Abstract:
An implant that has a predefined ratio of load bearing surface area to the overall size of the implant body and specifically to the surface area of a centrally located opening of a vertical aperture which connects to a center void area or passage defined by the implant body shape. This invention discloses a critical ratio or balance between loading of contained graft material through the hollow center or central passage of the implant and the implant&#39;s frictional load bearing or contact area with e.g., adjacent vertebral bones. Application of this invention to a spinal implant provides an improved integration and integration rate of the graft material or the fusion of the adjacent vertebral bone structures. The ratio between implant load bearing surface area and implant central opening area maximizes implant internal volume and allows a large passage disposed medial laterally to allow for radiographic verification of fusion growth.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation-in-part of pending U.S. patent application Ser. No. 12/151,198 filed on May 5, 2008 titled “Composite Interbody Spinal Implant Having Openings of Predetermined Size and Shape” which is a continuation-in-part of application Ser. No. 11/123,359, filed on May 6, 2005 titled “Anterior Interbody Spinal Implant”, now U.S. Pat. No. 7,662,186, both of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to interbody spinal implants and methods of using such implants and, more particularly, to an implant having one or more openings of predetermined sizes and shapes to achieve design trade offs depending upon a particular application. 
     BACKGROUND OF THE INVENTION 
     In the simplest terms, the spine is a column made of vertebrae and discs. The vertebrae provide the support and structure of the spine while the spinal discs, located between the vertebrae, act as cushions or “shock absorbers.” These discs also contribute to the flexibility and motion of the spinal column. Over time, the discs may become diseased or infected, may develop deformities such as tears or cracks, or may simply lose structural integrity (e.g., the discs may bulge or flatten). Impaired discs can affect the anatomical functions of the vertebrae, due to the resultant lack of proper biomechanical support, and are often associated with chronic back pain. 
     Several surgical techniques have been developed to address spinal defects, such as disc degeneration and deformity. Spinal fusion has become a recognized surgical procedure for mitigating back pain by restoring biomechanical and anatomical integrity to the spine. Spinal fusion techniques involve the removal, or partial removal, of at least one intervertebral disc and preparation of the disc space for receiving an implant by shaping the exposed vertebral endplates. An implant is then inserted between the opposing endplates. 
     Spinal fusion procedures can be achieved using a posterior or an anterior approach. Anterior interbody fusion procedures generally have the advantages of reduced operative times and reduced blood loss. Further, anterior procedures do not interfere with the posterior anatomic structure of the lumbar spine. Anterior procedures also minimize scarring within the spinal canal while still achieving improved fusion rates, which is advantageous from a structural and biomechanical perspective. These generally preferred anterior procedures are particularly advantageous in providing improved access to the disc space, and thus correspondingly better endplate preparation. 
     Several interbody implant systems have been introduced to facilitate interbody fusion. Traditional threaded implants involve at least two cylindrical bodies, each typically packed with bone graft material, surgically placed on opposite sides of the mid-sagittal plane through pre-tapped holes within the intervertebral disc space. This location is not the preferable seating position for an implant system, however, because only a relatively small portion of the vertebral endplate is contacted by these cylindrical implants. Accordingly, these implant bodies will likely contact the softer cancellous bone rather than the stronger cortical bone, or apophyseal rim, of the vertebral endplate. The seating of these threaded cylindrical implants may also compromise biomechanical integrity by reducing the area in which to distribute mechanical forces, thus increasing the apparent stress experienced by both the implant and vertebrae. Still further, a substantial risk of implant subsidence (defined as sinking or settling) into the softer cancellous bone of the vertebral body may arise from such improper seating. 
     In contrast, open ring-shaped cage implant systems are generally shaped to mimic the anatomical contour of the vertebral body. Traditional ring-shaped cages are generally comprised of allograft bone material, however, harvested from the human femur. Such allograft bone material restricts restrict the usable size and shape of the resultant implant. For example, many of these femoral ring-shaped cages generally have a medial-lateral width of less than 25 mm. Therefore, these cages may not be of a sufficient size to contact the strong cortical bone, or apophyseal rim, of the vertebral endplate. These size-limited implant systems may also poorly accommodate related instrumentation such as drivers, reamers, distractors, and the like. For example, these implant systems may lack sufficient structural integrity to withstand repeated impact and may facture during implantation. Still further, other traditional non-allograft ring-shaped cage systems may be size-limited due to varied and complex supplemental implant instrumentation which may obstruct the disc space while requiring greater exposure of the operating space. These supplemental implant instrumentation systems also generally increase the instrument load upon the surgeon. 
     The surgical procedure corresponding to an implant system should preserve as much vertebral endplate bone surface as possible by minimizing the amount of bone removed. This vertebral endplate bone surface, or subchondral bone, is generally much stronger than the underlying cancellous bone. Preservation of the endplate bone stock ensures biomechanical integrity of the endplates and minimizes the risk of implant subsidence. Thus, proper interbody implant design should provide for optimal seating of the implant while utilizing the maximum amount of available supporting vertebral bone stock. 
     Traditional interbody spinal implants generally do not seat properly on the preferred structural bone located near the apophyseal rim of the vertebral body, which is primarily composed of preferred dense subchondral bone. Accordingly, there is a need in the art for interbody spinal implants which better utilize the structurally supportive bone of the apophyseal rim. 
     In summary, at least ten, separate challenges can be identified as inherent in traditional anterior spinal fusion devices. Such challenges include: (1) end-plate preparation; (2) implant difficulty; (3) materials of construction; (4) implant expulsion; (5) implant subsidence; (6) insufficient room for bone graft; (7) stress shielding; (8) lack of implant incorporation with vertebral bone; (9) limitations on radiographic visualization; and (10) cost of manufacture and inventory. Each of these challenges is addressed in turn. 
     End-Plate Preparation 
     There are three traditional end-plate preparation methods. The first is aggressive end-plate removal with box chisel types of tools to create a nice match of end-plate geometry with implant geometry. In the process of aggressive end-plate removal, however, the end-plates are typically destroyed. Such destruction means that the load-bearing implant is pressed against soft cancellous bone and the implant tends to subside. 
     The second traditional end-plate preparation method preserves the end-plates by just removing cartilage with curettes. The end-plates are concave; hence, if a flat implant is used, the implant is not very stable. Even if a convex implant is used, it is very difficult to match the implant geometry with the end-plate geometry, as the end-plate geometry varies from patient-to-patient and on the extent of disease. 
     The third traditional end-plate preparation method uses threaded fusion cages. The cages are implanted by reaming out corresponding threads in the end-plates. This method also violates the structure. 
     2. Implant Difficulty 
     Traditional anterior spinal fusion devices can also be difficult to implant. Some traditional implants with teeth have sharp edges. These edges can bind to the surrounding soft tissue during implantation, creating surgical challenges. 
     Typically, secondary instrumentation is used to keep the disc space distracted during implantation. The use of such instrumentation means that the exposure needs to be large enough to accommodate the instrumentation. If there is a restriction on the exposure size, then the maximum size of the implant available for use is correspondingly limited. The need for secondary instrumentation for distraction during implantation also adds an additional step or two in surgery. Still further, secondary instrumentation may sometimes over-distract the annulus, reducing the ability of the annulus to compress a relatively undersized implant. The compression provided by the annulus on the implant is important to maintain the initial stability of the implant. 
     For anterior spinal surgery, there are traditionally three trajectories of implants: anterior, antero-lateral, and lateral. Each approach has its advantages and drawbacks. Sometimes the choice of the approach is dictated by surgeon preference, and sometimes it is dictated by patient anatomy and biomechanics. A typical traditional implant has design features to accommodate only one or two of these approaches in a single implant, restricting intra-operative flexibility. 
     3. Materials of Construction 
     Other challenges raised by traditional devices find their source in the conventional materials of construction. Typical devices are made of PEEK or cadaver bone. Materials such as PEEK or cadaver bone do not have the structural strength to withstand impact loads required during implantation and may fracture during implantation. 
     PEEK is an abbreviation for polyetherether-ketone, a high-performance engineering thermoplastic with excellent chemical and fatigue resistance plus thermal stability. With a maximum continuous working temperature of 480° F., PEEK offers superior mechanical properties. Superior chemical resistance has allowed PEEK to work effectively as a metal replacement in harsh environments. PEEK grades offer chemical and water resistance similar to PPS (polyphenylene sulfide), but can operate at higher temperatures. PEEK materials are inert to all common solvents and resist a wide range of organic and inorganic liquids. Thus, for hostile environments, PEEK is a high-strength alternative to fluoropolymers. 
     The use of cadaver bone has several drawbacks. The shapes and sizes of the implants are restricted by the bone from which the implant is machined. Cadaver bone carries with it the risk of disease transmission and raises shelf-life and storage issues. In addition, there is a limited supply of donor bone and, even when available cadaver bone inherently offers inconsistent properties due to its variability. Finally, as mentioned above, cadaver bone has insufficient mechanical strength for clinical application. 
     4. Implant Expulsion 
     Traditional implants can migrate and expel out of the disc space, following the path through which the implant was inserted. Typical implants are either “threaded” into place, or have “teeth” which are designed to prevent expulsion. Both options can create localized stress risers in the end-plates, increasing the chances of subsidence. The challenge of preventing implant expulsion is especially acute for PEEK implants, because the material texture of PEEK is very smooth and “slippery.” 
     5. Implant Subsidence 
     Subsidence of the implant is a complex issue and has been attributed to many factors. Some of these factors include aggressive removal of the end-plate; an implant stiffness significantly greater than the vertebral bone; smaller sized implants which tend to seat in the center of the disc space, against the weakest region of the end-plates; and implants with sharp edges which can cause localized stress fractures in the end-plates at the point of contact. The most common solution to the problem of subsidence is to choose a less stiff implant material. This is why PEEK and cadaver bone have become the most common materials for spinal fusion implants. PEEK is softer than cortical bone, but harder than cancellous bone. 
