Abstract:
An improved bone graft is provided for human implantation, bone graft includes a substrate block of high strength biocompatible material having a selected size and shape to fit the anatomical space, and a controlled porosity analogous to natural bone. The substrate block may be coated with a bio-active surface coating material such as hydroxyapatite or a calcium phosphate to promote bone ingrowth and enhanced bone fusion. Upon implantation, the bone graft provides a spacer element having a desired combination of mechanical strength together with osteoconductivity and osteoinductivity to promote bone ingrowth and fusion, as well as radiolucency for facilitated post-operative monitoring. The bone graft may additionally carry one or more natural or synthetic therapeutic agents for further promoting bone ingrowth and fusion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application is a continuation-in-part of copending U.S. Ser. No. 10/137,106, filed Apr. 30, 2002, which in turn claims the benefit of U.S. Provisional Application No. 60/287,824, filed May 1, 2001. 
     
    
       [0002]     This invention relates generally to improvements in bone grafts such as spinal fusion cages of the type designed for human implantation between adjacent spinal vertebrae, to maintain the vertebrae in substantially fixed spaced relation while promoting interbody bone ingrowth and fusion therebetween. More particularly, this invention relates to an implantable bone graft having an improved combination of enhanced mechanical strength together with osteoinductive and osteoconductive properties, in a device that additionally and beneficially provides visualization of bone growth for facilitated post-operative monitoring.  
         [0003]     Implantable bone grafts are known in the art and are routinely used by orthopedic surgeons to keep skeletal structures in a desired spaced-apart relation while bone ingrowth and fusion takes place. Such grafts are also used to provide weight bearing support between adjacent skeletal bodies and thus correct clinical problems. Such grafts are indicated for surgical treatment to reinforce weak bony tissue. These conditions have been treated by using constructs, typically made from metals such as titanium or cobalt chrome alloys such as used in orthopedic implants, and allograft (donor) or autograft (patient) bone to promote bone ingrowth and fusion.  
         [0004]     Typical bone grafts, such as plugs for example, have hollow or open spaces that are usually filled with bone graft material, either autogenous bone material provided by the patient or allogenous bone material provided by a third party donor. These devices also have lateral slots or openings which are primarily used to promote ingrowth of blood supply and grow active and live bone. These implants may also have a patterned exterior surface such as a ribbed or serrated surface or a screw thread to achieve enhanced mechanical interlock between skeletal structures, with minimal risk of implant dislodgement from the site. See, for example, U.S. Pat. Nos. 5,785,710; and 5,702,453. Typical materials of construction for such devices include bio-compatible carbon fiber reinforced polymers, cobalt chrome alloys, and stainless steels or titanium alloys. See, for example, U.S. Pat. No. 5,425,772.  
         [0005]     Most state-of-the-art bone grafts are made from titanium alloy and allograft (donor) bone, and have enjoyed clinical success as well as rapid and widespread use due to improved patient outcomes. However, traditional titanium-based implant devices exhibit poor radiolucency characteristics, presenting difficulties in post-operative monitoring and evaluation of the fusion process due to the radio-shadow produced by the non-lucent metal. There is also clinical evidence of bone subsidence and collapse which is believed to be attributable to mechanical incompatibility between natural bone and the metal implant material. Moreover, traditional titanium-based implant devices are primarily load bearing but are not osteoconductive, i.e., not conducive to direct and strong mechanical attachment to patient bone tissue, leading to potential graft necrosis, poor fusion and stability. By contrast, allograft bone implants exhibit good osteoconductive properties, but can subside over time as they assimilate into natural bone. Further, they suffer from poor pull out strength resulting in poor stability, primarily due to the limited options in machining the contact surfaces. Allograft bone implants also have variable materials properties and, perhaps most important of all, are in very limited supply. A small but finite risk of disease transmission with allograft bone is a factor as well. In response to these problems some developers are attempting to use porous tantalum-based metal constructs, but these have met with limited success owing to the poor elastic modulii of porous metals.  
