGS.  3 A,  3 B, and  3 C  show several views of a cage related to the present disclosure with screws and which is used in ALIF procedures; 
         FIGS.  4 A,  4 B, and  4 C  are illustrations of a disc replacement device related to the present disclosure and which includes a keel to anchor the device; 
         FIGS.  4 D,  4 E,  4 F and  4 G  show another disc replacement device of the present disclosure that is anchored by teeth that project from the upper and lower endplates; 
         FIGS.  5 A,  5 B,  5 C and  5 D  illustrate an interbody device of one embodiment of the present disclosure; 
         FIGS.  6 A,  6 B,  6 C,  6 D and  6 E  illustrate another interbody device according to embodiments of the present disclosure and which includes a keel to anchor the interbody device to adjacent vertebrae; 
         FIG.  6 F  is an anterior (or front) elevation schematic view of interbody devices of the present disclosure positioned within disc spaces between vertebrae; 
         FIGS.  7 A,  7 B,  7 C and  7 D  are views of still another interbody device according to embodiments of the present disclosure and which includes teeth to anchor the interbody device to adjacent vertebrae; 
         FIGS.  8 A,  8 B, and  8 C  illustrate embodiments of interbody devices of the present disclosure which include one or more open endplates or open front/rear plates; 
         FIGS.  9 A and  9 B  illustrate additional embodiments of interbody devices with open endplates and/or open front/rear plates; 
         FIG.  10    is a front elevation view of an interbody device similar to the interbody device of  FIG.  6    and which includes an upper endplate with a dome; 
         FIG.  11    is another front elevation view of an interbody device of the present disclosure which includes domes on the upper and lower endplates; 
         FIG.  12    is a front elevation view of an interbody device similar to the interbody device of  FIG.  7    and which includes an upper endplate with a dome; 
         FIG.  13    is another front elevation view of an interbody device of the present disclosure which includes domes on the upper and lower endplates; 
         FIGS.  14  and  15    are perspective views of an interbody device according to another embodiment of the present disclosure; 
         FIG.  16    is a top plan view of the interbody device of  FIG.  14   ; 
         FIG.  17    is a bottom plan view of the interbody device of  FIG.  14   ; 
         FIG.  18    is a front elevation view of the interbody device of  FIG.  14   ; 
         FIG.  19    is a rear elevation view of the interbody device of  FIG.  14   ; 
         FIG.  20    is a side elevation view of a first lateral side of the interbody device of  FIG.  14   , the second lateral side being substantially the same; 
         FIG.  21    is a cross-sectional side elevation view of the interbody device of  FIG.  14   , the cross-section taken along a median plane that bisects the interbody device; 
         FIG.  22    is a cross-sectional perspective view of the interbody device of  FIG.  14    taken along a transverse plane that bisects the interbody device; 
         FIG.  23    is a perspective view of interbody devices according to the embodiment of  FIG.  14   , the interbody devices being of different sizes; 
         FIG.  24 A  is a graph of the performance of samples of an interbody device of the present disclosure in a static expulsion test conducted with simulated bone of Grade 5; 
         FIG.  24 B  is a graph of the performance of three samples of an AIS-C Cervical Stand-Alone System produced by Genesys Spine in the static expulsion test conducted with simulated bone of Grade 5; 
         FIG.  25 A  is a graph of the performance of three samples of the interbody device of the present disclosure in the static expulsion test conducted with simulated bone of Grade 15; 
         FIG.  25 B  is a graph of the performance of three samples of the AIS-C Cervical Stand-Alone System produced by Genesys Spine in the static expulsion test conducted with simulated bone of Grade 15; 
         FIG.  26 A  is a graph of the performance of three samples of the interbody device of the present disclosure in the static expulsion test conducted with simulated bone of Grade 40; and 
         FIG.  26 B  is a graph of the performance of three samples of the AIS-C Cervical Stand-Alone System produced by Genesys Spine in the static expulsion test conducted with simulated bone of Grade 40. 
     
    
    
     The drawings are not necessarily (but may be) to scale. In certain instances, details that are not necessary for an understanding of the disclosure or that render other details difficult to perceive may have been omitted. It should be understood, of course, that the disclosure is not necessarily limited to the embodiments illustrated herein. As will be appreciated, other embodiments are possible using, alone or in combination, one or more of the features set forth above or described below. For example, it is contemplated that various aspects, features and devices shown and/or described with respect to one embodiment may be combined with or substituted for aspects, features or devices of other embodiments regardless of whether or not such a combination or substitution is specifically shown or described herein. 
     The following is a listing of components according to various embodiments of the present disclosure, and as shown in the drawings: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Number 
                 Component 
               
               
                   
                   
               
             
            
               
                   
                  2 
                 Vertebrae 
               
               
                   
                  4 
                 Groove or slot 
               
               
                   
                  6 
                 Disc space 
               
               
                   
                  8 
                 Plate 
               
               
                   
                  10 
                 Plate screw 
               
               
                   
                  12 
                 Cage 
               
               
                   
                  14 
                 Cage screw 
               
               
                   
                  16 
                 Blade or shim 
               
               
                   
                  18 
                 Disc replacement device 
               
               
                   
                  20A 
                 Cranial (upper) endplate 
               
               
                   
                  20B 
                 Caudal (lower) endplate 
               
               
                   
                  22 
                 Medial element 
               
               
                   
                  24 
                 Keel 
               
               
                   
                  26 
                 Teeth 
               
               
                   
                  30 
                 Interbody device 
               
               
                   
                  32 
                 Median plane 
               
               
                   
                  34 
                 Endplate 
               
               
                   
                  34A 
                 Superior endplate 
               
               
                   
                  34B 
                 Inferior endplate 
               
               
                   
                  36 
                 Orifice in endplate 
               
               
                   
                  38 
                 Strut 
               
               
                   
                  40 
                 Dome 
               
               
                   
                  42 
                 Anterior surface 
               
               
                   
                  44 
                 Engagement feature 
               
               
                   
                  46 
                 Support 
               
               
                   
                  48 
                 Posterior surface 
               
               
                   
                  50 
                 Lateral surface 
               
               
                   
                  52 
                 Apertures in lateral surface 
               
               
                   
                  54 
                 Friction element 
               
               
                   
                  56 
                 Keel 
               
               
                   
                  58 
                 Projection 
               
               
                   
                  60 
                 Teeth 
               
               
                   
                  62 
                 Interior cavity 
               
               
                   
                 130 
                 Interbody device 
               
               
                   
                 131 
                 Coronal (or vertical) plane 
               
               
                   
                 132 
                 Median plane 
               
               
                   
                 133 
                 Transverse (or horizontal) plane 
               
               
                   
                 134 
                 Endplate 
               
               
                   
                 134A 
                 Superior endplate 
               
               
                   
                 134B 
                 Inferior endplate 
               
               
                   
                 136 
                 Orifice in endplate 
               
               
                   
                 142 
                 Anterior wall 
               
               
                   
