Process for fabricating bioactive vertebral endplate bone-contacting surfaces on a spinal implant

Abstract

An interbody spinal implant including a body having a top surface, a bottom surface, opposing lateral sides, and opposing anterior and posterior portions. At least a portion of the top surface, the bottom surface, or both surfaces has a roughened surface topography including both micro features and nano features, without sharp teeth that risk damage to bone structures, adapted to grip bone through friction generated when the implant is placed between two vertebrae and to inhibit migration of the implant. The roughened surface topography typically further includes macro features and the macro features, micro features, and nano features overlap. Also disclosed are methods of using such implants and processes of fabricating a roughened surface topography on a surface of an implant. The process includes separate and sequential macro processing, micro processing, and nano processing steps.

Claims

1. A process for fabricating bioactive vertebral endplate bone-contacting surfaces on a spinal implant, comprising: providing a spinal implant comprising titanium or an alloy of titanium, aluminum, and vanadium having one or more vertebral endplate bone-contacting surfaces; acid-etching or machining the one or more vertebral endplate bone-contacting surfaces to form macro-scale structural features; following the acid etching or machining step, abrasive media blasting the one or more vertebral endplate bone-contacting surfaces to form micro-scale structural features that overlap the macro-scale structural features; and following the abrasive media blasting step, mildly acid-etching the one or more vertebral endplate bone-contacting surfaces to form nano-scale structural features that overlap the macro-scale structural features and the micro-scale structural features, thereby forming the final and completed bioactive vertebral endplate bone-contacting surfaces on the spinal implant, provided that the process does not include at any time applying a coating to any of the vertebral endplate bone-contacting surfaces and does not include at any time embedding surface contaminants into any of the vertebral endplate bone-contacting surfaces.

2. The process according to claim 1, wherein the abrasive media blasting comprises blasting the one or more vertebral endplate bone-contacting surfaces with media comprising aluminum oxide particles.

3. The process according to claim 1, wherein the spinal implant is configured for use in a lumbar fusion procedure.

4. The process according to claim 1, wherein the spinal implant is configured for use in a cervical fusion procedure.

5. The process according to claim 1, wherein the macro-scale structural features are roughly spherical in shape.

6. The process according to claim 5, wherein the macro-scale structural features lack undercuts or protruding sharp edges.

7. The process according to claim 1, wherein the macro-scale features, the micro-scale features, and the nano-scale features are oriented in opposition to the insertion direction of the implant.

8. The process according to claim 1, wherein the one or more bioactive vertebral endplate bone-contacting surfaces are irregular.

9. The process according to claim 1, wherein the step of acid-etching or machining comprises acid-etching or machining a first-cut to produce macro-scale features having the smallest diameter and greatest depth, then acid-etching or machining a second cut to produce macro-scale features having an intermediate diameter and depth, and then acid-etching or machining a third cut to produce macro-scale features having the greatest diameter and smallest depth.

10. The process according to claim 1, wherein the macro-scale structural features have an amplitude of from about 20 to about 200 microns, a peak-to-valley height of from about 40 to about 500 microns, and a mean spacing of from about 400 to about 2000 microns.

11. The process according to claim 1, wherein the micro-scale structural features have an amplitude of from about 1 to about 20 microns, a peak-to-valley height of from about 2 to about 40 microns, and a mean spacing of from about 20 to about 400 microns.

12. The process according to claim 1, wherein the nano-scale structural features have an amplitude of from about 0.010 to about 1 microns, a peak-to-valley height of from about 0.2 to about 2 microns, and a mean spacing of from about 0.5 to about 20 microns.

13. The process according to claim 1, wherein the step of mildly acid etching comprises etching the one or more vertebral endplate bone-contacting surfaces with aqueous hydrochloric acid to form the nano-scale structural features.