     6. Insufficient Room for Bone Graft 
     Cadaver bone implants are restricted in their size by the bone from which they are machined. Their wall thickness also has to be great to create sufficient structural integrity for their desired clinical application. These design restrictions do not leave much room for filling the bone graft material into cortical bone implants. The exposure-driven limitations on implant size narrow the room left inside the implant geometry for bone grafting even for metal implants. Such room is further reduced in the case of PEEK implants because their wall thickness needs to be greater as compared to metal implants due to structural strength needs. 
     7. Stress Shielding 
     For fusion to occur, the bone graft packed inside the implant needs to be loaded mechanically. Typically, however, the stiffness of the implant material is much greater than the adjacent vertebral bone and takes up a majority of the mechanical loads, “shielding” the bone graft material from becoming mechanically loaded. The most common solution is to choose a less stiff implant material. Again, this is why PEEK and cadaver bone have become the most common materials for spinal fusion implants. As noted above, although harder than cancellous bone, PEEK is softer than cortical bone. 
     8. Lack of Implant Incorporation with Vertebral Bone 
     In most cases, the typical fusion implant is not able to incorporate with the vertebral bone, even years after implantation. Such inability persists despite the use of a variety of different materials used to construct the implants. There is a perception that cadaver bone is resorbable and will be replaced by new bone once it resorbs. Hedrocel is a composite material composed of carbon and tantalum, an inert metal that has been used as a material for spinal fusion implants. Hedrocel is designed to allow bone in-growth into the implant. In contrast, PEEK has been reported to become surrounded by fibrous tissue which precludes it from incorporating with surrounding bone. There have also been reports of the development of new bio-active materials which can incorporate into bone. The application of such bio-active materials has been limited, however, for several reasons, including biocompatibility, structural strength, and lack of regulatory approval. 
     9. Limitations on Radiographic Visualization 
     For implants made out of metal, the metal prevents adequate radiographic visualization of the bone graft. Hence it is difficult to assess fusion, if it is to take place. PEEK is radiolucent. Traditional implants made of PEEK need to have radiographic markers embedded into the implants so that implant position can be tracked on an X-ray. Cadaver bone has some radiopacity and does not interfere with radiographic assessment as much as metal implants. 
     10. Cost of Manufacture and Inventory 
     The requirements of spinal surgery dictate that manufacturers provide implants of various foot-prints, and several heights in each foot-print. This requirement means that the manufacturer needs to carry a significant amount of inventory of implants. Because there are so many different sizes of implants, there are setup costs involved in the manufacture of each different size. The result is increased implant costs, which the manufacturers pass along to the end users by charging high prices for spinal fusion implants. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to interbody spinal implants and to methods of using such implants. The implants can be inserted, using methods of the present invention, from a variety of vantages, including anterior, anterolateral, and lateral implantation. Certain embodiments of the present invention provide an anatomically shaped spinal implant for improved seating in the disc space, particularly in the mediallateral aspect of the disc space, and improved utilization of the vertebral apophyseal rim. Certain embodiments of the present invention further have a highly radiused posterior portion and sides which allow for ease of implantation. Thus, the posterior portion may have a generally blunt nosed profile. Certain embodiments also allow for improved visualization of the disc space during surgical procedures while minimizing exposure of the operating space. Certain aspects of the invention reduce the need for additional instrumentation—such as chisels, reamers, or other tools—to prepare the vertebral endplate, thus minimizing the instrument load upon the surgeon. 
     Certain embodiments of the interbody implant are substantially hollow and have a generally oval-shaped transverse cross-sectional area. Substantially hollow, as used in this document, means at least about 33% of the interior volume of the interbody spinal implant is vacant. Further embodiments of the present invention include a body having a top surface, a bottom surface, opposing lateral sides, and opposing anterior and posterior portions. The implant includes at least one aperture that extends the entire height of the body. Thus, the aperture extends from the top surface to the bottom surface. The implant may further include at least one aperture that extends the entire transverse length of the implant body. 
     Still further, the substantially hollow portion may be filled with cancellous autograft bone, allograft bone, demineralized bone matrix (DBM), porous synthetic bone graft substitute, bone morphogenic protein (BMP), or combinations of those materials. The implant further includes a roughened surface topography on at least a portion of its top surface, its bottom surface, or both surfaces. The anterior portion, or trailing edge, of the implant is preferably generally greater in height than the opposing posterior portion, or leading edge. In other words, the trailing edge is taller than the leading edge. The posterior portion and lateral sides may also be generally smooth and highly radiused, thus allowing for easier implantation into the disc space. Thus, the posterior portion may have a blunt nosed profile with large radii in the transitional areas from the top surface to the outside walls and nose portion. The anterior portion of the implant may preferably be configured to engage a delivery device, a driver, or other surgical tools. The anterior portion may also be substantially flat. 
     According to certain embodiments, the present invention provides an interbody spinal implant including a body having a top surface, a bottom surface, opposing lateral sides, opposing anterior and posterior portions, a substantially hollow center, and a single vertical aperture. The single vertical aperture extends from the top surface to the bottom surface, has a size and shape predetermined to maximize the surface area of the top surface and the bottom surface available proximate the anterior and posterior portions while maximizing both radiographic visualization and access to the substantially hollow center, and defines a transverse rim. The body may comprise metal or be non-metallic and may form one component of a composite implant; the other component is a metal plate disposed on at least one of the top and bottom surfaces of the body. 
     According to a further embodiment, there exists a critical ratio between implant load bearing external working surface area (A) and the surface area of the implant&#39;s hollow center opening (O). It is to be understood by this terminology that the surface area of the opening is the area of a plan, the perimeter of which is defined by the adjacent implant structure. Specifically, expressing the implant load bearing surface area in mm 2  and the surface area of hollow center or the central opening or in mm 2 , the ratio of area to volume (A/O) should fall in the range of 15% to 50%, preferably 35% to 50% and most preferably 40% to 48%. 
     Application of the critical A/O ratio of this invention to an interbody spinal implant obtains implant integration, i.e., stronger or more robust, in a shorter period of time. Thus, surgical outcome success rate is surprisingly and unexpectedly increased. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
         FIG. 1  shows a perspective view of a first embodiment of the interbody spinal implant having a generally oval shape and roughened surface topography on the top surface; 
         FIG. 2  depicts a top view of the first embodiment of the interbody spinal implant; 
         FIG. 3  depicts an anterior view of the first embodiment of the interbody spinal implant; 
         FIG. 4  depicts a posterior view of the first embodiment of the interbody spinal implant; 
         FIG. 5A  depicts a first post-operative radiograph showing visualization of an embodiment of the interbody spinal implant; 
         FIG. 5B  depicts a second post-operative radiograph showing visualization of an embodiment of the interbody spinal implant; 
         FIG. 5C  depicts a third post-operative radiograph showing visualization of an embodiment of the interbody spinal implant; 
         FIG. 6  shows an exemplary surgical tool (implant holder) to be used with certain embodiments of the interbody spinal implant; 
         FIG. 7  shows an exemplary distractor used during certain methods of implantation; 
         FIG. 8  shows an exemplary rasp used during certain methods of implantation; 
         FIG. 9  shows a perspective view from the front of another embodiment of the interbody spinal implant according to the present invention; 
         FIG. 10  shows a perspective view from the rear of the embodiment of the interbody spinal implant illustrated in  FIG. 9 ; 
         FIG. 11  is a top view of the interbody spinal implant illustrated in  FIGS. 9 and 10 ; 
         FIG. 12  shows a perspective view from the rear, like  FIG. 10 , of the interbody spinal implant illustrated in  FIGS. 9-11  highlighting an alternative transverse aperture; 
         FIG. 13  shows a perspective view from the front of yet another embodiment of the interbody spinal implant according to the present invention; 
         FIG. 14  is a top view of the interbody spinal implant illustrated in  FIG. 13 ; 
         FIG. 15  shows a perspective view from the rear of the embodiment of the interbody spinal implant illustrated in  FIG. 13  highlighting an alternative transverse aperture; 
         FIG. 16  shows a perspective view from the side of one component of a composite embodiment of the interbody spinal implant; 
         FIG. 17  is a top view of the composite embodiment of the interbody spinal implant illustrated in  FIG. 16  with the components attached; 
         FIG. 18  shows an exemplary mechanism by which the two components of the composite embodiment of the interbody spinal implant illustrated in  FIGS. 16 and 17  may be attached; 
         FIG. 19  shows a perspective view of a final embodiment of the interbody spinal implant having a generally oval shape and being especially well adapted for use in a cervical spine surgical procedure; and 
         FIG. 20  shows a perspective view of the final implant having a generally box shape. 
         FIG. 21  shows external surface or working surfaces  500 , and  502  and an central opening surface area or hole  510  and  512  of two devices of this invention; 
         FIGS. 22A-22E  show ALIF, Cervical, TLIF, TO and TAS implants according to this invention. 
         FIGS. 23A and 23B  show anterior and top or “cranial” perspective views of a TA (and TAS) implant of this invention, as deployed. 
         FIGS. 24A and 24B  show anterior and cranial perspective views of a TC implant of this invention, as deployed. 
         FIGS. 25A and 25B  show anterior (posterior insertion) and cranial perspective views of a TT implant of this invention, as deployed. 
         FIGS. 26A and 26B  show anterior (posterior insertion) and cranial perspective views of a TO (PLIF) implant of this invention, as deployed. 
         FIGS. 27A and 27B  show anterior and cranial views of a TO (oblique) implant of this invention, as deployed. 
         FIG. 28  illustrates the contact surface or face  520 , axial column  522  (central opening surface area) and foot print  524  as used to compute the numbers shown herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain embodiments of the present invention may be especially suited for placement between adjacent human vertebral bodies. The implants of the present invention may be used in procedures such as Anterior Lumbar Interbody Fusion (ALIF), Posterior Lumbar Interbody Fusion (PLIF), Transforaminal Lumbar Interbody Fusion (TLIF), and cervical fusion. Certain embodiments do not extend beyond the outer dimensions of the vertebral bodies. 