                 143 
                 Height of anterior wall 
               
               
                   
                 144 
                 Engagement feature 
               
               
                   
                 148 
                 Posterior wall 
               
               
                   
                 149 
                 Height of posterior wall 
               
               
                   
                 150 
                 Lateral plate 
               
               
                   
                 152 
                 Apertures in lateral plate 
               
               
                   
                 154 
                 Friction element 
               
               
                   
                 156 
                 Anterior end of friction element 
               
               
                   
                 158 
                 Posterior end of friction element 
               
               
                   
                 160 
                 Teeth 
               
               
                   
                 162 
                 Interior cavity 
               
               
                   
                 164 
                 Corner 
               
               
                   
                 166 
                 Angle of friction element 
               
               
                   
                 168 
                 Angle of lateral plate 
               
               
                   
                 170 
                 Aperture of engagement feature 
               
               
                   
                 172 
                 Thread of engagement feature 
               
               
                   
                 174 
                 Maximum width 
               
               
                   
                 176 
                 Minimum width 
               
               
                   
                 178 
                 Depth of interbody device 
               
               
                   
                 180 
                 Length of a friction element 
               
               
                   
                 182 
                 First side of tooth 
               
               
                   
                 184 
                 Long side of tooth 
               
               
                   
                 186 
                 Fixed end of tooth 
               
               
                   
                 188 
                 Free end of tooth 
               
               
                   
                 190 
                 Height of tooth 
               
               
                   
                 192A 
                 First plane 
               
               
                   
                 192B 
                 Second plane 
               
               
                   
                 194 
                 Angle of endplate 
               
               
                   
                 196 
                 Maximum height of interbody device 
               
               
                   
                 198 
                 Maximum distance between teeth 
               
               
                   