14. A process for fabricating bioactive vertebral endplate bone-contacting surfaces on a spinal implant, comprising: providing a spinal implant comprising titanium or an alloy of titanium, aluminum, and vanadium having one or more vertebral endplate bone-contacting surfaces; acid-etching or machining the one or more vertebral endplate bone-contacting surfaces to form macro-scale structural features; following the acid etching or machining step, abrasive media blasting the one or more vertebral endplate bone-contacting surfaces to form micro-scale structural features that overlap the macro-scale structural features, such overlap avoiding undercuts or protruding sharp edges; and following the abrasive media blasting step, mildly acid-etching the one or more vertebral endplate bone-contacting surfaces to form nano-scale structural features that overlap the macro-scale structural features and the micro-scale structural features, such overlap avoiding undercuts or protruding sharp edges, thereby forming the final and completed bioactive vertebral endplate bone-contacting surfaces on the spinal implant, provided that the process does not include at any time applying a coating to any of the vertebral endplate bone-contacting surfaces and does not include at any time embedding surface contaminants into any of the vertebral endplate bone-contacting surfaces.

15. The process according to claim 14, wherein the macro-scale structural features are roughly spherical in shape.

16. The process according to claim 15, wherein the macro-scale structural features lack undercuts or protruding sharp edges.

17. The process according to claim 14, wherein the macro-scale features, the micro-scale features, and the nano-scale features are oriented in opposition to the insertion direction of the implant.

18. The process according to claim 14, wherein the one or more bioactive vertebral endplate bone-contacting surfaces are irregular.

19. The process according to claim 14, wherein the step of acid-etching or machining comprises add-etching or machining a first-cut to produce macro-scale features having the smallest diameter and greatest depth, then acid-etching or machining a second cut to produce macro-scale features having an intermediate diameter and depth, and then acid-etching or machining a third cut to produce macro-scale features having the greatest diameter and smallest depth.

20. A process for fabricating bioactive vertebral endplate bone-contacting surfaces on a spinal implant, comprising: providing a spinal implant comprising titanium or an alloy of titanium, aluminum, and vanadium having one or more vertebral endplate bone-contacting surfaces; acid-etching or machining the one or more vertebral endplate bone-contacting surfaces to form macro-scale structural features that are roughly spherical in shape and lack undercuts or protruding sharp edges; following the acid etching or machining step, abrasive media blasting the one or more vertebral endplate bone-contacting surfaces to form micro-scale structural features that overlap the macro-scale structural features; and following the abrasive media blasting step, mildly acid-etching the one or more vertebral endplate bone-contacting surfaces to form nano-scale structural features that overlap the macro-scale structural features and the micro-scale structural features, thereby forming the final and completed bioactive vertebral endplate bone-contacting surfaces on the spinal implant, provided that the process does not include at any time applying a coating to any of the vertebral endplate bone-contacting surfaces and does not include at any time embedding surface contaminants into any of the vertebral endplate bone-contacting surfaces, wherein the macro-scale features, the micro-scale features, and the nano-scale features are oriented in opposition to the insertion direction of the implant.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) 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:

(2) FIG. 1A is a perspective view of a traditional implant having sharp teeth on its top and bottom surfaces;

(3) FIG. 1B is a side view of the traditional implant illustrated in FIG. 1A;

(4) FIG. 1C is an expanded view of a portion of the traditional implant illustrated in FIG. 1B;

(5) FIG. 2 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;

(6) FIG. 3 depicts a top view of the first embodiment of the interbody spinal implant;

(7) FIG. 4 depicts an anterior view of the first embodiment of the interbody spinal implant;

(8) FIG. 5 depicts a posterior view of the first embodiment of the interbody spinal implant;

(9) FIG. 6A depicts a first post-operative radiograph showing visualization of an embodiment of the interbody spinal implant;

(10) FIG. 6B depicts a second post-operative radiograph showing visualization of an embodiment of the interbody spinal implant;

(11) FIG. 6C depicts a third post-operative radiograph showing visualization of an embodiment of the interbody spinal implant;

(12) FIG. 7 shows an exemplary surgical tool (implant holder) to be used with certain embodiments of the interbody spinal implant;

(13) FIG. 8 shows an exemplary rasp used during certain methods of implantation;

(14) FIG. 9 shows an exemplary distractor used during certain methods of implantation;

(15) FIG. 10A is a perspective view of an implant having a roughened topography according to another embodiment of the present invention;

(16) FIG. 10B is a side view of the implant illustrated in FIG. 10A;

(17) FIG. 10C is an expanded view of a portion of the implant illustrated in FIG. 10B;

(18) FIG. 11A is a perspective view illustrating the result of a first step in forming the macro features of the roughened topography of the implant according to one embodiment of the present invention;

(19) FIG. 11B is a perspective view illustrating the result of a second step in forming the macro features of the roughened topography of the implant according to one embodiment of the present invention;