     The ability to achieve spinal fusion is directly related to the available vascular contact area over which fusion is desired, the quality and quantity of the fusion mass, and the stability of the interbody spinal implant. Interbody spinal implants, as now taught, allow for improved seating over the apophyseal rim of the vertebral body. Still further, interbody spinal implants, as now taught, better utilize this vital surface area over which fusion may occur and may better bear the considerable biomechanical loads presented through the spinal column with minimal interference with other anatomical or neurological spinal structures. Even further, interbody spinal implants, according to certain aspects of the present invention, allow for improved visualization of implant seating and fusion assessment. Interbody spinal implants, as now taught, may also facilitate osteointegration with the surrounding living bone. 
     Anterior interbody spinal implants in accordance with certain aspects of the present invention can be preferably made of a durable material such as stainless steel, stainless steel alloy, titanium, or titanium alloy, but can also be made of other durable materials such as, but not limited to, polymeric, ceramic, and composite materials. For example, certain embodiments of the present invention may be comprised of a biocompatible, polymeric matrix reinforced with bioactive fillers, fibers, or both. Certain embodiments of the present invention may be comprised of urethane dimethacrylate (DUDMA)/tri-ethylene glycol dimethacrylate (TEDGMA) blended resin and a plurality of fillers and fibers including bioactive fillers and E-glass fibers. Durable materials may also consist of any number of pure metals, metal alloys, or both. Titanium and its alloys are generally preferred for certain embodiments of the present invention due to their acceptable, and desirable, strength and biocompatibility. In this manner, certain embodiments of the present interbody spinal implant may have improved structural integrity and may better resist fracture during implantation by impact. Interbody spinal implants, as now taught, may therefore be used as a distractor during implantation. 
     Referring now to the drawing, in which like reference numbers refer to like elements throughout the various figures that comprise the drawing,  FIG. 1  shows a perspective view of a first embodiment of the interbody spinal implant  1  especially well adapted for use in an ALIF procedure. The interbody spinal implant  1  includes a body having a top surface  10 , a bottom surface  20 , opposing lateral sides  30 , and opposing anterior  40  and posterior  50  portions. One or both of the top surface  10  and the bottom surface  20  has a roughened topography  80 . Distinguish the roughened topography  80 , however, from the disadvantageous teeth provided on the surfaces of some conventional devices. 
     Certain embodiments of the interbody spinal implant  1  are substantially hollow and have a generally ovalshaped transverse cross-sectional area with smooth, rounded, or both smooth and rounded lateral sides and posterior-lateral corners. As used in this document, “substantially hollow” means at least about 33% of the interior volume of the interbody spinal implant  1  is vacant. The implant  1  includes at least one vertical aperture  60  that extends the entire height of the implant body. As illustrated in the top view of  FIG. 2 , the vertical aperture  60  further defines a transverse rim  100  having a greater posterior portion thickness  55  than an anterior portion thickness  45 . 
     In at least one embodiment, the opposing lateral sides  30  and the anterior portion  40  have a rim thickness of about 5 mm, while the posterior portion  50  has a rim thickness of about 7 mm. Thus, the rim posterior portion thickness  55  may allow for better stress sharing between the implant  1  and the adjacent vertebral endplates and helps to compensate for the weaker posterior endplate bone. In certain embodiments, the transverse rim  100  has a generally large surface area and contacts the vertebral endplate. The transverse rim  100  may act to better distribute contact stresses upon the implant  1 , and hence minimize the risk of subsidence while maximizing contact with the apophyseal supportive bone. It is also possible for the transverse rim  100  to have a substantially constant thickness (i.e., for the anterior portion thickness  45  to be substantially the same as the posterior portion thickness  55 ) or, in fact, for the posterior portion  50  to have a rim thickness less than that of the opposing lateral sides  30  and the anterior portion  40 . Some studies have challenged the characterization of the posterior endplate bone as weaker. 
     It is generally believed that the surface of an implant determines its ultimate ability to integrate into the surrounding living bone. Without being limited by theory, it is hypothesized that the cumulative effects of at least implant composition, implant surface energy, and implant surface roughness play a major role in the biological response to, and osteointegration of, an implant device. Thus, implant fixation may depend, at least in part, on the attachment and proliferation of osteoblasts and like-functioning cells upon the implant surface. Still further, it appears that these cells attach more readily to relatively rough surfaces rather than smooth surfaces. In this manner, a surface may be bioactive due to its ability to facilitate cellular attachment and osteointegration. The surface roughened topography  80  may better promote the osteointegration of certain embodiments of the present invention. The surface roughened topography  80  may also better grip the vertebral endplate surfaces and inhibit implant migration upon placement and seating. 
     Accordingly, the implant  1  further includes the roughened topography  80  on at least a portion of its top and bottom surfaces  10 ,  20  for gripping adjacent bone and inhibiting migration of the implant  1 . The roughened topography  80  may be obtained through a variety of techniques including, without limitation, chemical etching, shot peening, plasma etching, laser etching, or abrasive blasting (such as sand or grit blasting). In at least one embodiment, the interbody spinal implant  1  may be comprised of titanium, or a titanium alloy, having the surface roughened topography  80 . The surfaces of the implant  1  are preferably bioactive. 
     In a preferred embodiment of the present invention, the roughened topography  80  is obtained via the repetitive masking and chemical or electrochemical milling processes described in U.S. Pat. No. 5,258,098; No. 5,507,815; No. 5,922,029; and No. 6,193,762. Each of these patents is incorporated in this document by reference. Where the invention employs chemical etching, the surface is prepared through an etching process which utilizes the random application of a maskant and subsequent etching of the metallic substrate in areas unprotected by the maskant. This etching process is repeated a number of times as necessitated by the amount and nature of the irregularities required for any particular application. Control of the strength of the etchant material, the temperature at which the etching process takes place, and the time allotted for the etching process allow fine control over the resulting surface produced by the process. The number of repetitions of the etching process can also be used to control the surface features. 
     By way of example, an etchant mixture of nitric acid (HN03) and hydrofluoric (HF) acid may be repeatedly applied to a titanium surface to produce an average etch depth of about 0.53 mm. Interbody spinal implants, in accordance with preferred embodiments of the present invention, may be comprised of titanium, or a titanium alloy, having an average surface roughness of about 100 um. Surface roughness may be measured using a laser profilometer or other standard instrumentation. 
     In another example, chemical modification of the titanium implant surfaces can be achieved using HF and a combination of hydrochloric acid and sulfuric acid (HQ/H2S04). In a dual acid etching process, the first exposure is to HF and the second is to HCl/H2S04. Chemical acid etching alone of the titanium implant surface has the potential to greatly enhance osteointegration without adding particulate matter (e.g., hydroxyapatite) or embedding surface contaminants (e.g., grit particles). 
     Certain embodiments of the implant  1  are generally shaped to reduce the risk of subsidence, and improve stability, by maximizing contact with the apophyseal rim of the vertebral endplates. Embodiments may be provided in a variety of anatomical footprints having a medial-lateral width ranging from about 32 mm to about 44 mm. Interbody spinal implants, as now taught, generally do not require extensive supplemental or obstructive implant instrumentation to maintain the prepared disc space during implantation. Thus, the interbody spinal implant  1  and associated implantation methods, according to presently preferred aspects of the present invention, allow for larger sized implants as compared with the size-limited interbody spinal implants known in the art. This advantage allows for greater medial-lateral width and correspondingly greater contact with the apophyseal rim. 
       FIG. 3  depicts an anterior view, and  FIG. 4  depicts a posterior view, of an embodiment of the interbody spinal implant  1 . As illustrated in  FIGS. 1 and 3 , the implant  1  has an opening  90  in the anterior portion  40 . As illustrated in  FIGS. 3 and 4 , in one embodiment the posterior portion  50  has a similarly shaped opening  90 . In another embodiment, as illustrated in  FIG. 1 , only the anterior portion  40  has the opening  90  while the posterior portion  50  has an alternative opening  92  (which may have a size and shape different from the opening  90 ). 
     The opening  90  has a number of functions. One function is to facilitate manipulation of the implant  1  by the caretaker. Thus, the caretaker may insert a surgical tool into the opening  90  and, through the engagement between the surgical tool and the opening  90 , manipulate the implant  1 . The opening  90  may be threaded to enhance the engagement. 
       FIG. 6  shows an exemplary surgical tool, specifically an implant holder  2 , to be used with certain embodiments of the interbody spinal implant  1 . Typically, the implant holder  2  has a handle  4  that the caretaker can easily grasp and an end  6  that engages the opening  90 . The end  6  may be threaded to engage corresponding threads in the opening  90 . The size and shape of the opening  90  can be varied to accommodate a variety of tools. Thus, although the opening  90  is substantially square as illustrated in  FIGS. 1 ,  3 , and  4 , other sizes and shapes are feasible. 
     The implant  1  may further include at least one transverse aperture  70  that extends the entire transverse length of the implant body. As shown in  FIGS. 5A-5C , these transverse apertures  70  may provide improved visibility of the implant  1  during surgical procedures to ensure proper implant placement and seating, and may also improve post-operative assessment of implant fusion. Still further, the substantially hollow area defined by the implant  1  may be filled with cancellous autograft bone, allograft bone, DBM, porous synthetic bone graft substitute, BMP, or combinations of these materials (collectively, bone graft materials), to facilitate the formation of a solid fusion column within the spine of a patient. 
     The anterior portion  40 , or trailing edge, of the implant  1  is preferably generally greater in height than the opposing posterior portion  50 . Accordingly, the implant  1  may have a lordotic angle to facilitate sagittal alignment. The implant  1  may better compensate, therefore, for the generally less supportive bone found in the posterior regions of the vertebral endplate. The posterior portion  50  of the interbody implant  1 , preferably including the posterior-lateral corners, may also be highly radiused, thus allowing for ease of implantation into the disc space. Thus, the posterior portion  50  may have a generally blunt nosed profile. The anterior portion  40  of the implant  1  may also preferably be configured to engage a delivery device, driver, or other surgical tool (and, therefore, may have an opening  90 ). 