         [0006]     A typical titanium alloy bone graft device is constructed from a hollow cylindrical and threaded metal cage-like construct with fenestrations that allow communication of the cancellous host tissue with the hollow core, which is packed with morselized bone graft material. This design, constrained by the materials properties of titanium alloys, relies on bony ingrowth into the fenestrations induced by the bone graft material. However, the titanium-based structure can form a thin fibrous layer at the bone/metal interface, which degrades bone attachment to the metal. In addition, the hollow core into which the graft material is packed may have sub-optimal stress transmission and vascularization, thus eventually leading to failure to incorporate the graft. Mechanical stability, transmission of fluid stress, and the presence of osteoinductive agents are required to stimulate the ingrowth of vascular buds and proliferate mesenchymal cells from the cancellous host tissue into the graft material. However, most titanium-based bone graft devices in use today have end caps or lateral solid walls to prevent egress of the graft outwardly from the core and ingress of remnant disc tissue and fibroblasts into the core.  
         [0007]     Autologous (patient) bone fusion has been used in the past and has a theoretically ideal mix of osteoconductive and osteoinductive properties. However, supply of autologous bone material is limited and significant complications are known to occur from bone harvesting. Moreover, the costs associated with harvesting autograft bone material are high, requiring two separate incisions, with the patient having to undergo more pain and recuperation due to the harvesting and implantation processes. Additionally, autologous cancellous bone material has inadequate mechanical strength to support musculoskeletal forces by itself, whereby the bone material is normally incorporated with a metal-based construct.  
         [0008]     Ceramic materials provide potential alternative structures for use in spinal fusion implant devices. In this regard, monolithic ceramic constructs have been proposed, formed from conventional materials such as hydroxyapatitie (HAP) and/or tricalcium phosphate (TCP). See, for example, U.S. Pat. No. 6,037,519. However, while these ceramic materials may provide satisfactory osteoconductive and osteoinductive properties, they have not provided the mechanical strength necessary for the implant.  
         [0009]     Thus, a significant need exists for further improvements in and to the design of bone grafts, particularly to provide a high strength implant having high bone ingrowth and fusion characteristics, together with substantial radiolucency for effective and facilitated post-operative monitoring.  
         [0010]     Hence, it is an object of the present invention to provide an improved bone graft made from a bio-compatible open pore structure, which has a radiolucency similar to that of the surrounding bone. It is also an object of the present invention to provide a substrate of adequate bio-mechanical strength for carrying biological agents which promote bone ingrowth, healing and fusion. It is a further objective of the present invention to provide a fusion device which has mechanical properties that substantially match that of natural bone.  
       SUMMARY OF THE INVENTION  
       [0011]     In accordance with the invention, an improved bone graft is provided for human implantation into the space between a pair of adjacent skeletal structures, to maintain the adjacent skeletal anatomy in a predetermined and substantially fixed spaced relation while promoting bone ingrowth and fusion. In this regard, the improved bone graft of the present invention is designed for use in addressing clinical problems indicated by surgical treatment of bone fractures, skeletal non-unions, weak bony tissue, degenerative disc disease, discogenic lower back pain, and spondylolisthesis.  
         [0012]     The improved bone graft comprises a substrate block formed from a bio-compatible material composition having a relatively high bio-mechanical strength and load bearing capacity, substantially equivalent to natural cortical bone. This substrate may be porous, open-celled, or dense solid. A preferred material of the high strength substrate block comprises a ceramic material. The substrate block may be porous, having a porosity of about 10% to about 80% by volume with open pores distributed throughout and a pore size range of from about 5 to about 500 microns. When the substrate is porous, the porosity of the substrate block is gradated from a first relatively low porosity region emulating or mimicking the porosity of cortical bone to a second relatively higher porosity region emulating or mimicking the porosity of cancellous bone. In a second embodiment, the substrate block is a dense solid comprised of a ceramic, metal or polymer material such as PEEK, carbon fiber reinforced polymer, PMMA, PLA or other bioresorbable polymer, or composition thereof. This dense solid substrate would then be attached to a second highly porous region emulating or mimicking the porosity of cancellous bone. Preferably, the porous region would be formed around the substrate.  
         [0013]     In the method where a dense, solid material is used as the substrate block, the block will be externally coated with a bio-active surface coating material selected for relatively high osteoconductive and osteoinductive properties, such as a hydroxyapatite or a calcium phosphate material. The porous portion is internally and externally coated with a bio-active surface coating material selected for relatively high osteoconductive and osteoinductive properties, such as a hydroxyapatite or a calcium phosphate material. The porous region, however, may be in and of itself a bio-active material selected for relatively high osteoconductive and osteoinductive properties, such as a hydroxyapatite or a calcium phosphate material.  