(20) FIG. 11C is a perspective view illustrating the result of a third step in forming the macro features of the roughened topography of the implant according to one embodiment of the present invention;

(21) FIG. 12A is a perspective, simulated view of the implant following completion of a first process step in forming the macro features of the roughened topography of the implant according to one embodiment of the present invention, highlighting the diameter of the feature;

(22) FIG. 12B is a side view corresponding to the perspective view of FIG. 12A;

(23) FIG. 13A is a perspective, simulated view of the implant following completion of a first process step in forming the macro features of the roughened topography of the implant according to one embodiment of the present invention, highlighting the edges of the feature;

(24) FIG. 13B is a side view corresponding to the perspective view of FIG. 13A;

(25) FIG. 14A is a perspective, simulated view of the implant following completion of a first process step in forming the macro features of the roughened topography of the implant according to one embodiment of the present invention, highlighting the pattern of repeating features;

(26) FIG. 14B is a side view corresponding to the perspective view of FIG. 14A;

(27) FIG. 15A is a perspective, simulated view of the implant following completion of the three process steps in forming the macro features of the roughened topography of the implant according to one embodiment of the present invention;

(28) FIG. 15B is a side view corresponding to the perspective view of FIG. 15A;

(29) FIG. 16 is a top, simulated view showing each of the first cut pattern, the second cut pattern, and the third cut pattern individually and, in an area of overlapping features, the roughened topography following completion of the three, sequential processing steps and combining (in an overlapping pattern) the first cut pattern, the second cut pattern, and the third cut pattern of the macro features;

(30) FIG. 17 is a perspective view of the implant illustrated in FIG. 16;

(31) FIG. 18 is a side view illustrating the measurement of the average amplitude, Ra, for the third cut pattern of the implant shown in FIGS. 16 and 17;

(32) FIG. 19A is another side view of the implant shown in FIGS. 16 and 17;

(33) FIG. 19B is an expanded view of a portion of the implant illustrated in FIG. 19A illustrating the measurement of the mean spacing, Sm, for the second cut pattern;

(34) FIG. 20A is yet another side view of the implant shown in FIGS. 16 and 17;

(35) FIG. 20B is an expanded view of a portion of the implant illustrated in FIG. 20A illustrating the measurement of the maximum peak-to-valley height, Rmax, for the first cut pattern;

(36) FIG. 21 illustrates three parameters, namely, Ra, Rmax, and Sm, used to measure surface roughness for the macro features of an implant;

(37) FIG. 22 illustrates the parameters Ra, Rmax, and Sm for the completed macro and nano surface features of the implant according to an embodiment of the present invention;

(38) FIG. 23 illustrates one set of process steps that can be used to form macro, micro, or nano processes;

(39) FIG. 24 graphically represents the average amplitude, Ra;

(40) FIG. 25 graphically represents the average peak-to-valley roughness, Rz;

(41) FIG. 26 graphically represents the maximum peak-to-valley height, Rmax;

(42) FIG. 27 graphically represents the total peak-to-valley of waviness profile; and

(43) FIG. 28 graphically represents the mean spacing, Sm.

DETAILED DESCRIPTION OF THE INVENTION

(44) 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.

(45) 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.

(46) 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.

(47) Referring now to the drawing, in which like reference numbers refer to like elements throughout the various figures that comprise the drawing, FIG. 2 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.

(48) Certain embodiments of the interbody spinal implant 1 are substantially hollow and have a generally oval-shaped 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. 3, the vertical aperture 60 further defines a transverse rim 100 having a greater posterior portion thickness 55 than an anterior portion thickness 45.

(49) 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.

(50) 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.

(51) FIG. 4 depicts an anterior view, and FIG. 5 depicts a posterior view, of an embodiment of the interbody spinal implant 1. As illustrated in FIGS. 2 and 4, the implant 1 has an opening 90 in the anterior portion 40. As illustrated in FIGS. 4 and 5, in one embodiment the posterior portion 50 has a similarly shaped opening 90. In another embodiment, as illustrated in FIG. 2, 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).

(52) 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.

(53) FIG. 7 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. 2, 4, and 5, other sizes and shapes are feasible.

(54) 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. 6A, 6B, and 6C, 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.

(55) 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).

(56) As illustrated in FIG. 2, 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.