     As illustrated in  FIG. 1 , the anterior portion  40  of the implant  1  is substantially flat. Thus, the anterior portion  40  provides a face that can receive impact from a tool, such as a surgical hammer, to force the implant  1  into position. The implant  1  has a sharp edge  8  where the anterior portion  40  meets the top surface  10 , where the anterior portion  40  meets the bottom surface  20 , or in both locations. The sharp edge or edges  8  function to resist pullout of the implant  1  once it is inserted into position. 
     Certain embodiments of the present invention are particularly suited for use during interbody spinal implant procedures (or vertebral body replacement procedures) and may act as a final distractor during implantation, thus minimizing the instrument load upon the surgeon. For example, in such a surgical procedure, the spine may first be exposed via an anterior approach and the center of the disc space identified. The disc space is then initially prepared for implant insertion by removing vertebral cartilage. Soft tissue and residual cartilage may then also be removed from the vertebral endplates. 
     Vertebral distraction may be performed using trials of various-sized embodiments of the interbody spinal implant  1 . The determinatively sized interbody implant  1  may then be inserted in the prepared disc space for final placement. The distraction procedure and final insertion may also be performed under fluoroscopic guidance. The substantially hollow area within the implant body may optionally be filled, at least partially, with bone fusion-enabling materials such as, without limitation, cancellous autograft bone, allograft bone, DBM, porous synthetic bone graft substitute, BMP, or combinations of those materials. Such bone fusion-enabling material may be delivered to the interior of the interbody spinal implant  1  using a delivery device mated with the opening  90  in the anterior portion  40  of the implant  1 . Interbody spinal implants  1 , as now taught, are generally larger than those currently known in the art, and therefore have a correspondingly larger hollow area which may deliver larger volumes of fusion-enabling bone graft material. The bone graft material may be delivered such that it fills the full volume, or less than the full volume, of the implant interior and surrounding disc space appropriately. 
     As noted above,  FIG. 1  shows a perspective view of one embodiment of the present invention, the interbody spinal implant  1 , which is especially well adapted for use in an ALIF procedure. Other embodiments of the present invention are better suited for PLIF, TLIF, or cervical fusion procedures. Specifically,  FIGS. 9 and 10  show perspective views, from the front and rear, respectively, of an embodiment of an interbody spinal implant  101  especially well adapted for use in a PLIF procedure. The interbody spinal implant  101  includes a body having a top surface  110 , a bottom surface  120 , opposing lateral sides  130 , and opposing anterior  140  and posterior  150  portions. One or both of the top surface  110  and the bottom surface  120  has a roughened topography  180  for gripping adjacent bone and inhibiting migration of the implant  101 . 
     Certain embodiments of the interbody spinal implant  101  are substantially hollow and have a generally rectangular shape with smooth, rounded, or both smooth and rounded lateral sides and anterior-lateral corners. As best shown in  FIG. 10 , the anterior portion  140  may have a tapered nose  142  to facilitate insertion of the implant  101 . To further facilitate insertion, the implant  101  has chamfers  106  at the corners of its posterior portion  150 . The chamfers  106  prevent the implant  101  from catching upon insertion, risking potential damage such as severed nerves, while still permitting the implant  101  to have a sharp edge  108 . 
     As illustrated in  FIG. 9 , the posterior portion  150  of the implant  101  is substantially flat. Thus, the posterior portion  150  provides a face that can receive impact from a tool, such as a surgical hammer, to force the implant  101  into position. The implant  101  has a sharp edge  108  between the chamfers  106  where the posterior portion  150  meets the top surface  110 , where the posterior portion  150  meets the bottom surface  120 , or in both locations. The sharp edge or edges  108  function to resist pullout of the implant  101  once it is inserted into position. 
     The implant  101  includes at least one vertical aperture  160  that extends the entire height of the implant body. As illustrated in the top view of  FIG. 11 , the vertical aperture  160  further defines a transverse rim  200 . The size and shape of the vertical aperture  160  are carefully chosen to achieve a preferable design trade off for the particular application envisioned for the implant  101 . Specifically, the vertical aperture  160  seeks to maximize the surface area of the top surface  110  and the bottom surface  120  available proximate the anterior  140  and posterior  150  portions while maximizing both radiographic visualization and access to the bone graft material toward the center of the top  110  and bottom  120  surfaces. Thus, the size and shape of the vertical aperture  160  are predetermined by the application. By “predetermined” is meant determined beforehand, so that the predetermined size and shape are determined, i.e., chosen or at least known, before the implant  101  is selected for insertion. 
     In the particular example shown in  FIGS. 9-11 , the width of the implant  101  between the two lateral sides  130  is approximately 9 mm. The shape of the vertical aperture  160  approximates, in cross section, that of an American football. The center of the vertical aperture  160 , which defines the maximum width of the vertical aperture  160 , is about 5 mm. Thus, the rim thickness  200  on either side of the vertical aperture  160  adjacent the center of the vertical aperture  160  is about 2 mm. These dimensions permit ample engagement between the bone graft material contained within the implant  101  and bone. 
     The vertical aperture  160  tapers from its center to its ends along a longitudinal distance of about 7.75 mm (thus, the total length of the vertical aperture  160  is about 15.5 mm). This shape leaves intact much of the rim thickness  200  in the areas around the ends of the vertical aperture  160 . These areas may allow for better stress sharing between the implant  101  and the adjacent vertebral endplates. Thus, the transverse rim  200  has a generally large surface area and contacts the vertebral endplate. 
     As illustrated in  FIG. 9 , the implant  101  has an opening  190  in the posterior portion  150 . The opening  190  has a number of functions. One function is to facilitate manipulation of the implant  101  by the caretaker. Thus, the caretaker may insert a surgical tool ( FIG. 6  shows an exemplary surgical tool, the implant holder  2 ) into the opening  190  and, through the engagement between the surgical tool and the opening  190 , manipulate the implant  101 . The opening  190  may be threaded to enhance the engagement. 
     The implant  101  may also have an Implant Holding Feature (IHF)  194  instead of or in addition to the opening  190 . As illustrated in  FIG. 9 , the IHF  194  is located proximate the opening  190  in the posterior portion  150 . In this particular example, the IHF  194  is a U-shaped notch. Like the opening  190 , the IHF  194  has a number of functions, one of which is to facilitate manipulation of the implant  101  by the caretaker. Other functions of the opening  190  and the IHF  194  are to increase visibility of the implant  101  during surgical procedures and to enhance engagement between bone graft material and adjacent bone. 
     The implant  101  may further include at least one transverse aperture  170 . Like the vertical aperture  160 , the size and shape of the transverse aperture  170  are carefully chosen (and predetermined) to achieve a preferable design trade off for the particular application envisioned for the implant  101 . Specifically, the transverse aperture  170  should have minimal dimensions to maximize the strength and structural integrity of the implant  101 . On the other hand, the transverse aperture  70  should have maximum dimensions to (a) improve the visibility of the implant  101  during surgical procedures to ensure proper implant placement and seating, and to improve post-operative assessment of implant fusion, and (b) to facilitate engagement between bone graft material and adjacent bone. The substantially hollow area defined by the implant  101  may be filled with bone graft materials to facilitate the formation of a solid fusion column within the spine of a patient. 
     As shown in  FIGS. 9 and 10 , the transverse aperture  170  extends the entire transverse length of the implant body and nearly the entire height of the implant body. Thus, the size and shape of the transverse aperture  170  approach the maximum possible dimensions for the transverse aperture  170 . Like  FIG. 10 ,  FIG. 12  shows a perspective view from the rear of the interbody spinal implant  101 .  FIG. 12  highlights, however, an alternative transverse aperture  170 . 
     As illustrated in  FIG. 12 , the transverse aperture  170  is broken into two, separate sections by an intermediate wall  172 . Thus, the dimensions of the transverse aperture  170  shown in  FIG. 12  are much smaller than those for the transverse aperture  170  shown in  FIG. 10 . The section of the transverse aperture  170  proximate the IHF  194  is substantially rectangular in shape; the other section of the transverse aperture  170  has the shape of a curved arch. Other shapes and dimensions are suitable for the transverse aperture  170 . In particular, all edges of the transverse aperture  170  may be rounded, smooth, or both. The intermediate wall  172  may be made of the same material as the remainder of the implant  101  (e.g., metal), or it may be made of another material (e.g., PEEK) to form a composite implant  101 . The intermediate wall  172  may offer one or more of several advantages, including reinforcement of the implant  101  and improved bone graft containment. 
     The embodiment of the present invention illustrated in  FIGS. 9-12  is especially well suited for a PLIF surgical procedure. TLIF surgery is done through the posterior (rear) part of the spine and is essentially like an extended PLIF procedure. The TLIF procedure was developed in response to some of the technical problems encountered with a PLIF procedure. The main difference between the two spine fusion procedures is that the TLIF approach to the disc space is expanded by removing one entire facet joint; a PLIF procedure is usually done on both sides by only taking a portion of each of the paired facet joints. 
     By removing the entire facet joint, visualization into the disc space is improved and more disc material can be removed. Such removal should also provide for less nerve retraction. Because one entire facet is removed, the TLIF procedure is only done on one side: removing the facet joints on both sides of the spine would result in too much instability. With increased visualization and room for dissection, one or both of a larger implant and more bone graft can be used in the TLIF procedure. Theoretically, these advantages can allow the spine surgeon to distract the disc space more and realign the spine better (re-establish the normal lumbar lordosis). 
     Although the TLIF procedure offers some improvements over a PLIF procedure, the anterior approach in most cases still provides the best visualization, most surface area for healing, and the best reduction of any of the approaches to the disc space. These advantages must be weighed, however, against the increased morbidity (e.g., unwanted aftereffects and postoperative discomfort) of a second incision. Probably the biggest determinate in how the disc space is approached is the comfort level that the spine surgeon has with an anterior approach for the spine fusion surgery. Not all spine surgeons are comfortable with operating around the great vessels (aorta and vena cava) or have access to a skilled vascular surgeon to help them with the approach. Therefore, choosing one of the posterior approaches for the spine fusion surgery is often a more practical solution. 