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION 
     Referring now to  FIGS.  5 A- 5 D , an embodiment of an interbody device  30 A of the present disclosure is generally illustrated. The interbody device  30  includes superior and inferior endplates  34 A,  34 B, an anterior surface  42 , a posterior surface  48 , and lateral surfaces  50 . Friction elements  54  are formed on the endplates  34 . The friction elements  54  are configured to engage a superior vertebral body above the interbody device and an inferior vertebral body below the interbody device. In some embodiments, the friction elements comprise one or more of a bump, a ridge, a post, a point, a hook, a tooth, or other projections that extend from the endplates to frictionally engage a vertebral body. In this manner, the friction elements  54  prevent inadvertent or unintended movement of the interbody device  30  relative to the vertebral bodies. 
     The friction elements provide a pull out strength that is at least equal to the screws  14  or shims  16  used with cages  12 A,  12 B. One of skill in the art will appreciate that the interbody device  30  of the present disclosure can be placed in a disc space quicker and with less risk than cages that require screws for retention because no screws are required to anchor the interbody device into the vertebral bodies. Moreover, the friction provided by the friction elements  54  eliminates the need to install a plate into the superior and inferior vertebrae as required with cages. By eliminating the plate, the interbody device  30  of the present disclosure further reduces the risk to the patient and the possibility of damage to the vertebrae. For example, screws can damage the vertebrae and may weaken the vertebrae. Further, screws can become loose or move relative to the vertebrae. Eliminating the plate also reduces the time required to plan and perform the surgery which is beneficial to both the patient, the surgeon, and the medical facility, and reduces the material costs associated with the plate and screws used in the surgery. Additionally, by facilitating faster surgeries, the interbody devices of the present disclosure frees operating rooms for other procedures. 
     Optionally, one or more surface (such as an endplate  34 , the anterior surface  42 , the posterior surface  48 , and a lateral surface  50 ) of the interbody device  30  are treated to facilitate bone in-growth. In this manner, the interbody device  30  of the present disclosure can optionally be used without inserting bone graft material into the disc space. This is beneficial because bone graft material is expensive and some commercially supplied bone graft materials can lead to complications. 
     In some embodiments, a surface of the interbody device  30  has a micro-porous structure for bone ingrowth or adherence. In another embodiment, one or more of the surfaces comprises a mesh. A surface of the interbody device  30  may include a plurality of apertures such that it is between approximately 10% open and approximately 90% open. In embodiments, the superior endplate  34 A is between approximately 10% open and approximately 90% open. Additionally, or alternatively, the inferior endplate  34 B is between approximately 10% open and approximately 90% open. 
     In embodiments, the lateral surfaces  50 A,  50 B are approximately parallel. However, other configurations are contemplated in which the lateral surfaces are not parallel. The lateral surfaces  50  can include an aperture  52  to access an interior cavity  62  within the interbody device. Accordingly, the interior cavity  62  can optionally be filled with bone graft material to facilitate fusion of the adjacent vertebrae. However, the interbody device  30  can optionally be positioned in the disc space without inserting either bone graft material or a bone graft substitute within the disc space. 
     In some embodiments, the anterior surface  42  has a height of between approximately 5 mm and approximately 12 mm, or approximately 8 mm. In other embodiments, the anterior surface  42  has a width of between approximately 10 mm and approximately 22 mm, or approximately 16 mm. 
     The superior endplate  34 A is oriented in a diverging relationship to the inferior endplate  34 B in embodiments of the present disclosure. More specifically, in embodiments, the endplates  34  are not parallel and are oriented at a predetermined angle. Optionally, one or more of the endplates  34  is generally planar. 
     Referring now to  FIG.  5 C , one or more of the superior endplate  34 A and the inferior endplate  34 B extend continuously between the lateral surfaces. Additionally, or alternatively, one or more of the endplates  34  can include an orifice  36  such as generally illustrated in  FIG.  5 D . 
     Optionally, the anterior surface  42  includes an engagement feature  44 . The engagement feature  44  is adapted to interface with a surgical tool, such as an inserter, used to position the interbody device  30  within the disc space. In some embodiments, the engagement feature  44  comprises an aperture or bore in the anterior surface. The engagement feature  44  may be generally circular. Threads may optionally be formed in the engagement feature  44  to releasably connect the interbody device to the surgical tool. Other configurations and locations for the engagement feature are contemplated. More specifically, the interbody device  30  of the present disclosure can include an engagement feature  44  configured to connect to any prior art surgical tool or insertion device for use in a fusion procedure. 
     The interbody devices  30  of all embodiments of the present disclosure can be made of any suitable material. In embodiments, the interbody device  30  optionally comprises a plastic. The plastic may be a thermoplastic polymer. Optionally, the plastic is a polyether ether ketone (PEEK). Additionally, or alternatively, in other embodiments, the interbody device optionally comprises a metal. In some embodiments, the metal is titanium or a titanium alloy. 
     In embodiments, the interbody device  30  is produced as a casting. Optionally, the interbody device  30  is forged. Additionally, or alternatively, the interbody device  30  may be produced or finished by an additive manufacturing process. In some embodiments, the interbody device is produced by a 3-D printing process. In embodiment, the interbody device is treated to produce micro- and nanoscale surface roughness. For example, the interbody device  30  may be formed by casting and/or forging and the micro- or nanoscale roughness is created with a 3-D additive process. In any embodiment, the treatment of the interbody device optionally includes acid etching to promote the micro- and nanoscale surface roughness. Additionally, or alternatively, the interbody device is heat treated to promote the micro- and nanoscale surface roughness. 
     Notably, unlike the related disc replacement devices  18 , the interbody device is rigid. In embodiments, the interbody device is of a one-piece construction. For example, in some embodiments the interbody device has no moving parts or pieces. 
     Referring now to  FIGS.  6 A- 6 E , another embodiment of an interbody device  30 B of the present disclosure is generally illustrated. The interbody device  30 B can include any or all of the features of the interbody device  30 A. Moreover, the interbody device  30 B may be formed of the same or similar materials as the interbody device  30 A. 
     Notably, the friction element  54  of the interbody device includes keels  56  that extend from the endplates. Optionally, the keels  56  are generally linear. The keels  56  extend generally parallel to a median plane  32  that bisects the interbody device. As generally shown in  FIGS.  6 C,  6 E , the keels extend along a predetermined length of the endplates. In some embodiments, a keel  56  extends between about 50% and about 100% of the length of the endplate. 
     In embodiments, a keel of the present disclosure has a height of less than about 2 mm. For example, the height of the keel can be about 1.5 mm, or less. 
     Optionally, the keel  56  includes a micro-porous surface to encourage bone in-growth. 
     Referring now to  FIG.  6 D , the keels  56  optionally include projections to engage a vertebral body. The projections  58  can extend from the lateral surfaces and/or the axial surface of the keel. 
     In some embodiments, the projections are flexible. For example, the projections can be configured to bent or pivot into a first position during placement of the interbody device in the disc space. In the first position, the projections provide minimal resistance to movement in a first direction relative to the vertebral body, such as when the interbody device is pushed in a posterior direction into the disc space. Continuing this example, the projections can move to a second position to increase resistance to movement in a second direction, such as to resist movement in an axial direction out of the disc space. In embodiments, the projections are integrally formed with the keel and the interbody device  30 B. 
     In embodiments, a projection has a first end fixed to a lateral surface of a keel as generally illustrated in  FIG.  6 D . A second end of the projection extends away from the keel. One embodiment of the first position of the projections is generally illustrated on the right side of the keel in  FIG.  6 D . In the first position, the second end of the projection is positioned proximate to the lateral surface of the keel. The second position of the projections is shown according to another embodiment on the left side of  FIG.  6 D . In some embodiments, in the second position the second end of the projection is positioned distal to the lateral surface of the keel. 
     The endplates  34 A,  34 B can include any number of keels  56 . In some embodiments, the superior endplate  34 A includes 1 or 2 keels  56 A. The inferior endplate  34 B can also include 1 or 2 keels  56 B. However, in other embodiments, the inferior endplate  34 B has a different number of keels than the superior endplate. For example, in some embodiments, the superior endplate has two keels and the inferior endplate has one keel. In other embodiments, the superior endplate has one keel and the inferior endplate has two keels. 