(57) 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.

(58) 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.

(59) 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. In at least one embodiment, the interbody spinal implant 1 is formed of metal. In a more preferred embodiment, the implant 1 is comprised of titanium, or a titanium alloy, having the surface roughened topography 80. The surfaces of the implant 1 are preferably bioactive. An oxide layer naturally forms on titanium alloy. In a preferred embodiment, however, the base material of the implant 1 includes the elements Ti, Al, and V without any coatings.

(60) In a specific embodiment of the present invention, the roughened topography 80 is obtained by combining separate macro processing, micro processing, and nano processing steps. The term “macro” typically means relatively large; for example, in the present application, dimensions measured in millimeters (mm). The term “micro” typically means one millionth (10.sup.−6); for example, in the present application, dimensions measured in microns (μm) which correspond to 10.sup.−6 meters. The term “nano” typically means one billionth (10.sup.−9); for example, in the present application, dimensions measured in manometers (nm) which correspond to 10.sup.−9 meters.

(61) FIGS. 10A, 10B, and 10C illustrate the interbody implant 1 with one embodiment of the roughened topography 80 according to the present invention. Specifically, FIG. 10A is a perspective view of the implant 1. FIG. 10B is a side view of the implant 1. And FIG. 1C is an expanded view of a portion of the implant 1 taken along the detail 10C illustrated in FIG. 1B, highlighting the pattern of the example embodiment of the roughened topography 80.

(62) The interbody implant 1 has a roughened topography 80 with predefined surface features that (a) engage the vertebral endplates with a friction fit and, following an endplate preserving surgical technique, (b) attain initial stabilization, and (c) benefit fusion. The composition of the endplate is a thin layer of notch-sensitive bone that is easily damaged by features (such as teeth) that protrude sharply from the surface of traditional implants. Avoiding such teeth and the attendant risk of damage, the roughened topography 80 of the implant 1 does not have teeth or other sharp, potentially damaging structures; rather, the roughened topography 80 has a pattern of repeating features of predetermined sizes, smooth shapes, and orientations. By “predetermined” is meant determined beforehand, so that the predetermined characteristic of the implant 1 must be determined, i.e., chosen or at least known, before use of the implant 1.

(63) The shapes of the frictional surface protrusions of the roughened topography 80 are formed using processes and methods commonly applied to remove metal during fabrication of implantable devices such as chemical, electrical, electrochemical, plasma, or laser etching; cutting and removal processes; casting; forging; machining; drilling; grinding; shot peening; abrasive media blasting (such as sand or grit blasting); and combinations of these subtractive processes. Additive processes such as welding and thermal and optical melt additive processes are also suitable. The resulting surfaces either can be random in the shape and location of the features or can have repeating patterns. This flexibility allows for the design and production of surfaces that resist motion induced by loading in specific directions that are beneficial to the installation process and resist the opposing forces that can be the result of biologic or patient activities such as standing, bending, or turning or as a result of other activities. The shapes of the surface features when overlapping work to increase the surface contact area but do not result in undercuts that generate a cutting or aggressively abrasive action on the contacting bone surfaces.

(64) These designed surfaces are composed of various sizes of features that, at the microscopic level, interact with the tissues and stimulate their natural remodeling and growth. At a larger scale these features perform the function of generating non-stressful friction that, when combined with a surgical technique that retains the most rigid cortical bone structures in the disc space, allow for a friction fit that does not abrade, chip, perforate, or compromise the critical endplate structures. The features are typically divided into three size scales: nano, micro, and macro. The overlapping of the three feature sizes can be achieved using manufacturing processes that are completed sequentially and, therefore, do not remove or degrade the previous method.

(65) The first step in the process is either mechanical (e.g., machining though conventional processes) or chemical bulk removal to generate macro features, roughly spherical in shape without undercuts or protruding sharp edges. Other shapes are possible, such as ovals, polygons (including rectangles), and the like. These features are overlapped with the next scale (micro) of features using either chemical or mechanical methods (e.g., AlO.sub.2 blasting) in predetermined patterns which also do not result in undercuts or protruding sharp edges. The third and final process step is completed through more mild (less aggressive) etching (e.g., HCl acid etching) that, when completed, generates surface features in both the micro and nano scales over both of the features generated by the two previous steps. The nano layer dictates the final chemistry of the implant material.