     The embodiment of the present invention illustrated in  FIGS. 13-15  is especially well suited when the spine surgeon elects a TLIF procedure. Many of the features of the implant  101   a  illustrated in  FIGS. 13-15  are the same as those of the implant  101  illustrated in  FIGS. 9-12 . Therefore, these features are given the same reference numbers, with the addition of the letter “a,” and are not described further. 
     There are several differences, however, between the two embodiments. For example, unlike the substantially rectangular shape of the implant  101 , the implant  101   a  has a curved shape. Further, the chamfers  106  and sharp edges  108  of the implant  101  are replaced by curves or rounded edges for the implant  101   a . Still further, the TLIF procedure often permits use of a larger implant  101   a  which, in turn, may affect the size and shape of the predetermined vertical aperture  160   a.    
     The effect of the larger (relative to the implant  101 ) implant  101   a  is shown in  FIG. 14 , which illustrates a top view of the implant  101   a . The substantially constant 9 mm width of the transverse rim  200  of the implant  101  is replaced with a larger, curved transverse rim  200   a . The width of the transverse rim  200   a  is 9 mm in the regions adjacent the anterior  140   a  and posterior  150   a  portions. That width gradually increases to 11 mm, however, near the center of the transverse rim  200   a . The additional real estate provided by the transverse rim  200   a  (relative to the transverse rim  200 ) allows the shape of the vertical aperture  160   a  to change, in cross section, from approximating a football to approximating a boomerang. Maintaining the thickness of the transverse rim  200   a  on either side of the vertical aperture  160   a  adjacent the center of the vertical aperture  160   a  at about 2 mm, similar to the dimensions of the implant  101 , the center of the vertical aperture  160   a , which defines the maximum width of the vertical aperture  160   a , is increased (from 5 mm for the implant  101 ) to about 7 mm. 
     The implant  101   a  may also have a lordotic angle to facilitate alignment. As illustrated in  FIG. 14 , the lateral side  130   a  depicted at the top of the implant  101   a  is preferably generally greater in height than the opposing lateral side  130   a . Therefore, the implant  101   a  may better compensate for the generally less supportive bone found in certain regions of the vertebral endplate. 
     As shown in  FIG. 13 , the transverse aperture  170   a  extends the entire transverse length of the implant body and nearly the entire height of the implant body.  FIG. 15  highlights an alternative transverse aperture  170   a . As illustrated in  FIG. 15 , the transverse aperture  170   a  is broken into two, separate sections by an intermediate wall  172   a . Thus, the dimensions of the transverse aperture  170   a  shown in  FIG. 15  are much smaller than those for the transverse aperture  170   a  shown in  FIG. 13 . The two sections of the alternative transverse aperture  170   a  are each illustrated as substantially rectangular in shape and extending nearly the entire height of the implant body; other sizes and shapes are possible for one or both sections of the alternative transverse aperture  170   a.    
     The intermediate wall  172   a  may be made of the same material as the remainder of the implant  101   a  (e.g., metal), or it may be made of another material (e.g., PEEK) to form a composite implant  101   a . It is also possible to extend the intermediate wall  172   a , whether made of metal, PEEK, ultra-high molecular weight polyethylene (UHMWPE), or another material, to eliminate entirely the transverse aperture  170   a . Given the reinforcement function of the intermediate wall  172   a , the length of the vertical aperture  160   a  can be extended (as shown in  FIG. 15 ) beyond the top surface  110   a  and into the anterior portion  140   a  of the implant  101   a.    
     Also important is that the top surface  110   a  of the implant  101   a  shown in  FIG. 14  differs from the top surface  110   a  of the implant  101   a  shown in  FIGS. 13 and 15  in that the former does not include the roughened topography  180   a  of the latter. This difference permits the implant  101   a , at least for certain applications, to be made entirely of a non-metal material. Suitable materials of construction for the implant  101   a  of such a design (which would not be a composite) include PEEK, hedrocel, UHMWPE, other radiolucent soft plastics, and additional materials as would be known to an artisan. 
     Another embodiment of a composite implant  101   a  is illustrated in  FIGS. 16 and 17 .  FIG. 16  shows a perspective view from the side of one component of the composite implant  101   a : an all plastic body  152 . The body  152  may preferably be injection molded. PEEK is a suitable material for the body  152 . In order to retain the advantages of a metal surface, including strength and an acid-etched roughened topography  180   a , a second component of the composite implant  101   a  is provided: one or more metal strips or plates  162 . The plates  162  may be provided on the top surface  110   a , on the bottom surface  120   a , or on both surfaces  110   a  and  120   a.    
     Thus, the composite implant  101   a  combines the benefits of two, separate components: a body  152  and a plate  162 . The composite structure of implant  101   a  advantageously permits the engineering designer of the implant  101   a  to balance the mechanical characteristics of the overall implant  101   a . This allows the implant  101   a  to achieve the best balance, for example, of strength, resistance to subsidence, and stress transfer to bone graft. Moreover, although it is a relatively wide device designed to engage the ends of the vertebrae, the implant  101   a  can be inserted with minimal surgical modification. This combination of size and minimal surgical modification is advantageous. 
     The two components that form the composite implant  101   a  must be connected. As illustrated in  FIG. 16 , the body  152  of the composite implant  101   a  has a recessed upper surface  154 . The recessed upper surface  154  is recessed below the top surface  110   a  of the composite implant  101   a  by an amount corresponding to the thickness of the plate  162  that will be installed over the recessed upper surface  154  to create a substantially flat top surface  110   a .  FIG. 17  is a top view of the composite interbody spinal implant  101   a  with the plate  162  installed. As illustrated in  FIG. 17 , the plate  162  has a roughened topography  180   a . A corresponding second plate  162  may be installed on the bottom surface  120   a  of the composite implant  101   a.    
     Any suitable connection mechanism, as would be known to an artisan, will suffice to install the plate  162  on the recessed upper surface  154  of the body  152 . One connection mechanism is illustrated in  FIGS. 16 and 18 .  FIG. 16  shows that a plurality of holes  156  are provided in the recessed upper surface  154  of the body  152 . The holes  156  receive a corresponding plurality of legs  164  on the plate  162 . The legs  164  are positioned on the plate  162  so that, when the plate  162  is installed over the recessed upper surface  154  of the body  152 , each of the legs  164  engages one of the holes  156 . Preferably, the legs  164  are integral with the remainder of the plate  162 . By “integral” is meant a single piece or a single unitary part that is complete by itself without additional pieces, i.e., the part is of one monolithic piece formed as a unit with another part. 
     As shown in  FIG. 18 , the legs  164  may be configured to prevent them from exiting the holes  156 . Thus, as shown, the legs  164  have a toothed periphery. If the plate  162  is metal and the body  152  is plastic, the body  152  may be injection molded around the legs  164  of the plate  162 . In some applications, for example were the body  152  and the plate  162  both made of metal, it may be possible to provide corresponding threads on the legs  164  and holes  156 . 
     The embodiments of the present invention described above are best suited for one or more of the ALIF, PLIF, and TLIF surgical procedures. Another embodiment of the present invention is better suited for cervical fusion procedures. This embodiment is illustrated in  FIGS. 19 and 20  as the interbody spinal implant  201 . 
     Because there is not a lot of disc material between the vertebral bodies in the cervical spine, the discs are usually not very large. The space available for the nerves is also not that great, however, which means that even a small cervical disc herniation may impinge on the nerve and cause significant pain. There is also less mechanical load on the discs in the cervical spine as opposed to the load that exists lower in the spine. Among others, these differences have ramifications for the design of the implant  201 . 
     The implant  201  is generally smaller in size than the other implant embodiments. In addition, the lower mechanical load requirements imposed by the cervical application typically render a composite implant unnecessary. Therefore, the implant  201  is generally made entirely of metal (e.g., titanium) and devoid of other materials (e.g., PEEK). 
     With specific reference to  FIG. 19 , the implant  201  includes a body having a top surface  210 , a bottom surface  220 , opposing lateral sides  230 , and opposing anterior  240  and posterior  250  portions. One or both of the top surface  210  and the bottom surface  220  has a roughened topography  280  for gripping adjacent bone and inhibiting migration of the implant  201 . The implant  201  is substantially hollow and has a generally oval shape with smooth, rounded, or both smooth and rounded edges. 
     The implant  201  includes at least one vertical aperture  260  that extends the entire height of the implant body. The vertical aperture  260  further defines a transverse rim  300 . The size and shape of the vertical aperture  260  are carefully chosen to achieve a preferable design trade off for the particular application envisioned for the implant  201 . Specifically, the vertical aperture  260  seeks to maximize the surface area of the top surface  210  and the bottom surface  220 , to allow for better stress sharing between the implant  201  and the adjacent vertebral endplates, while maximizing access to the bone graft material provided within the implant  201 . Thus, the size and shape of the vertical aperture  260  are predetermined by the application. 
     As illustrated in  FIG. 19 , the implant  201  has an opening  290  in the posterior portion  250 . The opening  290  has a number of functions. One function is to facilitate manipulation of the implant  201  by the caretaker. Thus, the caretaker may insert a surgical tool ( FIG. 6  shows an exemplary surgical tool, the implant holder  2 ) into the opening  290  and, through the engagement between the surgical tool and the opening  290 , manipulate the implant  201 . The opening  290  may be threaded to enhance the engagement. 
     The implant  201  may further include at least one transverse aperture  270 . Like the vertical aperture  260 , the size and shape of the transverse aperture  270  are carefully chosen (and predetermined) to achieve a preferable design trade off for the particular application envisioned for the implant  201 . For example, as shown in  FIG. 19 , the transverse aperture  270  may extend the entire transverse length of the implant body and nearly the entire height of the implant body. Thus, the size and shape of the transverse aperture  270  approach the maximum possible dimensions for the transverse aperture  270 . 
     As illustrated in  FIG. 19 , the implant  201  may be provided with a solid rear wall  242 . The rear wall  242  extends the entire width of the implant body and nearly the entire height of the implant body. Thus, the rear wall  242  essentially closes the anterior portion  240  of the implant  201 . The rear wall  242  may offer one or more of several advantages, including reinforcement of the implant  201  and improved bone graft containment. In the cervical application, it may be important to prevent bone graft material from entering the spinal canal. 