     Other configurations of the keels are contemplated. 
     Referring now to  FIG.  6 F , in use, a groove or slot  4  can optionally be formed in the superior and inferior vertebrae  2 A,  2 B to receive the keels  56  of the interbody device  30 B. The groove  4  can be formed in any suitable manner known to those of skill in the art. In some embodiments, the keel  56  of the interbody device  30 B is adapted to cut or form a slot  4  in the vertebral body during insertion into the disc space. 
     Notably, in embodiments, the keels  56 A of the superior endplate  34 A are offset relative to the medial plane  32  from the keels  56 B of the inferior endplate  34 B. More specifically, in embodiments, the keels of the endplates  34  are not coplanar. This is beneficial when the interbody device  30  is positioned at multiple adjacent levels of a patient&#39;s spine as generally illustrated in  FIG.  6 F . Because the superior keels  56 A are offset from the inferior keels  56 B, if grooves are formed in a vertebrae between two interbody devices, the superior groove  4 A in the vertebrae will also be offset from the inferior grooves  4 B as shown in vertebrae  2 A,  2 B. Offsetting the superior and inferior grooves  4 A,  4 B in the vertebrae reduces the risk of damaging the vertebrae. 
     In contrast, the keels  24  of the disc replacement device  18  described in conjunction with  FIG.  4    are generally coplanar. Accordingly, grooves formed in vertebrae to receive the disc replacement device will also be coplanar. When a groove is formed in a superior surface of a vertebrae that is aligned (or coplanar) with a groove formed in an inferior surface of the vertebrae, the aligned grooves combine to reduce the thickness of the vertebrae in a plane. This can weaken the superior and inferior vertebrae when disc replacement devices are positioned at several levels. For example, the vertebrae may crack or fracture between two coplanar grooves. 
     Referring now to  FIG.  7   , still another embodiment of an interbody device  30 C of the present disclosure is generally illustrated. The interbody device  30 C includes many of the same, or similar, features as the interbody devices  30 A,  30 B. 
     The interbody device  30 C has a friction element  54  which comprises teeth  60 . The teeth  60  are similar to the keels  56  and can have a similar arrangement and position on the endplates  34  of the interbody device  30 C. 
     In any of the embodiments, the teeth  60  are optionally formed in a row that extends approximately parallel to a median plane  32  that bisects the interbody device. The rows extend a predetermined length of the endplates. In some embodiments, a row of teeth  60  extends at least 50% of the length of the endplate. Optionally, the row of teeth can extend from proximate to the anterior surface  42  to proximate to the posterior surface  48 . 
     The teeth  60  generally include a free end that projects away from an associated endplate. In embodiments, the teeth have a fixed end engaged with an endplate that is wider than the free end. Optionally, the free end is pointed. 
     In embodiments, the free end of one or more of the teeth is oriented toward the anterior surface  42  of the interbody device. Additionally, or alternatively, one or more of the teeth have a free end that is oriented toward the posterior surface  48  of the interbody device. 
     Optionally, the teeth in a row of teeth have free ends that are spaced two or more distances from median plane. More specifically, a first tooth in a row can have a free end that is spaced a first distance laterally from the median plane. A second tooth in the row may have a free end that is spaced a second distance laterally from the median plane. In this manner, the free ends of the teeth are not coplanar to increase pull out resistance of the interbody device. Accordingly, in some embodiments, a first tooth in a row of teeth  60  has a free end oriented toward a first lateral surface  50  of the interbody device. A second tooth in the row of teeth  60  has a free end oriented toward a second lateral surface of the interbody device. 
     In some embodiments, the teeth are rigid. Alternatively, the teeth are flexible. The teeth can be integrally formed with the interbody device  30 C. 
     Optionally, the teeth are configured to permit movement in a first direction (such as in the posterior direction when the interbody device is being positioned in the disc space) and resist movement in a second direction (for example, in the anterior direction). In embodiments, the teeth can bend or flex into a retracted positioned during movement in the first direction. Additionally, or alternatively, the teeth can bend or flex into an extended position during movement of the interbody device in the second direction. 
     The endplates  34 A,  34 B can have any number of rows of teeth  60 . In some embodiments, the superior endplate  34 A includes 1 or 2 rows of teeth  60 A. The inferior endplate  34 B can also include 1 or 2 rows of teeth  60 B. However, in embodiments, the inferior endplate  34 B has a different number of rows of teeth than the superior endplate. For example, in some embodiments, the superior endplate has two rows of teeth and the inferior endplate has one row of teeth. In other embodiments, the superior endplate has one row of teeth and the inferior endplate has two rows of teeth. Optionally, the superior endplate and the inferior endplate both have two rows of teeth. 
     In one configuration, the superior endplate  34 A has two rows of teeth  60 A that are positioned proximate to the lateral surfaces  50  of the interbody device. Alternatively, the two rows of teeth of the superior endplate are spaced from the lateral surfaces of the interbody device. 
     Optionally, the inferior endplate  34 B has two rows of teeth  60 B that are positioned proximate to the lateral surfaces  50  of the interbody device. Alternatively, the two rows of teeth of the inferior endplate are spaced from the lateral surfaces of the interbody device. 
     Similar to the keels of the interbody device  30 B, the teeth  60 A of the superior endplate  34 A are optionally offset medially from the teeth  60 B of the inferior endplate as generally illustrated in  FIG.  7 A . As described above in conjunction with  FIGS.  6 A- 6 F , this is beneficial because if grooves  4 A,  4 B are formed in superior and inferior surfaces of a vertebral body to receive rows of teeth of two interbody devices, the superior grooves  4 A will be offset from the inferior grooves  4 B. 
     Referring now to  FIG.  8   , another embodiment of an interbody device  30 D of the present disclosure is illustrated. The interbody device  30 D is similar to interbody devices  30 A- 30 C. Notably, one or more of the superior endplate  34  and the inferior endplate  34  is substantially open. 
     The interbody device  30 D includes one or more keels  56 . The keels are supported by a strut  38 . 
     Additionally, or alternatively, the interbody device  30 D can also include an anterior surface  42  and/or a posterior surface (not illustrated) that is substantially open. In some embodiments, the engagement feature  44  is fixed to a support  46 . 
     The open surfaces of the endplates and the anterior/posterior surfaces facilitate the optional packing an interior cavity  62  of the interbody device with bone graft material. 
     Yet another embodiment of an interbody device  30 E of the present disclosure is shown in  FIG.  9   . The interbody device  30 E is similar to the interbody device  30 D with one or more endplate  34 A,  34 B that is substantially open. However, the interbody device  30 E includes teeth  60  similar to the interbody device  30 C. 
     Additionally, or alternatively, the interbody device  30 E can also include an anterior surface  42  and/or a posterior surface (not illustrated) that is substantially open. In some embodiments, the engagement feature  44  is fixed to a support  46 . 
     Referring now to  FIGS.  10 - 13   , embodiments of interbody devices  30  with one or more domes  40  are generally illustrated. More specifically, embodiments of the interbody devices of the present disclosure can have a dome that extends from at least one of the superior endplate  34 A and the inferior endplate  34 B. In one configuration, the interbody devices  30  have one dome. Optionally, the interbody devices  30  have two domes  40 . 
     The domes  40  are configured to fit into a concave surface of a superior or inferior vertebral body facing a disc space. More specifically, the domes  40  are anatomically shaped to engage the concave surfaces of vertebral bodies. The dome  40  increases the surface area of the interbody device engaged with the vertebral body. In this manner, the dome  40  increases the pull-out resistance of the interbody device. 
     Referring now to  FIGS.  14 - 23   , anther embodiment of an interbody device  130  of the present disclosure is generally illustrated. The interbody device  130  is similar to other embodiments of interbody devices  30  described herein and includes many of the same or similar features. 
     The interbody device  130  is adapted for use in ACDF and ALIF procedures. It includes features to frictionally engage an upper vertebral body and a lower vertebral body. Moreover, the interbody device  130  is configured to provide sufficient friction (or pull-out resistance) once positioned in a disc space such that deployable fixation devices (e.g., screws, pins, rods, anchors, plates and the like) are not required for use with the interbody device. In some embodiments, the interbody device  130  uses static friction elements  154  to engage the adjacent vertebral bodies. Optionally, for any of the embodiments, the interbody device  130  is wholly static, meaning it does not include moving parts, deployable anchors (such as screws, pins, and the like), or interaction with screws or other fixation devices to reduce (or prevent) unintended or inadvertent movement. 