         [0014]     The thus-formed bone graft can be made in a variety of shapes and sizes to suit different specific implantation requirements. Preferred shapes include a generally rectangular or cylindrical block with a tapered cross section to suit the required skeletal anatomy. The exterior superior and inferior surfaces of the body may include ridges or teeth for facilitated engagement with the adjacent skeletal structures. Alternative preferred shapes include a generally oblong, rectangular or cylindrical block which may also include serrations or the like on one or more exterior faces thereof, and/or may have a tapered cross section for improved fit into the skeletal anatomy. A further preferred shape may include a crescent shape block which may also include serrations or the like on one or more exterior faces thereof, and/or may have a tapered cross section for improved. The bone graft may desirably include notches for releasable engagement with a suitable insertion tool. In addition, the bone graft may also include one or more laterally open recesses or bores for receiving and supporting osteoconductive bone graft material, such as allograft (donor) or autograft (patient) material.  
         [0015]     Further alternative bone graft configurations may include a dense substrate region substantially emulating cortical bone, to define a high strength loading bearing zone or strut for absorbing impaction and insertion load, in combination with one or more relatively high porosity second regions substantially emulating cancellous bone for contacting adjacent patient bone for enhanced bone ingrowth and fusion.  
         [0016]     The resultant bone graft exhibits relatively high mechanical strength for load bearing support, while additionally and desirably providing high osteoconductive and osteoinductive properties to achieve enhanced bone ingrowth and interbody fusion. Importantly, these desirable characteristics are achieved in a structure which is substantially radiolucent so that the implant does not interfere with post-operative radiographic monitoring of the fusion process.  
         [0017]     In accordance with a further aspect of the invention, the bone graft may additionally carry one or more therapeutic agents for achieving further enhanced bone fusion and ingrowth. Such therapeutic agents may include natural or synthetic therapeutic agents such as bone morphogenic proteins (BMPs), growth factors, bone marrow aspirate, stem cells, progenitor cells, antibiotics, or other osteoconductive, osteoinductive, osteogenic, or any other fusion enhancing material or beneficial therapeutic agent.  
         [0018]     Other features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The accompanying drawings illustrate the invention. In such drawings:  
         [0020]      FIG. 1  is a perspective view showing the one preferred embodiment of bone graft;  
         [0021]      FIG. 2  is a perspective view showing the load bearing portion of the device of  FIG. 1  with anterior and posterior load bearing walls connected by a strut, relieved in the superior and inferior aspects;  
         [0022]      FIG. 3  is a perspective view depicting one alternative preferred and generally rectangular bone graft;  
         [0023]      FIG. 4  is a perspective view depicting the load bearing portion of the device of  FIG. 3  with anterior and posterior load bearing walls connected by a strut, relieved in the superior and inferior aspects;  
         [0024]      FIG. 5  is a perspective view showing still another alternative preferred form of the invention, comprising a generally oblong, rectangular bone graft;  
         [0025]      FIG. 6  is a perspective view depicting the load bearing portion of the device of  FIG. 5  with anterior and posterior load bearing walls connected by a strut, relieved in the superior and inferior aspects;  
         [0026]      FIG. 7  is an axial view of still another alternative form of the invention, comprising a generally crescent shaped device conforming to the natural of the pelvis;  
         [0027]      FIG. 8  is a perspective view of the device of  FIG. 7 , showing a porous posterior margin;  
         [0028]      FIG. 9  is a perspective view of the load bearing portion of the device of  FIG. 7 , showing a anterior and lateral load bearing walls connected by a central strut, relieved in the superior and inferior aspects;  
         [0029]      FIG. 10  is an axial view of a further preferred alternative embodiment of the invention, comprising of a generally rectangular shape with macro-pores;  
         [0030]      FIG. 11  is a perspective view of the device of  FIG. 10  showing the interconnection of the macro-pores; and  
         [0031]      FIG. 12  is a sectional view of the device of  FIG. 10  taken generally along the mid-transverse plane  12 - 12  of  FIG. 10  of the device;  
         [0032]      FIG. 13  is a perspective view depicting the bone graft in the inter-vertebral space;  
         [0033]      FIG. 14  is a perspective view depicting the device in  FIG. 7  in the iliac crest of the pelvis; and  
         [0034]      FIG. 15  is a perspective view depicting the bone graft in the femur.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     As shown in the exemplary drawings, a radiolucent bone graft referred to generally in  FIGS. 1-2  by the reference numeral  10  is provided for seated implantation between a pair of adjacent patient bones  12  ( FIG. 13 ) to maintain the skeletal tissues or structures in spaced relation while promoting interbody bone ingrowth and fusion. In general, the improved bone graft  10  comprises a bio-compatible substrate having a porous construction to define an open lattice conducive to interbody bone ingrowth and fusion, while providing a strong mechanical load bearing structure analogous to the load bearing properties of cortical and cancellous bone. This open-celled substrate is coated internally and externally with a bio-active surface coating selected for relatively strong osteoconductive and osteoinductive properties, whereby the coated substrate provides a scaffold conducive to cell attachment and proliferation to promote interbody bone ingrowth and fusion attachment. The substrate may also carry one or more selected therapeutic agents suitable for bone repair, augmentation and other orthopedic uses.  