     Alternative shapes for the implant  201  are possible. As illustrated in  FIG. 20 , for example, the implant  201  may have a generally box shape which gives the implant  201  increased cortical bone coverage. Like the implant  201  shown in  FIG. 19 , the implant  201  shown in  FIG. 20  has a curved transverse rim  300  in the area of the anterior portion  240 . The shape of the posterior portion  250  of the implant  201  is substantially flat, however, and the shape of the transverse rim  300  in the area of the posterior portion  250  is substantially square. Thus, the posterior portion  250  provides a face that can receive impact from a tool, such as a surgical hammer, to force the implant  201  into position. 
     The implant  201  may also have a lordotic angle to facilitate alignment. As illustrated in  FIGS. 19 and 20 , the anterior portion  240  is preferably generally greater in height than the posterior portion  250 . Therefore, the implant  201  may better compensate for the generally less supportive bone found in certain regions of the vertebral endplate. As an example, four degrees of lordosis may be built into the implant  201  to help restore balance to the spine. 
     Certain embodiments of the implant  1 ,  101 ,  101   a , and  201  are generally shaped (i.e., made wide) to maximize contact with the apophyseal rim of the vertebral endplates. They are designed to be impacted between the endplates, with fixation to the endplates created by an interference fit and annular tension. Thus, the implants  1 , 101 , 101   a , and  201  are shaped and sized to spare the vertebral endplates and leave intact the hoop stress of the endplates. A wide range of sizes are possible to capture the apophyseal rim, along with a broad width of the peripheral rim, especially in the posterior region. It is expected that such designs will lead to reduced subsidence. As much as seven degrees of lordosis (or more) may be built into the implants  1 ,  101 ,  101   a , and  201  to help restore cervical balance. 
     When endplate-sparing spinal implant  1 , 101 , 101   a , and  201  seats in the disc space against the apophyseal rim, it should still allow for deflection of the endplates like a diaphragm. This means that, regardless of the stiffness of the spinal implant  1 , 101 , 101   a , and  201 , the bone graft material inside the spinal implant  1 , 101 , 101   a , and  201  receives load, leading to healthy fusion. The vertical load in the human spine is transferred though the peripheral cortex of the vertebral bodies. By implanting an apophyseal-supporting inter-body implant  1 , 101 , 101   a , and  201 , the natural biomechanics may be better preserved than for conventional devices. If this is true, the adjacent vertebral bodies should be better preserved by the implant  1 , 101 , 101   a , and  201 , hence reducing the risk of adjacent segment issues. 
     In addition, the dual-acid etched roughened topography  80 , 180 , 180   a , and  280  of the top surface  30 , 130 , 130   a , and  230  and the bottom surface  40 , 140 , 140   a , and  240  along with the broad surface area of contact with the end-plates, is expected to yield a high pull-out force in comparison to conventional designs. As enhanced by the sharp edges  8  and  108 , a pull-out strength of up to 3,000 nt may be expected. The roughened topography  80 ,  180 ,  180   a , and  280  creates a biological bond with the end-plates over time, which should enhance the quality of fusion to the bone. Also, the in-growth starts to happen much earlier than the bony fusion. The center of the implant  1 , 101 , 101   a , and  201  remains open to receive bone graft material and enhance fusion. Therefore, it is possible that patients might be able to achieve a full activity level sooner than for conventional designs. 
     The spinal implant  1 , 101 , 101   a , and  201  according to the present invention offers several advantages relative to conventional devices. Such conventional devices include, among others, ring-shaped cages made of allograft bone material, threaded titanium cages, and ring-shaped cages made of PEEK or carbon fiber. Several of the advantages are summarized with respect to each conventional device, in turn, as follows. 
     1. Advantages Over Allograft Bone Material Cages 
     The spinal implant  1 , 101 , 101   a , and  201  is easier to use than ring-shaped cages made of allograft bone material. For example, it is easier to prepare the graft bed, relative to the allograft cage, for the spinal implant  1 ,  101 ,  101   a , and  201 . And ring allograft cages typically are not sufficiently wide to be implanted on the apophasis. The spinal implant  1 ,  101 ,  101   a , and  201  offers a large internal area for bone graft material and does not require graft preparation, cutting, or trimming. The central aperture  60 , 160 , 160   a , and  260  of the spinal implant  1 ,  101 ,  101   a , and  201  can be filled with cancellous allograft, porous synthetic bone graft substitute (such as the material offered by Orthovita, Inc., Malvern, Pa., under the Vitoss trademark), or BMP. The process of healing the bone can proceed by intra-membranous ossification rather than the much slower process of enchondral ossification. 
     The spinal implant  1 ,  101 ,  101   a , and  201  is generally stronger than allograft cages. In addition, the risk of osteolysis (or, more generally, disease transmission) is minimal with the spinal implant  1 ,  101 ,  101   a , and  201  because titanium is osteocompatible. The titanium of the spinal implant  1 ,  101 ,  101   a , and  201  is unaffected by BMP; there have been reports that BMP causes resorption of allograft bone. 
     2. Advantages Over Threaded Titanium Cages 
     In contrast to conventional treaded titanium cages, which offer little bone-to-bone contact (about 9%), the spinal implant  1 ,  101 ,  101   a , and  201  has a much higher bone-to-bone contact area and commensurately little metal-to-bone interface. Unlike threaded titanium cages which have too large a diameter, the spinal implant  1 , 101 , 101   a , and  201  can be relatively easily used in “tall” disc spaces. The spinal implant  1 ,  101 ,  101   a , and  201  can also be used in either a “stand alone” manner in collapsed discs or as an adjunct to a 360-degree fusion providing cervical column support. 
     The spinal implant  1 ,  101 ,  101   a , and  201  offers safety advantages over conventional threaded titanium cages. The spinal implant  1 ,  101 ,  101   a , and  201  is also easier to implant, avoiding the tubes necessary to insert some conventional cages, and easier to center. Without having to put a tube into the disc space, the vein can be visualized by both the spine surgeon and the vascular surgeon while working with the spinal implant  1 ,  101 ,  101   a , and  201 . Anterior-posterior (AP) fluoroscopy can easily be achieved with trial before implanting the spinal implant  1 , 101 , 101   a , and  201 , ensuring proper placement. The smooth and rounded edges of the spinal implant  1 ,  101 ,  101   a , and  201  facilitate insertion and enhance safety. No reaming of the endplate, which weakens the interface between the endplate and the cage, is necessary for the spinal implant  1 ,  101 ,  101   a , and  201 . Therefore, no reamers or taps are generally needed to insert and position the spinal implant  1 ,  101 ,  101   a , and  201 . 
     Referring to  FIG. 21  two devices of this invention are shown, their working surfaces being designated  500  and  502 , respectively. Also shown in  FIG. 21  is the central opening surface area or internal “hole”  510  and  512  defined by the device into which bone material may be deployed. It is the ratio of the area of the working surface (designed “A”) to the central opening area (designed “O”) of the device that provides the advantageous medical outcomes of surgery using devices defined herein. 
       FIGS. 22A through 22E  show similar structures having working surfaces  600 ,  602 ,  604 ,  606  and  608 , respectively, with areas A and central openings  620 ,  622 ,  624 ,  626 , and  628 , with areas “O” according to this invention. 
     In accordance with this invention the A/O ratio discussed herein has been computed for various sizes of several typical implants designated ALIF, Cervical, TLIF, and TO as shown in the following tables. Typical ALIF, Cervical, TLIF and TO implant configurations are shown in  FIG. 22A through 22D . 
     Surgical Site and Procedure 
     TA, TAS and TC implants are used in an anterior approach to the lumbar (TA and TAS) and cervical (TC) spine. Prior to surgery, the patient is positioned to maintain lordosis of the spine and to facilitate fluoroscopy. Additionally, the surgical area must be cleansed, prepped and draped prior to making an incision at anatomical locations relevant to the target disc space. Access to the appropriate disc space is performed using surgeon specific techniques. A pin is usually placed in the disc space. Once the annulus is identified, a rectangular portion is excised to permit access to the disc space. Following the discectomy the adjacent inferior and superior endplates are prepared removing diseased disk materials, but preserve the thin endplate bone. Surgical instrumentation compatible with the implants is used to grip the device. Prior to placement, bone graft should be placed in the cage. After placement, additional graft is placed around the cage especially in the anterior zone of the implant. Following distraction and placement, the retractors are removed to allow the vessels and muscles to relax toward their normal position. The Endoskeleton® TC implant is indicated to be used with supplemental fixation. Supplemental fixation is optional for the Endoskeleton® TA implant. The Endoskeleton® TAS implant is a stand-alone device and does not require supplemental fixation apart from the three screws included for use with the device. Following stabilization, the surgical wound is closed with standard practice. 
     TO and TT implants are placed in the lumbar spine using a posterior/posterolateral (TO) or transforaminal (TT) surgical approach. TO—Oblique and TLIF implants are placed in the interdisk space and then rotated or pushed into final position verified by imaging techniques. Patients are placed in the prone position and the surgical area is cleaned, prepped and draped. The surgeon begins the procedure through a midline incision. Access to the correct levels and the position of the midline are confirmed via fluoroscopy. A facetectomy is usually completed with resection of the ascending and descending articular processes to allow visualization of the disc space and decompression of the ipsilateral neural structures. Once the annulus is identified, a rectangular portion of the annulus is incised and removed. Following the discectomy, the adjacent inferior and superior endplates are prepared removing diseased disk materials, but preserve the thin endplate bone. The surgical instrumentation compatible with the implants are used to grip the device. Prior to placement, additional bone graft should be placed in and around the cage especially in the anterior zone of the implant. Following distraction and placement, the retractors are removed to allow the spine, muscles, nerve structures and tissues and any blood vessels to relax toward their normal position. TO and TT implants are indicated to be used with supplemental fixation. Following stabilization, the surgical wound is closed with standard practice. 
     Tables 1, 2, 3 and 4 below show data extracted from the implants shown in  FIG. 22A through 22D . Of particular note in those Tables are the ratios shown in the respective Hole to FP and Face to FP columns. These are the A/O ratios discussed above. 
     3. Advantages Over PEEK/Carbon Fiber Cages 
     Cages made of PEEK or carbon fiber cannot withstand the high impact forces needed for implantation, especially in a collapsed disc or spondylolisthesis situation, without secondary instruments. In contrast, the spinal implant  1 ,  101 ,  101   a , and  201  avoids the need for secondary instruments. Moreover, relative to PEEK or carbon fiber cages, the spinal implant  1 ,  101 , 101   a , and  201  provides better distraction through endplate sparing and being designed to be implanted on the apophysis (the bony protuberance of the human spine). The titanium of the top surface  10 , 110 , 110   a , and  210  and the bottom plate  20 ,  120 ,  120   a , and  220  of the spinal implant  1 ,  101 ,  101   a , and  201  binds to bone with a mechanical (knawling) and a chemical (a hydrophilic) bond. In contrast, bone repels PEEK and such incompatibility can lead to locked pseudoarthrosis. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 ALIF Implants 
               