     The interbody device generally includes a superior endplate  134 A opposite to an inferior endplate  134 B, an anterior wall  142  opposite to a posterior wall  148 , lateral plates  150  extending between the endplates  134 A,  134 B, an engagement feature  144 , and friction elements  154 . In some embodiments, corners  164  between the lateral plates  150  and the anterior and posterior walls  142 ,  148  are optionally rounded with a predetermined radius of curvature. Although described herein as a superior endplate  134 A and an inferior endplate  134 B, in use the interbody device  130  can be positioned within a disc space with either endplate  134 A,  134 B oriented cranially. 
     An orifice  136  may be formed through one or more of the superior and inferior endplates  134 A,  134 B. Additionally, or alternatively, an aperture  152  may also be formed through one or more of the lateral plates  150 . The orifice  136  and/or the aperture  152  may extend to an interior cavity  162  formed within the interbody device  130 . In this manner, the interior cavity  162  can optionally be filled with bone graft material during a surgical procedure to facilitate fusion of the adjacent vertebrae. Further, after the interbody device  130  is positioned in a patient&#39;s disc space, the bone graft material can flow out of the interior cavity  162  outwardly through one or more of the orifice  136  and the aperture  152  and into the disc space. However, the interbody device  130  can optionally be positioned in a disc space without inserting either bone graft material or a bone graft substitute within the disc space or the interior cavity  162 . 
     The orifice  136  and the aperture  152  may have any size or shape. Optionally, for any of the embodiments, one or more of the orifice  136  and the aperture  152  have a shape that is generally pentagonal. The orifice and the aperture may have sizes selected to meet minimum requirements for access to the interior cavity  162 . More specifically, the orifice  136  has a size selected to maximize a surface area of the endplate  134 . As will be understood by one of skill in the art, the endplates  134  will frictionally engage superior or inferior surfaces of adjacent vertebrae and increase the pull-out strength of the interbody device. Accordingly, the orifice  136  is adapted to provide access to the interior cavity  162  without unnecessarily decreasing the surface area of the endplates  134  below a predetermined surface area. 
     Optionally, for any of the embodiments, one or more of the endplates  134 A,  134 B are adapted to facilitate osseointegration between the interbody device  130  and adjacent vertebrae. For example, one or both of the endplates  134  can include micro porous areas to allow for bone in-grow. One or more surfaces of the interbody device  130  may include a plurality of apertures such that the surface is between approximately 10% open and approximately 90% open. In embodiments, one or more of the superior endplate  134 A and the inferior endplate  134 B is between approximately 10% open and approximately 90% open. 
     Any suitable method of enabling osseointegration between the vertebrae and the interbody device  130  known to those of skill in the art may be used. In some embodiments, at least a portion of each endplate  134  is porous to facilitate bone growth and integration with boney endplate of the adjacent vertebrae. The anterior wall  142 , posterior wall  148 , and lateral plates  150  may also be adapted for osseointegration. 
     A friction element  154  extends from one or more of the endplates  134 . The friction element is configured to increase the pull-out resistance of the interbody device and to prevent or reduce unintended or inadvertent movement of the interbody device. The endplates  134 A,  134 B can have any number of friction elements  154 . In some embodiments, the interbody device has from two to six friction elements  154 . Optionally, for any of the embodiments, the friction elements  154  are static and do not move relative to the interbody device  130 . 
     In some embodiments, the superior endplate  134 A includes one or two friction elements  154 . The inferior endplate  134 B can also include one or two friction elements  154 . However, in some embodiments, the inferior endplate  134 B has a different number of friction elements  154  than the superior endplate. For example, in embodiments, the superior endplate has two friction elements  154  and the inferior endplate has one friction element  154 . In other embodiments, the superior endplate has one friction element  154  and the inferior endplate has two friction elements. Optionally, the superior endplate and the inferior endplate both have two friction elements  154 . In embodiments, the superior endplate has from two to six friction elements  154 . Additionally, or alternatively, the inferior endplate optionally includes from two to six friction elements  154 . 
     In some embodiments, the interbody device  130  has four friction elements  154 A,  154 B,  154 C and  154 D. Optionally, two friction elements  154 A,  154 B extend from the superior endplate  134 A and two friction elements  154 C,  154 D extend from the inferior endplate  134 B. 
     Referring now to  FIGS.  16  and  17   , each friction element is oriented at a predetermined angle  166  relative to a median plane  132  that bisects the interbody device  130  and that extends through the endplates  134 A,  134 B. The median plane  132  is substantially perpendicular to the plane of  FIG.  16   . Optionally, for any of the embodiments, one or more of the friction elements  154  is oriented at an oblique angle relative to the median plane  132 . In this manner, the friction elements  154  increase the force necessary to pull (or move) the interbody device in the anterior direction (or in a direction parallel to the median plane  132 ) when the interbody device is positioned in a disc space of a patient. 
     In some embodiments, the first and second friction elements  154 A,  154 B are optionally oriented at an oblique angle to each other. Additionally, or alternatively, the third friction element  154 C may optionally be oriented at an oblique angle to the fourth friction element  154 D. 
     The friction elements  154 A,  154 B of the superior endplate  134 A may be oriented such that anterior ends  156  of each friction element  154 A,  154 B are a first distance from the median plane  132 . In some embodiments, the posterior ends  158  are a second distance from the median plane that is greater than the first distance. 
     In contrast, the friction elements  154 C,  154 D of the inferior endplate  134 B can be oriented such that anterior ends  156  of each friction element  154 C,  154 D are a third distance from the median plane  132 . The posterior ends  158  of friction elements  154 C,  154 D are a fourth distance from the median plane. Optionally, in some embodiments, the third distance is greater than the fourth distance. 
     In some embodiments, the first friction element  154 A is approximately parallel to the third friction element  154 C. Additionally, or alternatively, the second friction element  154 B may be approximately parallel to the fourth friction element  154 D. 
     Referring now to  FIG.  16   , in embodiments, the first friction element  154 A is oriented at a first angle  166 A and the second friction element  154 B is oriented at a second angle  166 B with respect to the median plane  132 . In some embodiments the first and second angles are of approximately the same magnitude, but the first angle  166 A is a negative angle and the second angle  166 B is a positive angle in the perspective of  FIG.  16   . 
     The first angle  166 A of the first friction element is optionally between about −3.0° and about −5.0°, or about −3.75° relative to the median plane  132 . In some embodiments, the second angle  166 B of the second friction element is between about 3.0° and about 5.0°, or about 3.75° relative to the median plane  132 . 
     Similarly, as generally shown in  FIG.  17   , the third friction element  154 C may be oriented at a third angle  166 C and the fourth friction element  154 D can be oriented at a fourth angle  166 D relative to the median plane  132 . In some embodiments the third and fourth angles are of approximately the same magnitude, but the third angle  166 C is a positive angle and the fourth angle  166 D is a negative angle in the perspective of  FIG.  17   . 
     Optionally, the third angle  166 C of the third friction element is oriented at between about 3.0° and about 5.0°, or about 3.75° relative to the median plane  132 . In some embodiments, the fourth angle  166 D of the fourth friction element  154 D is between about −3.0° and about −5.0°, or about −3.75° relative to the median plane  132 . 
     These orientations of the friction elements  154  beneficially offset the friction elements  154 A,  154 B of the superior endplate in the vertical dimension from the friction elements  154 C,  154 D of the inferior endplate. For example, as generally illustrated in  FIGS.  18 ,  19   , the friction elements  154 A,  154 D are not coplanar. Similarly, friction elements  154 B,  154 C are not coplanar. This is beneficial because when a first interbody device  130  is positioned cranially relative to a superior surface of a vertebral body and a second interbody device  130  is positioned caudally relative to an inferior surface of the vertebral body, the friction elements will contact the vertebral body along four different planes to distribute forces applied to the vertebral body. 
     As generally illustrated in  FIGS.  16 ,  17   , the lateral plates  150 A,  150 B of the interbody device are optionally oriented at oblique angles  168  relative to the median plane  132 . Accordingly, the anterior wall  142  has a first width that is greater than a second width of the posterior wall  148 . In this manner, the interbody device  130  is configured with a geometry that approximates the shape of a disc to be replaced. In some embodiments, the first lateral plate  150 A is oriented at an angle  168 A of between about 1.20° and about 3.20°, or about 2.20° relative to the median plane  132 . Similarly, the second lateral plate  150 B may be oriented at an angle  168 B of between about −1.20° and about −3.20°, or about −2.20° relative to the median plane  132 . 