         [0036]      FIGS. 1-2  illustrate the improved bone graft  10  in accordance with one preferred embodiment, in the shape of a generally rectangular body having ridges formed on the top and bottom faces  14 . The lateral, anterior, and posterior walls of the body having notches  18  for the releasable engagement with an insertion tool.  
         [0037]     The preferred substrate composition comprises a relatively high strength block  16  ( FIG. 2 ). In accordance with one preferred form of the invention, this substrate block comprises a relatively dense  16  ceramic composition having a controlled porosity and having a suitable size and shape for seated implantation, such as into the inter-vertebral space in the case of the spinal fusion cage  10 . In a preferred form, the remainder of the substrate is comprised of a relatively porous ceramic  20  ( FIG. 1 ) having an open-celled controlled porosity.  
         [0038]     Moreover, in the preferred form, the pores are arranged with a variable porosity gradient to define a first region of relatively low or reduced porosity (less than about 5%) mimicking cortical bone structure and a second region of relatively large or increased porosity (ranging from about 30% to about 80%) mimicking cancellous bone structure. In one preferred configuration, the outer or external surfaces of the reticulated substrate block comprise the first or low porosity region for improved load bearing capacity, while the interior surfaces of the substrate block comprises the second or high porosity region mimicking cancellous bone for enhance bone ingrowth and fusion.  
         [0039]     This high strength substrate block is surface-coated internally and externally with a bio-active organic or inorganic surface coating material selected for relatively strong osteoconductive and osteoinductive properties to provide a nutrient rich environment for cellular activity to promote interbody bone ingrowth and fusion attachment. Preferred surface coating materials comprise a resorbable material such as hydroxyapatite or a calcium phosphate ceramic. Alternative glassy (amorphous) materials having a relatively rich calcium and phosphate composition may also be used, particularly wherein such materials incorporate calcium and phosphate in a ratio similar to natural bone or hydroxyapatite. Such glassy compositions may comprise a partially or fully amorphous osteoinductive material comprising a composite of a glass and osteoinductive calcium compound, with a composition varying from about 100% glass to 100% osteoinductive calcium compound. The surface coating may also comprise autologous bone marrow aspirates.  
         [0040]     The resultant bone graft  10  thus comprises the substrate block formed from the high strength material having bio-mimetic properties and which is nonresorbable, or slowly or infinitely slowly resorbable when implanted into the patient, in combination with the bio-active surface coating which is comparatively rapidly resorbable to promote rapid and vigorous bone ingrowth activity.  
         [0041]     The substrate block may also advantageously be coated or impregnated with one or more selected therapeutic agents, for example, such as autologous, synthetic or stem cell derived growth factors or proteins and growth factors such as bone morphogenic protein (BMP) or a precursor thereto, which further promotes healing, fusion and growth. Alternative therapeutic agents may also include an antibiotic, or natural therapeutic agents such as bone marrow aspirates, and growth factors or progenitor cells such as mesenchymal stem cells, hematopoietic cells, or embryonic stem cells, either alone or as a combination of different beneficial agents.  
         [0042]     The resultant illustrative bone graft  10  exhibits relatively high bio-mechanical strength similar to the load bearing characteristics of natural bone. In addition, the bone graft  10  exhibits relatively strong osteoconductive and osteoinductive characteristics attributable primarily to the surface coating, again similar to natural bone. Importantly, the bone graft  10  is also substantially radiolucent and non-magnetic, so that the fusion cage does not interfere with post-operative radiological or other imaging methods of analysis of interbody bone ingrowth and fusion.  