             
          
           
               
                   
                 Size 
                 Hole 
                 Face 
                 FP 
                 Hole to FP 
                 Face to FP 
               
               
                   
                   
               
             
          
           
               
                   
                 Std 
                 181.8 
                 252.9 
                 577.4 
                 31.5% 
                 43.8% 
               
               
                   
                 HS 
                 286.6 
                 264.2 
                 707.6 
                 40.5% 
                 37.3% 
               
               
                   
                 HL 
                 217.8 
                 289.4 
                 661.4 
                 32.9% 
                 43.8% 
               
               
                   
                 Lrg 
                 284.2 
                 294.3 
                 740.4 
                 38.4% 
                 39.7% 
               
               
                   
                 HX 
                 346.6 
                 300.7 
                 815.6 
                 42.5% 
                 36.9% 
               
               
                   
                 Hlrg 
                 406.6 
                 337.2 
                 923.6 
                 44.0% 
                 36.5% 
               
               
                   
                   
                   
                   
                 Average 
                 38.3% 
                 39.7% 
               
               
                   
                   
                   
                   
                 StdDev 
                 5.1% 
                 3.4% 
               
               
                   
                   
                   
                   
                 Max 
                 44.0% 
                 43.8% 
               
               
                   
                   
                   
                   
                 Min 
                 31.5% 
                 36.5% 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Cervical Implants 
               
             
          
           
               
                   
                 Size 
                 Hole 
                 Face 
                 FP 
                 Hole to FP 
                 Face to FP 
               
               
                   
                   
               
             
          
           
               
                   
                 Sm 
                 52.5 
                 60.8 
                 137.1 
                 38.3% 
                 44.4% 
               
               
                   
                 Med 
                 81.6 
                 88.4 
                 181.9 
                 44.9% 
                 48.6% 
               
               
                   
                 Lrg 
                 117.0 
                 120.7 
                 233.1 
                 50.2% 
                 51.8% 
               
               
                   
                   
                   
                   
                 Average 
                 44.5% 
                 48.2% 
               
               
                   
                   
                   
                   
                 StdDev 
                 6.0% 
                 3.7% 
               
               
                   
                   
                   
                   
                 Max 
                 50.2% 
                 51.8% 
               
               
                   
                   
                   
                   
                 Min 
                 38.3% 
                 44.4% 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 TLIF Implants 
               
             
          
           
               
                 Size 
                 Hole 
                 Face 
                 FP 
                 Hole to FP 
                 Face to FP 
               
               
                   
               
             
          
           
               
                 Std 25 
                 46.9 
                 104.4 
                 229.0 
                 20.5% 
                 45.6% 
               
               
                 Lrg 30 
                 66.3 
                 128.7 
                 283.1 
                 23.4% 
                 45.5% 
               
               
                 XLrg 35 
                 85.8 
                 120.7 
                 337.4 
                 25.4% 
                 35.8% 
               
               
                   
                   
                   
                 Average 
                 23.1% 
                 42.3% 
               
               
                   
                   
                   
                 StdDev 
                 2.5% 
                 5.6% 
               
               
                   
                   
                   
                 Max 
                 25.4% 
                 45.6% 
               
               
                   
                   
                   
                 Min 
                 20.5% 
                 35.8% 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 TO Implants 
               
             
          
           
               
                 Size 
                 Hole 
                 Face 
                 FP 
                 Hole to FP 
                 Face to FP 
               
               
                   
               
             
          
           
               
                 PLIF 22 
                 24.7 
                 75.8 
                 182.3 
                 13.5% 
                 41.6% 
               
               
                 PLIF 26 
                 35.3 
                 88.3 
                 218.3 
                 16.1% 
                 40.4% 
               
               
                 Oblique 31 
                 72.0 
                 131.7 
                 315.6 
                 22.8% 
                 41.7% 
               
               
                   
                   
                   
                 Average 
                 17.5% 
                 41.2% 
               
               
                   
                   
                   
                 StdDev 
                 4.8% 
                 0.7% 
               
               
                   
                   
                   
                 Max 
                 22.8% 
                 41.7% 
               
               
                   
                   
                   
                 Min 
                 13.5% 
                 40.4% 
               
               
                   
               
             
          
         
       
     