(66) FIG. 23 illustrates one set of process steps that can be used to form an embodiment of the roughened topography 80 according to the present invention. As illustrated, there is some overlap in the processes that can be applied to form each of the three types of features (macro, micro, and nano). For example, acid etching can be used to form the macro features, then the same or a different acid etching process can be used to form the micro features.

(67) The final and overall shapes and dimensions of the features of the roughened topography 80 of the implant 1 are balanced to achieve design goals. More specifically, the combination of a rough but not sharp surface and the atraumatic surgical procedure generates initial stabilization upon insertion of the implant 1 into the vertebral space. As healing begins, the tissue cells benefit from this combination and can more rapidly anchor the implant 1 in a growing bone or fusion. Focusing high loads in small areas is also known through Wolff's Law to cause remodeling of the osseous tissues where they dissolve under high loads.

(68) Wolff's law is a theory developed by the German anatomist and surgeon Julius Wolff in the 19th century. The theory states that bone in a healthy person or animal will adapt to the loads under which the bone is placed. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that loading. The internal architecture of the trabeculae undergoes adaptive changes, followed by secondary changes to the external cortical portion of the bone, perhaps becoming thicker as a result. The converse is true as well: if the loading on a bone decreases, the bone will become weaker due to turnover. It is less metabolically costly to maintain the bone and there is no stimulus for continued remodeling that is required to maintain bone mass.

(69) This remodeling can work against the goal of fusion both biologically and mechanically with the implant 1 if the contact points are too aggressive, having points or sharp edges that focus loading of the bone and cause stress-induced necrosis or resorbtion of the bone tissues. In contrast, overly smooth surfaces do not have the benefit of generating enough initial friction for the interbody device to stabilize and allow for fusion. The improved surface of the roughened topography 80 of the implant 1, the related design of the surgical instruments used to insert the implant 1, and the procedure for conducting surgery work in concert to generate sufficient frictional resistance to biological forces allowing for beneficial initial stabilization and rapid long-term fusion of the joint to the implant 1. Instruments and procedures are balanced between preparing the surgical site with an endplate-preserving procedure and allowing for an implant 1 with a roughened top and bottom surface to frictionally fit into the site.

(70) The macro features of the roughened topography 80 are relatively large machined or etched features preferably, although not necessarily, organized in regular repeating patters and overlapping each other. The macro features also are cut from the base material that was used to form the implant 1. In a preferred process, the macro features are formed in three, sequential steps.

(71) FIG. 11A illustrates the result of the first step in forming the macro features. Specifically, a first cut pattern 102 of the macro features is formed in a surface (i.e., in the top surface 10 or the bottom surface 20) of the implant 1. The “cut 1” features of the first cut pattern 102 cover about 20% of the total area of the surface, leaving about 80% of the original surface 104 remaining. The range of these percentages is about ±20%.

(72) FIG. 12A is a perspective view, and FIG. 12B is a side view, of a cut 1 feature. FIG. 13A is a perspective view, and FIG. 13B is a side view, highlighting the edges of a cut 1 feature. As shown, the “cut 1” features of the first cut pattern 102 do not have any undercuts. These “cut 1” features have the smallest diameter and greatest depth of the macro features that are formed during the sequential steps. FIG. 14A is a perspective view, and FIG. 14B is a side view, showing an example regular and repeating pattern of the cut 1 features that form the first cut pattern 102. See also FIG. 11A.

(73) FIG. 11B illustrates the result of the second step in forming the macro features. Specifically, a second cut pattern 106 of the macro features is formed in the surface of the implant 1. Together, the “cut 1” features of the first cut pattern 102 and the “cut 2” features of the second cut pattern 106 cover about 85% of the total area of the surface, leaving about 15% of the original surface 104 remaining. The range of these percentages is about ±10%. These “cut 2” features have both a diameter and a depth between those of the “cut 1” and “cut 3” features of the macro features that are formed during the first and third steps of the process of forming the macro features of the roughened topography 80.

(74) FIG. 11C illustrates the result of the third and final step in forming the macro features. Specifically, a third cut pattern 108 of the macro features is formed in the surface of the implant 1. Together, the “cut 1” features of the first cut pattern 102, the “cut 2” features of the second cut pattern 106, and the “cut 3” features of the third cut pattern 108 cover about 95% of the total area of the surface, leaving about 5% of the original surface 104 remaining. The range of these percentages is about ±1%. These “cut 3” features have the largest diameter and least depth of the macro features that are formed during the sequential process steps.