         [0043]     The relatively dense, high strength portion  16  is preferably formed in a manner with which to withstand the loading of the skeletal structures. In the preferred embodiment, the anterior and posterior walls of the device are formed as part of this high strength portion. This is done to allow the high strength region to interface with the cortical portion of the adjacent skeletal body  12 . Additionally, a strut  22  of the high strength material extends between the anterior and posterior walls, which beneficially provides a load bearing structure capable of withstanding impaction and insertion loading in the anterior-posterior direction. Consequently, the relatively porous portion is formed in-between the dense anterior-posterior walls and around the central strut. The porous portion thereby forms the remainder of the device, including a large region of the superior, inferior, and lateral aspects. The porous portion, being less dense in nature than the high strength regions of the device, is increasingly radiolucent, thus allowing for assessment of bone growth and bony attachment to the adjacent skeletal tissue such as adjacent vertebral bodies.  
         [0044]      FIGS. 3-9  illustrate alternative configurations for improved bone grafts constructed in accordance with the present invention, it being recognized and understood that the bone graft can be constructed in a wide range of different geometric sizes and shapes.  FIG. 3  shows a spinal fusion cage  110  having a generally rectangular shape similar to the fusion cage  10  shown and described in  FIGS. 1-2 , but the form is elongated, as for use in replacing an entire skeletal body. As shown, the bone graft  110  ( FIG. 4 ) has a relatively dense structure defined by a high strength substrate block  112  (as previously described) coated with the bio-active surface coating material, but wherein the relatively dense interior structure is defined multiple struts  116  with high strength for withstanding impaction and insertion loading in an anterior-posterior direction. The multiple struts  116  additionally create interior openings which provide for lateral fluid transmission and optimize bone growth laterally through the center of the implant.  FIG. 4  shows multiple dense struts, thereby demonstrating that the porous region is able to make contact with the adjacent superior and inferior vertebrae. The porous region  114  is more radiolucent than the surrounding dense portion and therefore provides enhanced visualization for analysis of bone growth and subsequent fusion with the adjacent skeletal structures. Each of the embodiments depicted in  FIGS. 1-12  has a height dimension and may be tapered in shape for enhanced anatomical fit.  
         [0045]      FIGS. 5-6  depicts still another alternative preferred embodiment of a generally oblong, rectangular or cylindrical geometry  410  having both a high strength, dense region  40 , as well as a relatively porous region  44  for bone in-growth. This geometry would be useful for surgical approaches in which it is necessary to place two implants next to each other. More particularly,  FIGS. 5-6  show a generally oblong, rectangular or cylindrical bone graft  410  having a tapered height dimension in the anterior-posterior direction. The substrate block is formed with the first region  40  of relatively low porosity substantially mimicking cortical bone to extend across the anterior and posterior faces and further to include at least one interconnecting load bearing strut  42  shown in the illustrative drawings to extend centrally in an anterior-posterior direction within the body of the substrate block. The remainder of the substrate block comprises the second portion  44  of relatively high porosity substantially mimicking cancellous bone. The harder first region  40  including the central strut  42  beneficially provides a hard and strong load bearing structure capable of withstanding impaction and insertion forces in the anterior-posterior direction without damage to the implant, while the softer second region  44  presents an exposed and large surface area for substantially optimized interknitting ingrowth and fusion with adjacent patient bone. In a spinal fusion cage application, the medial-lateral faces of the implant are advantageously defined by the softer second region  44 , wherein these regions are thus exposed to traditional medial-lateral X-ray imaging for post-operative radiological analysis of the implant/bone interface. Persons skilled in the art will recognize and appreciate that alternative configurations for the load bearing strut or struts  42  may be used, such as an X-shaped strut configuration extending in a cranial-caudial direction, in combination with or in lieu of the exterior faces  40  and/or the anterior-posterior central strut as shown.  
         [0046]      FIGS. 7-9  depict a further alternative preferred form of the invention, with a generally crescent shaped geometry  510 . The substrate block is formed of a relatively dense, high strength region  50  substantially mimicking cortical bone extending along the anterior and lateral walls. The dense portion  50  once again beneficially provides a strong load bearing structure capable of withstanding loads. Also, the high-strength region  50  is located along the anterior of the substrate, thereby interfacing with the load bearing cortical bone of the adjacent skeletal body. An integral dense strut  52  extends between the dense lateral walls providing a load bearing structure for impaction and insertion forces exhibited in a lateral approach. The superior, inferior, and posterior portions of the substrate are formed with a relatively porous material  54 . This provides for bone growth and increased radiolucency.  