     Example Surgical Methods 
     The following examples of surgical methods are included to more clearly demonstrate the overall nature of the invention. These examples are exemplary, not restrictive, of the invention. 
     Certain embodiments of the present invention are particularly suited for use during interbody spinal implant procedures currently known in the art. For example, the disc space may be accessed using a standard mini open retroperitoneal laparotomy approach. The center of the disc space is located by AP fluoroscopy taking care to make sure the pedicles are equidistant from the spinous process. The disc space is then incised by making a window in the annulus for insertion of certain embodiments of the spinal implant  1 , 101 ,  101   a , and  201  (a 32 or 36 mm window in the annulus is typically suitable for insertion). The process according to the present invention minimizes, if it does not eliminate, the cutting of bone. The endplates are cleaned of all cartilage with a curette, however, and a size-specific rasp (or broach) may then be used. 
       FIG. 8  shows an exemplary rasp  14  used during certain methods of implantation. Typically, either a 32 mm or a 36 mm rasp  14  is used. A single rasp  14  is used to remove a minimal amount of bone. A lateral c-arm fluoroscopy can be used to follow insertion of the rasp  14  in the posterior disc space. The smallest height rasp  14  that touches both endplates (e.g., the superior and inferior endplates) is first chosen. After the disc space is cleared of all soft tissue and cartilage, distraction is then accomplished by using distractors (also called implant trials or distraction plugs). It is usually possible to distract 2-3 mm higher than the rasp  14  that is used because the disk space is elastic. 
       FIG. 7  shows an exemplary distractor  12  used during certain methods of implantation. The implant trials, or distractors  12 , are solid polished blocks which have a peripheral geometry identical to that of the implant  1 , 101 , 101   a , and  201 . These distractor blocks may be made in various heights to match the height of the implant  1 , 101 , 101   a , and  201 . The disc space is adequately distracted by sequentially expanding it with distractors  12  of progressively increasing heights. The distractor  12  is then left in the disc space and the centering location may be checked by placing the c-arm back into the AP position. If the location is confirmed as correct (e.g., centered), the c-arm is turned back into the lateral position. The spinal implant  1 , 101 , 101   a , and  201  is filled with autologous bone graft or bone graft substitute. The distractor  12  is removed and the spinal implant  1 ,  101 ,  101   a , and  201  is inserted under c-arm fluoroscopy visualization. The process according to the present invention does not use a secondary distractor; rather, distraction of the disc space is provided by the spinal implant  1 ,  101 ,  101   a , and  201  itself (i.e., the implant  1 ,  101 ,  101   a , and  201  itself is used as a distractor). 
     Use of a size-specific rasp  14 , as shown in  FIG. 8 , preferably minimizes removal of bone, thus minimizing impact to the natural anatomical arch, or concavity, of the vertebral endplate while preserving much of the apophyseal rim. Preservation of the anatomical concavity is particularly advantageous in maintaining biomechanical integrity of the spine. For example, in a healthy spine, the transfer of compressive loads from the vertebrae to the spinal disc is achieved via hoop stresses acting upon the natural arch of the endplate. The distribution of forces, and resultant hoop stress, along the natural arch allows the relatively thin shell of subchondral bone to transfer large amounts of load. 
     During traditional fusion procedures, the vertebral endplate natural arch may be significantly removed due to excessive surface preparation for implant placement and seating. This is especially common where the implant is to be seated near the center of the vertebral endplate or the implant is of relatively small medial-lateral width. Breaching the vertebral endplate natural arch disrupts the biomechanical integrity of the vertebral endplate such that shear stress, rather than hoop stress, acts upon the endplate surface. This redistribution of stresses may result in subsidence of the implant into the vertebral body. 
     Preferred embodiments of the present surgical method minimize endplate bone removal on the whole, while still allowing for some removal along the vertebral endplate far lateral edges where the subchondral bone is thickest. Still further, certain embodiments of the present interbody spinal implant  1 , 101 ,  101   a , and  201  include smooth, rounded, and highly radiused posterior portions and lateral sides which may minimize extraneous bone removal for endplate preparation and reduce localized stress concentrations. Thus, interbody surgical implants  1 , 101 , 101   a , and  201  and methods of using them, as now taught, are particularly useful in preserving the natural arch of the vertebral endplate and minimizing the chance of implant subsidence. 
     Because the endplates are spared during the process of inserting the spinal implant  1 ,  101 ,  101   a , and  201 , hoop stress of the inferior and superior endplates is maintained. Spared endplates allow the transfer of axial stress to the apophasis. Endplate flexion allows the bone graft placed in the interior of the spinal implant  1 ,  101 ,  101   a , and  201  to accept and share stress transmitted from the endplates. In addition, spared endplates minimize the concern that BMP might erode the cancellous bone. 
     Interbody spinal implants  1 ,  101 ,  101   a , and  201  of the present invention are durable and can be impacted between the endplates with standard instrumentation. Therefore, certain embodiments of the present invention may be used as the final distractor during implantation. In this manner, the disc space may be under-distracted (e.g., distracted to some height less than the height of the interbody spinal implant  1 ,  101 ,  101   a , and  201 ) to facilitate press-fit implantation. Further, certain embodiments of the current invention having a smooth and rounded posterior portion (and lateral sides) may facilitate easier insertion into the disc space. Still further, those embodiments having a surface roughened topography  80 , 180 , 180   a , and  280 , as now taught, may lessen the risk of excessive bone removal during distraction as compared to implants having teeth, ridges, or threads currently known in the art even in view of a press-fit surgical distraction method. Nonetheless, once implanted, the interbody surgical implants  1 ,  101 ,  101   a , and  201 , as now taught, may provide secure seating and prove difficult to remove. Thus, certain embodiments of the present interbody spinal implant  1 , 101 ,  101   a , and  201  may maintain a position between the vertebral endplates due, at least in part, to resultant annular tension attributable to press-fit surgical implantation and, post-operatively, improved osteointegration at the top surface  10 ,  110 ,  110   a , and  210 , the bottom surface  20 , 120 , 120   a , and  220 , or both top and bottom surfaces. 
     As previously mentioned, surgical implants and methods, as now taught, tension the vertebral annulus via distraction. These embodiments and methods may also restore spinal lordosis, thus improving sagittal and coronal alignment. Implant systems currently known in the art require additional instrumentation, such as distraction plugs, to tension the annulus. These distraction plugs require further tertiary instrumentation, however, to maintain the lordotic correction during actual spinal implant insertion. If tertiary instrumentation is not used, then some amount of lordotic correction may be lost upon distraction plug removal. Interbody spinal implants  1 ,  101 ,  101   a , and  201 , according to certain embodiments of the present invention, are particularly advantageous in improving spinal lordosis without the need for tertiary instrumentation, thus reducing the instrument load upon the surgeon. This reduced instrument load may further decrease the complexity, and required steps, of the implantation procedure. 
     Certain embodiments of the spinal implants  1 , 101 ,  101   a , and  201  may also reduce deformities (such as isthmic spondylolythesis) caused by distraction implant methods. Traditional implant systems require secondary or additional instrumentation to maintain the relative position of the vertebrae or distract collapsed disc spaces. In contrast, interbody spinal implants  1 , 101 , 101   a , and  201 , as now taught, may be used as the final distractor and thus maintain the relative position of the vertebrae without the need for secondary instrumentation. 
     Certain embodiments collectively comprise a family of implants, each having a common design philosophy. These implants and the associated surgical technique have been designed to address the ten, separate challenges associated with the current generation of traditional anterior spinal fusion devices listed above in the Background section of this document. Each of these challenges is addressed in turn and in the order listed above. 
     1. End-Plate Preparation 
     Embodiments of the present invention allow endplate preparation with custom-designed rasps  14 . These rasps  14  have a geometry matched with the geometry of the implant. The rasps  14  conveniently remove cartilage from the endplates and remove minimal bone, only in the posterolateral regions of the vertebral end-plates. It has been reported in the literature that the end-plate is the strongest in posterolateral regions. 
     2. Implant Difficulty 
     After desired annulotomy and discectomy, embodiments of the present invention first adequately distract the disc space by inserting (through impaction) and removing sequentially larger sizes of very smooth distractors, which have size matched with the size of the available implants  1 ,  101 ,  101   a , and  201 . Once adequate distraction is achieved, the surgeon prepares the end-plate with a size-specific rasp  14 . There is no secondary instrumentation required to keep the disc space distracted while the implant  1 ,  101 , 101   a , and  201  is inserted, as the implant  1 ,  101 ,  101   a , and  201  has sufficient mechanical strength that it is impacted into the disc space. In fact, the height of the implant  1 , 101 , 101   a , and  201  is about 1 mm greater than the height of the rasp  14  used for end-plate preparation, to create some additional tension in the annulus by implantation, which creates a stable implant construct in the disc space. 
     The implant geometry has features which allow it to be implanted via any one of an anterior, anterolateral, or lateral approach, providing tremendous intra-operative flexibility of options. The implant  1 ,  101 ,  101   a , and  201  is designed such that all the impact loads are applied only to the titanium part of the construct. Thus, the implant  1 , 101 , 101   a , and  201  has adequate strength to allow impact. The sides of the implant  1 ,  101 ,  101   a , and  201  have smooth surfaces to allow for easy implantation and, specifically, to prevent “binding” of the implant  1 , 101 , 101   a , and  201  to soft tissues during implantation. 
     3. Materials of Construction 
     The present invention encompasses a number of different implants  1 ,  101 ,  101   a , and  201 , including a one piece, titanium-only implant and a composite implant formed of top and bottom plates  162  (components) made out of titanium. The surfaces exposed to the vertebral body are dual acid etched to allow for bony in-growth over time, and to provide resistance against expulsion. The top and bottom titanium plates  162  are assembled together with the implant body  152  that is injection molded with PEEK. The net result is a composite implant  101   a  that has engineered stiffness for its clinical application. The axial load is borne by the PEEK component of the construct. 
     It is believed that an intact vertebral end-plate deflects like a diaphragm under axial compressive loads generated due to physiologic activities. If a spinal fusion implant is inserted in the prepared disc space via a procedure which does not destroy the end-plates, and if the implant contacts the end-plates only peripherally, the central dome of the endplates can still deflect under physiologic loads. This deflection of the dome can pressurize the bone graft material packed inside the spinal implant, hence allowing it to heal naturally. The implant  1 ,  101 ,  101   a , and  201  designed according to certain embodiments of the present invention allows the vertebral end-plate to deflect and allows healing of the bone graft into fusion. 
     4. Implant Expulsion 
     Certain faces of the implant  1 ,  101 ,  101   a , and  201  according to certain embodiments of the present invention have sharp edges  8 , 180 . These edges  8 , 180  tend to dig “into” the end-plates slightly and help to resist expulsion. The top and bottom surfaces of the implant are made out of titanium and are dual acid etched. The dual acid etching process creates a highly roughened texture on these surfaces, which generates tremendous resistance to expulsion. The width of these dual acid etched surfaces is very broad and creates a large area of contact with the vertebral end-plates, further increasing the resistance to expulsion. 
     5. Implant Subsidence 
     The implant  1 ,  101 ,  101   a , and  201  according to certain embodiments of the present invention has a large foot-print, and offers several sizes. Because there is no secondary instrument required to maintain distraction during implantation, all the medial-lateral (ML) exposure is available as implantable ML width of the implant. This feature allows the implant to contact the vertebral end-plates at the peripheral apophyseal rim, where the end-plates are the strongest and least likely to subside. 
     Further, there are no teeth on the top and bottom surfaces (teeth can create stress risers in the end-plate, encouraging subsidence). Except for certain faces, all the implant surfaces have heavily rounded edges, creating a low stress contact with the end-plates. The wide rim of the top and bottom surfaces, in contact with the end-plates, creates a low-stress contact due to the large surface area. Finally, the implant construct has an engineered stiffness to minimize the stiffness mismatch with the vertebral body which it contacts. 
     6. Insufficient Room for Bone Graft 
     As mentioned, the implant  1 ,  101 ,  101   a , and  201  according to certain embodiments of the present invention has a large foot-print. In addition, titanium provides high strength for a small volume. In combination, the large footprint along with the engineered use of titanium allows for a large volume of bone graft to be placed inside the implant. 
     7. Stress Shielding 
     As stated above, it is believed that an intact vertebral end-plate deflects like a diaphragm under axial compressive loads generated due to physiologic activities. If a spinal fusion implant is inserted in the prepared disc space via a procedure which does not destroy the end-plate, and if the implant contacts the end-plates only peripherally, the central dome of the end-plates can still deflect under physiologic loads. This deflection of the dome can pressurize the bone graft material packed inside the spinal implant, hence allowing it to heal naturally. The implant  1 ,  101 ,  101   a , and  201  according to certain embodiments of the present invention allows the vertebral end-plate to deflect and facilitates healing of the bone graft into fusion. 
     8. Lack of Implant Incorporation with Vertebral Bone 
     The top and bottom surfaces of the implant  1 ,  101 ,  101   a , and  201  according to certain embodiments of the present invention are made of titanium and are dual acid etched. The dual acid etched surface treatment of titanium allows in-growth of bone to the surfaces. Hence, the implant  1 ,  101 ,  101   a , and  201  is designed to incorporate with the vertebral bone over time. It may be that the in-growth happens sooner than fusion. If so, there may be an opportunity for the patients treated with the implant  1 , 101 , 101   a , and  201  of the present invention to return to normal activity levels sooner than currently recommended by standards of care. 
     9. Limitations on Radiographic Visualization 
     Even the titanium-only embodiment of the present invention has been designed with large windows to allow for radiographic evaluation of fusion, both through AP and lateral X-rays. The composite implant  101   a  minimizes the volume of titanium, and localizes it to the top and bottom surfaces. The rest of the implant  101   a  is made of PEEK which is radiolucent and allows for free radiographic visualization. 
     10. Cost of Manufacture and Inventory 
     The cost to manufacture a single implant  1 ,  101 ,  101   a , and  201  according to the present invention is comparable to the cost to manufacture commercially available products. But a typical implant set for a conventional device can have three foot-prints and ten heights for each foot-print. Therefore, to produce one set, the manufacturer has to make thirty different setups if the implants are machined. In contrast, for the composite embodiment according to certain embodiments of the present invention, the manufacturer will have to machine only three sets of metal plates, which is six setups. The PEEK can be injection molded between the metal plates separated by the distance dictated by the height of the implant  101   a . Once the injection molds are made, the subsequent cost of injection molding is considerably less as compared to machining. This feature of the present invention can lead to considerable cost savings. 
     In addition, a significant expense associated with a dual acid etched part is the rate of rejects due to acid leaching out to surfaces which do not need to be etched. In the case of the composite implant  101   a  according to certain embodiments of the present invention, the criteria for acceptance of such a part will be lower because the majority of the surfaces are covered with PEEK via injection molding after the dual acid etching process step. This feature can yield significant manufacturing-related cost savings. 
     Although illustrated and described above with reference to certain specific embodiments and examples, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. It is expressly intended, for example, that all ranges broadly recited in this document include within their scope all narrower ranges which fall within the broader ranges. In addition, features of one embodiment may be incorporated into another embodiment.