(75) FIG. 15A is a perspective view, and FIG. 15B is a side view, of the macro features of the roughened topography 80 following completion of the three, sequential processing steps. As shown, the finished macro features comprise multiple patterns of the three, overlapping cuts: the first cut pattern 102, the second cut pattern 106, and the third cut pattern 108. FIG. 16 is a top, simulated view of the roughened topography 80 showing each of the first cut pattern 102, the second cut pattern 106, and the third cut pattern 108 individually and, in an area of overlapping features 110, the roughened topography 80 following completion of the three, sequential processing steps and combining (in an overlapping pattern) the first cut pattern 102, the second cut pattern 106, and the third cut pattern 108. FIG. 17 is a perspective view of the implant illustrated in FIG. 16.

(76) Several separate parameters can be used to characterize the roughness of an implant surface. Among those parameters are the average amplitude, Ra; the maximum peak-to-valley height, Rmax; and the mean spacing, Sm. Each of these three parameters, and others, are explained in detail below. Meanwhile, FIG. 18 is a side view illustrating the measurement of Ra for the third cut pattern 108 of the implant 1 shown in FIGS. 16 and 17. FIG. 19A is another side view of the implant 1 shown in FIGS. 16 and 17, and FIG. 19B is an expanded view of a portion of the implant 1 illustrated in FIG. 19A illustrating the measurement of Sm for the second cut pattern 106. FIG. 20A is yet another side view of the implant 1 shown in FIGS. 16 and 17, and FIG. 20B is an expanded view of a portion of the implant 1 shown in FIG. 20A illustrating the measurement of Rmax for the first cut pattern 102. FIG. 21 illustrates all three parameters, namely, Ra, Rmax, and Sm, for the macro features 112 of the implant 1.

(77) After the macro features 112 are formed in the implant 1, additional process steps are sequentially applied to the implant 1, in turn, to form the micro and the nano surface features of the roughened topography 80. After the micro features are formed, less than about 3% of the original surface 104 remains. The range of that percentage is about ±1%. After the nano features are formed, the roughened topography 80 covers substantially all of the top surface 10, the bottom surface 20, or both surfaces of the implant 1. In a preferred embodiment, the entire implant 1 is dipped in an etchant bath (without any masks) so that the roughened topography 80 covers substantially all surfaces of the implant 1.

(78) FIG. 22 illustrates the parameters Ra, Rmax, and Sm for the completed macro and nano surface features of the implant 1. As should be readily apparent to a skilled artisan, the process steps can be adjusted to create a mixture of depths, diameters, feature sizes, and other geometries suitable for a particular implant application. The orientation of the pattern of features can also be adjusted. Such flexibility is desirable, especially because the ultimate pattern of the roughened topography 80 of the implant 1 should be oriented in opposition to the biologic forces on the implant 1 and to the insertion direction. In one particular embodiment, for example, the pattern of the roughened topography 80 is modeled after an S-shaped tire tread.

(79) In addition to the parameters Ra, Rmax, and Sm mentioned above, at least two other parameters can be used to characterize the roughness of an implant surface. In summary, the five parameters are: (1) average amplitude, Ra; (2) average peak-to-valley roughness, Rz; (3) maximum peak-to-valley height, Rmax; (4) total peak-to-valley of waviness profile, Wt; and (5) mean spacing, Sm. Each parameter is explained in detail as follows.

(80) 1. Average Amplitude Ra

(81) In practice, “Ra” is the most commonly used roughness parameter. It is the arithmetic average height. Mathematically, Ra is computed as the average distance between each roughness profile point and the mean line. In FIG. 24, the average amplitude is the average length of the arrows.

(82) In mathematical terms, this process can be represented as

(83) $\mathrm{Ra}=\frac{1}{n}\underset{i=1}{\overset{n}{.Math.}}.Math.y_i.Math.$

(84) 2. Average Peak-to-Valley Roughness Rz

(85) The average peak-to-valley roughness, Rz, is defined by the ISO and ASME 1995 and later. Rz is based on one peak and one valley per sampling length. The RzDIN value is based on the determination of the peak-to-valley distance in each sampling length. These individual peak-to-valley distances are averaged, resulting in the RzDIN value, as illustrated in FIG. 25.