     Referring now to  FIGS.  18  and  19   , the anterior  142  and posterior  148  walls are generally illustrated. Optionally, for any of the embodiments, the anterior wall  142  is approximately parallel to the posterior wall  148 . 
     Notably, the anterior wall  142  has an engagement feature  144 . The engagement feature is adapted to interface with a surgical tool, such as an inserter, used to position the interbody device  130  within the disc space during a surgical procedure. Optionally, for any of the embodiments, the engagement feature  144  comprises an aperture or bore  170  extending at least partially into the anterior wall  142 . The aperture  170  may optionally extend through the anterior wall  141  and intersect an interior cavity  162  of the interbody device (as generally illustrated in  FIGS.  21  and  22   ). Accordingly, in some embodiments, bone graft material may be introduced into the interior cavity  162  through the aperture  170  of the engagement feature  144 . The aperture is optionally generally circular. 
     Optionally, for any of the embodiments, a thread  172  is formed in the engagement feature aperture  170  to releasably connect the interbody device  130  to the surgical tool. The aperture  170  and thread  172  may have any size and configuration. In embodiments, the aperture  170  has a diameter of between about 3 mm and about 6 mm, or about 4 mm. Additionally, or alternatively, the thread  172  optionally has a pitch of one thread per 0.7 mm. Accordingly, in some embodiments, the engagement feature  144  is configured to engage a tool with an M4×0.7 thread such as known to those of skill in the art. 
     Other configurations and locations for the engagement feature  144  are contemplated. More specifically, the interbody device  130  of the present disclosure can include an engagement feature  144  configured to connect to any prior art surgical tool or insertion device for use in a fusion procedure. 
       FIG.  18    also illustrates a maximum width  174  of the lateral plates  150  of the interbody device. The maximum width  174  is proximate to the anterior wall  142 . Optionally, for any of the embodiments, the maximum width  174  is between about 12.0 mm and about 17.0 mm, or about 14.9 mm. 
     The minimum width  176  of the lateral plates  150  is generally illustrated in  FIG.  19   . In some embodiments, the minimum width  176  is between about 11.0 mm and about 16.0 mm, or about 14.2 mm. 
     Referring now to  FIG.  20   , each friction element  154  extends a predetermined length  180 . In embodiments, the length  180  of a friction element extends at least 50% of a depth  178  of the interbody device  130 . In other embodiments, a friction element extends between about 60% and about 98% of the device depth  178 . Optionally, the length  180  of the friction element  154  is between about 9.0 mm and about 13.0 mm, or about 10.9 mm. Optionally, for any of the embodiments, the depth  178  of the interbody device is between about 10.0 mm and about 16.0 mm, or about 13.0 mm. 
     In some embodiments, the friction elements  154  comprises a keel  56  as described herein. Optionally, the friction element  154  includes a projection  58  as described herein. The projection may be flexible or otherwise moveable. 
     In embodiments, one or more of the friction elements  154  extends along a path that is substantially linear. Additionally, or alternatively, one or more of the friction elements  154  optionally extends along a path that is curved or arcuate. 
     For any of the embodiments, one or more of the friction elements  154  optionally comprises a plurality of teeth  160 . The teeth  160  are similar to the teeth  60  of other embodiments described herein. The teeth  160  of one or more of the friction elements  154  are optionally arranged in a row that generally extends from proximate the anterior wall  142  to proximate the posterior wall  148 . In embodiments, one or more of the friction elements  154  include teeth that are not arranged in a row. 
     Optionally, each friction element has from three to sixteen teeth  160 . In some embodiments, one or more of the friction elements has eight teeth  160 . 
     For any of the embodiments, the teeth  160  may optionally have a cross-sectional shape that is generally triangular. A first side  182  of a tooth extends away from an endplate  134 . A second or long side  184  of the tooth extends from the endplate and intersects the first side  182  to define a free end  188  of the tooth. A fixed end  186  of the tooth engaged with the endplate is wider than the free end  188 . In embodiments, the free end  188  defines a line, or is generally linear. Optionally, the free end is pointed. 
     In one embodiment, an edge of the first side  182  is oriented approximately parallel to a coronal plane  131 . The coronal plane  131  is oriented substantially perpendicular to the median plane  132 . Alternatively, the edge of the first side may be oriented at an oblique angle to the coronal plane  131 . 
     An edge of the long side  184  is optionally oriented at an oblique angle to the coronal plane  131 . In embodiments, the edge of the long side  184  is oriented at an angle of between about 35° and about 55°, or about 45° to the coronal plane  131 . 
     The teeth have a predetermined height  190  defined by a height of the first side  182  relative to an endplate. In some embodiments, the height  190  is between about 0.5 mm and about 2.5 mm. In another embodiment, the height  190  is about 1.2 mm. 
     In embodiments, the first side  182  of one or more of the teeth is positioned toward the anterior wall  142  of the interbody device  130 . Additionally, or alternatively, one or more of the teeth have a first side that is positioned toward the posterior wall  148  of the interbody device. 
     Notably, in some embodiments, the first sides  182  of each tooth define distinct planes. More specifically, any of the embodiments of the interbody device  130  may optionally include friction elements  154 A,  154 B,  154 C, and  154 D that are each oriented at a different angle  166  relative to the median plane  131  such that the first side  182  of a first tooth defines a first plane that is not coplanar with a first side of any other of the teeth  160 . This may be seen by comparing  FIGS.  16 ,  17  and  20   . For example,  FIG.  20    generally illustrates the second friction element  154 B oriented such that the long sides  184  of its teeth are partially visible while only one edge of the first sides  182  of the teeth of friction element  154 B are visible. In contrast, in  FIG.  20    the third friction element  154 C (illustrated below the second friction element  154 B) is shown with the first sides  182  of its teeth visible. However, only a single edge of the long sides  184  of the teeth of the third friction element  154 C are visible in  FIG.  20   . 
     Optionally, and referring again to  FIGS.  16 - 17   , the teeth  160  in a row of teeth may have free ends  188  that are spaced two or more distances from median plane  132 . More specifically, a first tooth in friction element  154 A may have a free end that is spaced a first distance laterally from the median plane. A second tooth in the friction element  154 A may have a free end that is spaced a second distance laterally from the median plane. This arrangement of the teeth increases pull out resistance of the interbody device  130 . 
     Optionally, for any of the embodiments, one or more of the teeth are rigid. Alternatively, in some embodiments, one or more of the teeth are flexible. The teeth can be integrally formed with the interbody device  130 . 
     Referring again to  FIG.  20   , the anterior wall  142  has a height  143  measured relative to the coronal plane  131  that is greater than a height  149  of the posterior wall  148 . In this manner, the shape of the interbody device  130  generally corresponds to the anterior and posterior walls of a disc to be replaced. Further, the larger height  143  of the anterior wall compared to the small height  149  of the posterior wall introduces a predetermined amount of curvature (or lordosis) between two adjacent vertebrae. In some embodiments, the height  143  of the anterior wall is between about 1.0 mm and about 2.0 mm, or about 1.4 mm greater than the height  149  of the posterior wall. 
     Because the anterior height  143  is greater than the posterior height  149 , at least portions of the endplates  134 A,  134 B are oriented at an oblique angle relative to a transverse plane  133 . The transverse plane  133  is substantially perpendicular to the median plane  132  and to the coronal plane  131 . 
     In embodiments, a first plane  192 A extending through superior ends of the anterior and posterior walls  142 ,  148  is oriented at an angle  194 A of between about 2.0° and about 4.0°, or about 3.0° relative to the transverse plane  133 . Additionally, or alternatively, a second plane  192 B extending through inferior ends of the anterior and posterior walls  142 ,  148  is oriented at an angle  194 B of between about −2.0° and about −4.0°, or about −3.0° relative to the transverse plane  133 . 
     One or more of the endplates  134 A,  134 B may also be curved or arcuate between the anterior and posterior walls  142 ,  148  as generally illustrated in  FIG.  20   . In this manner, the endplates  134  may have a “domed” or convex shape configured to generally conform to a concave surface of a superior or inferior vertebrae. In contrast, some prior art cages have linear surfaces that do not conform to the surfaces of a vertebrae. 
     Because of the convex shape of the anterior and posterior walls  142 ,  148  according to some embodiments, a maximum height  196  of the interbody device  130  occurs between the anterior and posterior walls. In embodiments, the maximum height  196  is between 4 mm and 16 mm. 
     In embodiments, the position of the maximum height  196  is closer to the anterior wall than to the posterior wall. Accordingly, a first distance between the position of the maximum height  196  and the anterior wall  142  is less than a second distance between the position of the maximum height  196  and the posterior wall  148 . 