         [0047]      FIGS. 10-13  depict a still further alternative preferred embodiment which is formed entirely of a relatively low porosity, high-strength substrate  610 . The subsequent porous structure  60  is created by drilling or boring a plurality of macro-pores  62  into the superior, inferior, and lateral faces of the device. This method allows the anterior and posterior walls to remain intact and thus be able to withstand the loading of the skeletal structures. The macro-pores are oriented in both the axial direction of the skeletal structures, as well as between the lateral walls of the device, thereby allowing bone to grow in the direction of the skeletal loading and laterally through the substrate. The macro-pores are positioned in such a manner as to allow for continuous interconnection  70 , thereby creating a meshwork of pores for bony ingrowth into the device. The macro-pores extend either from one face of the device to the opposite face  64 , or towards the center of the device, extended to a certain depth, and terminated therein  66 . The blind macro-pores  66  in turn create a portion in the center of the device which remains solid and is therefore a load bearing strut  68  extending from the anterior wall to the posterior wall and capable of withstanding impaction and insertion loads in the anterior-posterior direction. This macro-pore method can also be utilized with geometries similar to those depicted in  FIGS. 5-9 , such as the oblong rectangular  410  and the crescent  510 .  
         [0048]     In all of the embodiments of  FIGS. 1-12 , the substrate block comprises a high strength porous ceramic as previously described, and is coated with the bio-active surface coating material, again as previously described, to enhance bone ingrowth and fusion. The substrate block may also include one or more therapeutic agents. Persons skilled in the art will recognize and appreciate that the relatively low and high porosity regions  16  and  20  shown in  FIGS. 1-2  will be integrally joined by a suitable albeit relatively narrow gradient region wherein the porosity transitions there between.  
         [0049]      FIGS. 13-15  depict various embodiments of the bone graft in different skeletal structures. In  FIG. 13 , bone graft  10  is shown between two adjacent vertebral bodies  12  with the intent to enhance bone ingrowth and fusion. The bone graft  510  embodiment displayed in  FIG. 14  is depicted as replacing a defect in the iliac crest  712  of the pelvic bone. In this embodiment, the defect could be a result of tumor, trauma, or surgical intervention.  FIG. 15  shows a further embodiment of the bone graft  710  connecting two portions of a long bone, such as the femur  714 . This embodiment of the bone graft  710  is intended to enhance bone growth and fusion while providing structural support.  
         [0050]     The improved bone graft of the present invention thus comprises an open-celled substrate block structure which is coated with a bio-active surface coating, and has the strength required for the weight bearing capacity required of a fusion device. The capability of being infused with the appropriate biologic coating agent imparts desirable osteoconductive and osteoinductive properties to the device for enhanced interbody bone ingrowth and fusion, without detracting from essential load bearing characteristics. The radiolucent or non-magnetic characteristics of the improved device beneficially accommodate post-operative radiological or other diagnostic imaging examination to monitor the bone ingrowth and fusion progress, substantially without undesirable radio-shadowing. The external serrations or threads formed on the bone graft may have a variable depth to enable the base of the device to contact the cortical bone for optimal weight bearing capacity. In addition to these benefits, the present invention is easy to manufacture in a cost competitive manner. The invention thus provides a substantial improvement in addressing clinical problems indicated for surgical treatment of bone fractures, non-unions, weak bony tissue, degenerative disc disease, discogenic low back pain and spondylolisthesis.  
         [0051]     The bone graft of the present invention provides at least the following benefits over the prior art: 
        [a] a porous osteoconductive scaffold for enhanced fusion rates;     [b] a bio-mimetic load bearing superstructure providing appropriate stress transmission without fatigue failure;     [c] a pore structure and size suitable for ingrowth and vascularization,     [d] the ability to absorb and retain an osteoinductive agent such as autologous bone marrow aspirate or BMPs;     [e] bio-inert and bio-compatible with adjacent tissue and selected for ease of resorption;     [f] fabricatable and machinable into various shapes;     [g] sterilizable; and     [h] low manufacturing cost.        
 
         [0060]     A variety of further modifications and improvements in and to the bone graft of the present invention will be apparent to those persons skilled in the art. In this regard, it will be recognized and understood that the bone graft implant can be formed in the size and shape of a small pellet for suitable packing of multiple implants into a bone regeneration/ingrowth site. Accordingly, no limitation on the invention is intended by way of the foregoing description and accompanying drawings, except as set forth in the appended claims.