(86) 3. Maximum Peak-to-Valley Height Rmax

(87) The maximum peak-to-valley height, Rmax, is the maximum peak-to-valley distance in a single sampling length—as illustrated in the FIG. 26.

(88) 4. Total Peak-to-Valley of Waviness Profile Wt

(89) The total peak-to-valley of waviness profile (over the entire assessment length) is illustrated in FIG. 27.

(90) 5. Mean Spacing Sm

(91) The mean spacing, Sm, is the average spacing between positive mean line crossings. The distance between each positive (upward) mean line crossing is determined and the average value is calculated, as illustrated in FIG. 28.

(92) The parameters Sm, Rmax, and Ra can be used define the surface roughness following formation of each of the three types of features macro, micro, and nano. Such data are provided in Table 1 below.

(93) TABLE-US-00001 TABLE 1 EXAMPLE DATA BY PROCESS STEP Size (Sm) Depth (Rmax) Roughness (Ra) Surface Feature Size and Roughness (Metric) Macro (μm) Max. 2,000 500 200 Min. 400 40 20 Avg. 1,200 270 110 Surface Feature Size and Roughness (Metric) Micro (μm) Max. 400 40 20 MM. 20 2 1 Avg. 210 11 5.5 Surface Feature Size and Roughness (Metric) Nano (μm) Max. 20 2 1 Min. 0.5 0.2 0.01 Avg. 10.25 1.1 0.505

(94) From the data in Table 1, the following preferred ranges (all measurements in microns) can be derived for the macro features for each of the three parameters. The mean spacing, Sm, is between about 400-2,000, with a range of 750-1,750 preferred and a range of 1,000-1,500 most preferred. The maximum peak-to-valley height, Rmax, is between about 40-500, with a range of 150-400 preferred and a range of 250-300 most preferred. The average amplitude, Ra, is between about 20-200, with a range of 50-150 preferred and a range of 100-125 most preferred.

(95) The following preferred ranges (all measurements in microns) can be derived for the micro features for each of the three parameters. The mean spacing, Sm, is between about 20-400, with a range of 100-300 preferred and a range of 200-250 most preferred. The maximum peak-to-valley height, Rmax, is between about 2-40, with a range of 2-20 preferred and a range of 9-13 most preferred. The average amplitude, Ra, is between about 1-20, with a range of 2-15 preferred and a range of 4-10 most preferred.

(96) The following preferred ranges (all measurements in microns) can be derived for the nano features for each of the three parameters. The mean spacing, Sm, is between about 0.5-20, with a range of 1-15 preferred and a range of 5-12 most preferred. The maximum peak-to-valley height, Rmax, is between about 0.2-2, with a range of 0.2-1.8 preferred and a range of 0.3-1.3 most preferred. The average amplitude, Ra, is between about 0.01-1, with a range of 0.02-0.8 preferred and a range of 0.03-0.6 most preferred.

(97) Certain embodiments of the implant 1 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 implant 1 is 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 implant 1 to help restore cervical balance.

(98) When endplate-sparing spinal implant 1 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, the bone graft material inside the spinal implant 1 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, 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, hence reducing the risk of adjacent segment issues.

(99) In addition, the roughened topography 80 of the top surface 30 and the bottom surface 40, 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, a pull-out strength of up to 3,000 newtons may be expected. The roughened topography 80 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 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.

(100) The spinal implant 1 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.

EXAMPLE SURGICAL METHODS

(101) 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.

(102) Although removing the soft tissues of the diseased or damaged intervertebral disc is important for placement of the interbody fusion implant 1 so that the implant 1 primarily contacts the outer rim where vertebral discs are strongest, perforation or damage to the endplate can have degenerative effects to its healing after completion of the surgical procedure. Instrumentation used to remove the soft tissues is commonly known in the practice of orthopedic medicine, but the disc-preparation instruments used in connection with the implant 1 of the present invention are specifically designed and used in a prescribed method and aid in preserving the endplate structure.