       FIG.  20    also illustrates a maximum distance  198  between the teeth  160  of the interbody device  130 . The distance  198  is measured approximately parallel to the coronal plane  131 . In embodiments, the maximum distance  198  between teeth is between approximately 5 mm and 18 mm. 
     Referring now to  FIG.  23   , the interbody device  130  of the present disclosure may be formed with different maximum heights for use at different positions of a spine. Accordingly, a first interbody device  130 A may have a maximum height  196 A of between about 5.5 mm and about 6.0 mm. A maximum distance  198  between the teeth of the first interbody device is between 7.7 mm and 8.4 mm. The maximum height  196 B of a second interbody device  130 B is between about 6.5 mm and about 7.0 mm with a maximum distance  198  between the teeth of between 8.7 mm and 9.4 mm. A third interbody device  130 C has a maximum height  196 C of between about 7.5 mm and about 8.0 mm. The maximum distance  198  between the teeth of the third interbody device is between 9.7 mm and 10.4 mm. Another interbody device  130 D has a maximum height  196 D of between about 8.5 mm and about 9.0 mm and has a maximum distance  198  between its teeth of between 10.7 mm and 11.4 mm. The maximum height  196 F of a fifth interbody device  130 F is between about 9.5 mm and about 10.0 mm. The maximum distance  198  between the teeth of the fifth interbody device  130 F is between 11.7 mm and 12.4 mm. Other sizes of the interbody devices  130  with different maximum heights  196  and maximum distances  198  are contemplated. 
     For any of the embodiments, the interbody device  130  may optionally be made of any known material or any material developed in the future known to those of skill in the art. In some embodiments, the interbody device comprises a plastic. The plastic may be a thermoplastic polymer. Optionally, the plastic is a polyether ether ketone (PEEK). Additionally, or alternatively, in other embodiments, the interbody device  130  comprises a metal. Optionally, for any of the embodiments, the metal is titanium or a titanium alloy. The titanium alloy may optionally be Ti-6Al-4V known to those of skill in the art. 
     In embodiments, the interbody device  130  is produced by an additive manufacturing process. In some embodiments, the interbody device is produced by a 3-D printing process. 
     The interbody device is optionally treated to produce micro- and nanoscale surface roughness. In embodiments, the treatment of the interbody device includes acid etching to promote the micro- and nanoscale surface roughness. Additionally, or alternatively, the interbody device is heat treated to promote the micro- and nanoscale surface roughness. Optionally, the exterior surfaces of the interbody device  130  are finished by shot blasting with titanium shot. 
     The interbody devices  30 ,  130  of the present disclosure provide many benefits compared to cages and disc replacement devices. For example, the interbody devices  30 ,  130  frictionally engage the vertebral bodies to resist pull out and inadvertent movement. Accordingly, the interbody devices of the present disclosure are retained in a disc space without the use of screws that extend through the interbody device into the vertebral bodies. Further, due to the frictional engagement of the interbody device with the vertebral bodies, a plate is not required to be fixed to the vertebral bodies to hold the interbody device in place. 
     By eliminating the use of screws and plates, the interbody devices of the present disclosure reduce presurgical planning and materials used to perform a spinal fusion. Further, surgical time is reduced because the interbody devices can be positioned in the disc space more quickly. The surgical procedure is also simplified by eliminating the need for anchoring devices such as screws, rods, pins or anchors that must penetrate one or more vertebral body. Specifically, damage to vertebral bodies is reduced by eliminating the use of screws, rods, pins, deployable anchors and plates. Eliminating the use of secondary anchors, such as screws, pins, plates, and rods also reduces the number of instruments used during spinal fusion which decreases the risk of injury to critical structures in the patient&#39; neck or abdomen. 
     In some embodiments, the interbody devices  30 ,  130  of the present disclosure are static and do not include deployable or moveable anchors, pins, shims, or screws. In addition to reducing the number of instruments and tools used during surgery and reducing or eliminating the need to cut or penetrate a vertebral body, the static interbody devices also reduce the risk of failure of the interbody device. For example, in some prior art cages have moving parts that may inadvertently become loose or unlock and unintentionally allow the cage to move. Additionally, deployable fixation devices (such as screws, pins and anchors) may bend, shear, or break under anatomical loads. This can lead to migration of loose pieces of the fixation device in the patient&#39;s body. This requires revisional surgery to remove pieces of the fixation device as well as additional surgery to correct for potential pseudo arthrosis. In contrast, because there are no moving parts in the static interbody devices  30 ,  130  of embodiments of the present disclosure, there are no moving parts that can become loose, unlock, break, or otherwise fail. 
     The interbody devices will allow for fusion from one vertebral body to another without the use of either autologous or donor bone reducing the cost and/or risk of the surgery. The number of components of the interbody device is greatly reduced compared to cages allowing for further reduction of costs. Lastly, without the screws, deployable anchors or alternative fixation mechanism, the overall design can be more highly optimized to fit within the interbody space. 
     While various embodiments of the system have been described in detail, it is apparent that modifications and alterations of those embodiments will occur to those skilled in the art. It is to be expressly understood that such modifications and alterations are within the scope and spirit of the present disclosure. Features of one aspect or embodiment of the present disclosure may be combined with other aspects or embodiments disclosed herein. It is contemplated that various aspects, features and devices shown and/or described with respect to one embodiment may be combined with (or substituted for) aspects, features or devices of other embodiments regardless of whether or not such a combination or substitution is specifically shown or described herein. For example, the position, arrangement, and/or alignment of friction elements  54 ,  154  of embodiments of the present disclosure may be combined with any interbody device  30 ,  130  described herein. Additionally, or alternatively, a keel  56 , projection  58  and teeth  60 ,  160  can be used with any embodiment of the interbody devices  30 ,  130  of the present disclosure. 
     Further, it is to be understood that the phraseology and terminology used herein is for the purposes of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof, as well as, additional items. 
     To provide additional background, context, and to further satisfy the written description requirements of 35 U.S.C. § 112, the following references are incorporated by reference herein in their entireties: CONDUIT™ Interbody Platform EIT™ Cellular Titanium, Sales Sheet, available at https://www.jnjmedicaldevices.com/sites/default/files/user uploaded assets/pdf_assets/2019-05/Conduit%20Interbody-%20EIT%20Sales%20Sheet.pdf; U.S. Pat. Nos. 8,034,110; 8,083,799; 8,460,385; 10,098,755; 10,744,003; U.S. Pat. Pub. 2008/0269901; U.S. Pat. Pub. 2012/0277870; U.S. Pat. Pub. 2015/0305887; U.S. Pat. Pub. 2019/0105174; and PCT Pub. WO 2017/205623. 
     Example 
     Samples of three interbody devices  130  of the present disclosure were evaluated using a “static expulsion” method to determine the loads required to expulse or displace the interbody device  130  from simulated bone of varied densities. The samples were of the “6 mm” embodiment of the interbody device  130 B generally illustrated in  FIG.  23   . Each interbody device was tested three times using simulated bone of three different densities: Grade 5 (least dense), Grade 15 (medium density) and Grade 40 (most dense). 
     For comparison, the same static expulsion tests were performed using three samples of the AIS-C Cervical Stand-Alone System produced by Genesys Spine of Austin, Tex. The Genesys Spine AIS-C Cervical Stand-Alone System includes PEEK interbodies and titanium interbodies, which utilize an integrated titanium alloy locking mechanism. Both PEEK interbodies and titanium interbodies of the Genesys device are configured to be anchored to patient anatomy by two titanium alloy bone anchors. The titanium alloy cervical bone anchors provide integrated fixation for the system and are to be offered in various lengths. Notably, the bone anchors are movable and are deployed through channels in the interbody and embed into the adjacent cervical vertebrae. 
     This testing indicates that the interbody device  130  of the present disclosure performs similarly to the Genesys Spine AIS-C Cervical Stand-Alone System which has been approved for use by the U.S. Food and Drug Administration. Specifically, these results suggest that the interbody device  130  can resist pushout forces and the anticipated physiologic loads expected to be experienced after insertion in a disc space. Further, the testing shows that the interbody device  130  is operable to frictionally engage simulated bone using only static friction elements and withstand expulsion in a manner similar to the Genesys Spine AIS-C Cervical Stand-Alone System which includes deployable bone anchors. 
       FIG.  24 A  is a graph of the performance of the three samples of the interbody devices  130  in the static expulsion test conducted with simulated bone of Grade 5. Table 1, below, illustrates the results of the static expulsion test for the interbody devices  130  conducted with simulated bone of Grade 5. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Displacement 
               
               
                   
                   
                 Peak Load 
                 at Peak Load 
               
               
                   
                 Specimen 
                 (N) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Sample 1 
                 152 
                 1.91 
               
               
                   
                 Sample 2 
                 161 
                 1.23 
               
               
                   
                 Sample 3 
                 153 
                 1.44 
               
               
                   
                 Mean 
                 155 
                 1.53 
               
               
                   
                 Standard Deviation 
                 5.1 
                 0.348 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  25 A  is a graph of the performance of the three samples of the interbody devices  130  in the static expulsion test conducted with simulated bone of Grade 15. Table 2 illustrates the results of the static expulsion test for the interbody devices  130  conducted with simulated bone of Grade 15. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Displacement 
               
               
                   
                   
                 Peak Load 
                 at Peak Load 
               
               
                   
                 Specimen 
                 (N) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Sample 1 
                 189 
                 1.67 
               
               
                   
                 Sample 2 
                 212 
                 1.25 
               
               
                   
                 Sample 3 
                 206 
                 1.07 
               
               
                   
                 Mean 
                 202 
                 1.33 
               
               
                   
                 Standard Deviation 
                 11.9 
                 0.308 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  26 A  is a graph of the performance of the three samples of the interbody devices  130  in the static expulsion test conducted with simulated bone of Grade 40. Table 3 illustrates the results of the static expulsion test for the interbody devices  130  conducted with simulated bone of Grade 40. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 Displacement 
               
               
                   
                   
                 Peak Load 
                 at Peak Load 
               
               
                   
                 Specimen 
                 (N) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Sample 1 
                 982 
                 2.75 
               
               
                   
                 Sample 2 
                 1,000 
                 2.21 
               
               
                   
                 Sample 3 
                 880 
                 2.60 
               
               
                   
                 Mean 
                 954 
                 2.52 
               
               
                   
                 Standard Deviation 
                 64.7 
                 0.279 
               
               
                   
                   
               
            
           
         
       
     
     For comparison,  FIG.  24 B  is a graph of the performance of the three samples of the Genesys Spine AIS-C Cervical Stand-Alone System with simulated bone of Grade 5. Table 4 illustrates the results of the static expulsion test for the samples of the Genesys Spine AIS-C Cervical Stand-Alone System with simulated bone of Grade 5. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                   
                 Displacement 
               
               
                   
                   
                 Peak Load 
                 at Peak Load 
               
               
                   
                 Specimen 
                 (N) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Genesys sample 1 
                 125 
                 1.99 
               
               
                   
                 Genesys sample 2 
                 99 
                 1.58 
               
               
                   
                 Genesys sample 3 
                 107 
                 2.99 
               
               
                   
                 Mean 
                 110 
                 2.19 
               
               
                   
                 Standard Deviation 
                 13.3 
                 0.725 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  25 B  is a graph of the performance of the three samples of the Genesys Spine AIS-C Cervical Stand-Alone System with simulated bone of Grade 15. Table 5 illustrates the results of the static expulsion test for the samples of the Genesys Spine AIS-C Cervical Stand-Alone System with simulated bone of Grade 15. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                   
                 Displacement 
               
               
                   
                   
                 Peak Load 
                 at Peak Load 
               
               
                   
                 Specimen 
                 (N) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Genesys sample 1 
                 183 
                 1.52 
               
               
                   
                 Genesys sample 2 
                 202 
                 1.94 
               
               
                   
                 Genesys sample 3 
                 234 
                 1.43 
               
               
                   
                 Mean 
                 206 
                 1.63 
               
               
                   
                 Standard Deviation 
                 25.8 
                 0.272 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  26 B  is a graph of the performance of the three samples of the Genesys Spine AIS-C Cervical Stand-Alone System with simulated bone of Grade 40. Table 6 illustrates the results of the static expulsion test for the samples of the Genesys Spine AIS-C Cervical Stand-Alone System with simulated bone of 40. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                   
                 Displacement 
               
               
                   
                   
                 Peak Load 
                 at Peak Load 
               
               
                   
                 Specimen 
                 (N) 
                 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Genesys sample 1 
                 882 
                 3.46 
               
               
                   
                 Genesys sample 2 
                 867 
                 3.50 
               
               
                   
                 Genesys sample 3 
                 881 
                 3.47 
               
               
                   
                 Mean 
                 877 
                 3.47 
               
               
                   
                 Standard Deviation 
                 8.4 
                 0.021