(103) Once the soft tissues have been exposed, a rasp 14 with a specific tooth design is used to remove only the soft tissues remaining adhered to the endplate surface. The goal of the disc preparation with the rasp 14 is to allow for an implant 1 of sufficient size to be implanted with as close an approximation in shape to the site primarily on the above-described apophyseal rim, the strongest structural area of the vertebral body. The design of the teeth of the rasp 14 is not so aggressive as to allow for easy bone removal but will capture fragments of the soft tissue and remove such fragments from the implantation site.

(104) 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 disc space is elastic.

(105) 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.

(106) The next step in the procedure is to place a smooth-surfaced sizing instrument between the vertebrae and determine the height size of the required implant for the patient. Implantation of a fusion implant to stop the advance of a malady must not allow for the movement of the vertebrae relative to each other during both short-term and long-term healing. Early in the healing process, when the load transfer serves the purpose of loading a captured amount of bone growth enhancement materials within the implant 1, the stability of the implant 1 in the surgically implanted position is balanced with a frictional surface of sufficient surface area to absorb loading without this surface acting to abrade or damage the critical endplate of the vertebra.

(107) FIG. 9 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. These distractor blocks may be made in various heights to match the height of the implant 1. 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 is filled with autologous bone graft or bone graft substitute. The distractor 12 is removed and the spinal implant 1 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 itself (i.e., the implant 1 itself is used as a distractor).

(108) 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 (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.

(109) 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.

(110) 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 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 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.

(111) Because the endplates are spared during the process of inserting the spinal implant 1, 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 to accept and share stress transmitted from the endplates. In addition, spared endplates minimize the concern that BMP might erode the cancellous bone.

(112) Interbody spinal implants 1 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) 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, 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, as now taught, may provide secure seating and prove difficult to remove. Thus, certain embodiments of the present interbody spinal implant 1 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, the bottom surface 20, or both top and bottom surfaces.

(113) 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, 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.

(114) Certain embodiments of the spinal implants 1 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, 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.

(115) The implant 1 according to certain embodiments of the present invention has a surface with the roughened topography 80. The roughened topography 80 includes features in repeating patterns that can be used to resist biologic-induced motion after placement in a joint space in contact with bone structures. The surface features are generated through a subtractive process and are further refined to remove sharp edges that could abrade the ambient bone while still providing sufficient friction to resist expulsion or movement. Macro features of the roughened topography 80 can be aligned to allow for insertion in opposition to a surface and to resist reverse motion from frictional contact with this surface. Repeating patterns, depth of features, spacing of various shaped features, and arraignment and overlapping of them in respect to others of a similar size and shape can also be used to develop designed composite patterns. As healing advances, the micro and nano surface modifications work in concert with the ambient biological actions occurring during the healing and fusion process. Biological structures will be stimulated especially at the nano feature level to produce biologic products that cause hard tissue formation with connections to the implant structure.

(116) The implant 1 improves joint fusion through a balanced combination of structural features (a) designed and manufactured using a specific process, and (b) implanted using a surgical technique and method that results in initial mechanical fixation, allows for rapid bone growth during the healing process, and stimulates bone growth and fusion while reducing surgical treatment times. Some of implant surfaces have macro, micro and nano features; others have only micro and nano features. Regardless, the combinations of feature shapes aid in implantation and bone growth stimulation.

(117) The shapes of the features are combined in a balanced manner with a conservative surgical procedure to improve recovery and fusion rates and reduce surgical operatory time. Implants that are placed within joint spaces following the practice of preserving structural bone, for example preserving the vertebral endplates in intervertebral procedures, through a defined and conservative surgical technique using specifically designed instrumentation can enhance fusion of the joint space. Therefore, the present invention encompasses a method for implanting the interbody implant 1 using a surgical procedure that preserves the bone structure of the vertebral endplates during preparation of the implant socket using instruments having bone-preserving features and surfaces. The method preserves the bone structures which the implant 1 contacts throughout the healing process, but retains a level of friction between the implant 1 and the bones of the vertebrae contributing to the fusion and healing process.

(118) Existing implantation practices did not preserve critical structures, especially the vertebral endplates, during the surgical procedures. In addition, some of the existing implant devices are not designed with features that preserve critical bone structures during or after implantation. The structures and features of the implant 1 and the system of instruments used in connection with the implant 1, in accordance with the present invention, are designed to work in concert to preserve the endplate bone structures of the vertebral body. The surface preparation of the implant 1 provides for friction generation within the disc space but is not too aggressive which preserves the bone structures with which it is in contact.

(119) 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.