Abstract:
An interbody osteogenic fusion device is provided that includes opposite end pieces with an integral central element. The end pieces are sized to maintain the height of an intervertebral disc space. The central element has a much smaller diameter so that the osteogenic fusion device forms an annular pocke around the central element. An osteogenic material is disposed within the annular pocket between the opposite end pieces. In one embodiment,the osteogenic material constitutes a collagen sheet soaked in asolution containing a bone morphogenetic protein. The osteogenic fusion device is configured so that the osteogenic material is in direct contact with the adjacent vertebral bone. In addition to the enhanced area of contact between the vertebral bone and the fusion material, the inventive osteogenic fusion device reduces stress-shielding and minimizes the radiopacity of the implant so that growth of the fusion mass can be continuously assessed. In yet another embodiment, the osteogenic fusion device includes at least one end piece with a truncated surface. The osteogenic fusion devices of the present invention may be combined with other fusion devices to form an implant system. The implant system includes at least one load bearing member having a truncated surface configured to nest within another load bearing member, preferably the load bearing, osteogenic fusion device of the present invention. The invention also provides implant systems comprising adjacent load bearing members connected to one another to resist lateral separation. Methods of promoting fusion bone growth in the space between adjacent vertebrae utilizing devices and systems of the invention are also described.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 09/581,335, filed Sep. 25, 2000, which is the National Stage of International Application No. PCT/US98/26254 filed Dec. 10, 1998, published in English, which is a continuation-in-part of U.S. patent Ser. No. 08/988,142 filed Dec. 10, 1997, now U.S. Pat. No. 6,146,420, all of which are hereby incorporated by reference in their entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to an implant to be placed into the intervertebral space left after the removal of a damaged spinal disc. Specifically, the invention concerns an osteogenic fusion device that enhances arthrodesis or fusion between adjacent vertebrae while also maintaining the normal spinal anatomy at the instrumented vertebral level.  
           [0003]    In many cases, low back pain originates from damages or defects in the spinal disc between adjacent vertebrae. The disc can be herniated or can be affected by a variety of degenerative conditions. In many cases, these pathologies affecting the spinal disc can disrupt the normal anatomical function of the disc. In some cases, this disruption is significant enough that surgical intervention is indicated.  
           [0004]    In one such surgical treatment, the affected disc is essentially removed and the adjacent vertebrae are fused together. In this treatment, a discectomy procedure is conducted to remove the disc nucleus while retaining the annulus. Since the disc material has been removed, a body must be placed within the intervertebral space to prevent the space from collapsing.  
           [0005]    In early spinal fusion techniques, bone material, or bone osteogenic fusion devices, were simply disposed between adjacent vertebrae, typically at the posterior aspect of the vertebrae. In the early history of these osteogenic fusion devices, the osteogenic fusion devices were fanned of cortical-cancellous bone which was not strong enough to support the weight of the spinal column at the instrumented level. Consequently, the spine was stabilized by way of a plate or a rod spanning the affected vertebrae. With this technique, once fusion occurred across and incorporating the bone osteogenic fusion device, the hardware used to maintain the stability of the spine became superfluous.  
           [0006]    Following the successes of the early fusion techniques, focus was directed to modifying the device placed within the intervertebral space. Attention was then turned to implants, or interbody fusion devices, that could be interposed between the adjacent vertebrae, maintain the stability of the disc interspace, and still permit fusion or arthrodesis. These interbody fusion devices have taken many forms. For example, one prevalent form is a cylindrical hollow implant or “cage”. The outer wall of the cage creates an interior space within the cylindrical implant that is filled with bone chips, for example, or other bone growth-inducing material. Implants of this type are represented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. No. 4,961,740; and Michelson, U.S. Pat. No. 5,015,247. In some cases, the cylindrical implants included a threaded exterior to permit threaded insertion into a tapped bore formed in the adjacent vertebrae. Alternatively, some fusion implants have been designed to be impacted into the intradiscal space.  
           [0007]    Experience over the last several years with these interbody fusion devices has demonstrated the efficacy of these implants in yielding a solid fusion. Variations in the design of the implants have accounted for improvements in stabilizing the motion segment while fusion occurs. Nevertheless, some of the interbody fusion devices still have difficulty in achieving a complete fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of the devices are not structurally strong enough to support the heavy loads and bending moments applied at certain levels of the spine, namely those in the lumbar spine.  
           [0008]    Even with devices that do not have these difficulties, other less desirable characteristics exist. Recent studies have suggested that the interbody fusion implant devices, or cages as they are frequently called, lead to stress-shielding of the bone within the cage. It is well known that bone growth is enhanced by stressing or loading the bone material. The stress-shielding phenomenon relieves some or all of the load applied to the material to be fused, which can greatly increase the time for complete bone growth, or disturb the quality and density of the ultimately formed fusion mass. In some instances, stress-shielding can cause the bone chips or fusion mass contained within the fusion cage to resorb or evolve into fibrous tissue rather than into a bony fusion mass.  
           [0009]    A further difficulty encountered with many fusion implants is that the material of the implant is not radiolucent. Most fusion cages are formed of metal, such as stainless steel, titanium or porous tantalum. The metal of the cage shows up prominently in any radiograph (x-ray) or CT scan. Since most fusion devices completely surround and contain the bone graft material housed within the cage, the developing fusion mass within the metal cage between the adjacent vertebrae cannot be seen under traditional radiographic visualizing techniques and only with the presence of image scatter with CT scans. Thus, the spinal surgeon does not have a means to determine the progress of the fusion, and in some cases cannot ascertain whether the fusion was complete and successful.  
           [0010]    The field of spinal fusion can be benefited by an intervertebral fusion device that can support bone growth material within the intervertebral space, while still maintaining the normal height of the disc space. The device would beneficially eliminate the risk of stress-shielding the fusion mass, and would also provide for visualization of the fusion mass as the arthrodesis progresses.  
         SUMMARY OF INVENTION  
         [0011]    To address the current needs with respect to interbody fusion devices, the present invention contemplates a osteogenic fusion device that is configured to place as much of the bone growth-inducing material as possible into direct contact with the adjacent bone. In one embodiment, the osteogenic fusion device includes an elongated body having opposite first and second end pieces separated by an integral central element. The central element has a significantly smaller diameter than the two end pieces. The osteogenic fusion device thus forms an annular pocket between the end pieces and around the central element.  
           [0012]    In accordance with one aspect of the present invention, a bone growth-inducing material is disposed within the annular pocket around the central element of the osteogenic fusion device. In one specific embodiment, the bone growth-inducing material can constitute a sheet of a pharmaceutically suitable carrier for a bone growth factor, such as a bone morphogenetic protein. In this embodiment, the sheet can be a collagen sheet that is soaked with the BMP and then subsequently wrapped in spiral fashion around the central element of the osteogenic fusion device.  
           [0013]    In one feature of the present invention, the osteogenic fusion device can be implanted in a bi-lateral approach. Specifically, two such osteogenic fusion devices can be inserted into prepared bores formed in the endplates of adjacent vertebrae after completion of a discectomy. The spinal loads are borne by the two end pieces that are in direct contact with the adjacent vertebral bodies. Preferably, the osteogenic fusion device has a length sufficient to allow the end pieces to at least partially contact the harder bone at the apophysis of the adjacent vertebrae. With the osteogenic fusion device thus inserted, the bone growth-inducing material is in direct contact with the adjacent vertebral bodies. In addition, bone growth-inducing material can be placed within the bi-lateral space separating the two osteogenic fusion devices. When fusion occurs, a substantial fusion mass is produced that is virtually uninterrupted by the material of the osteogenic fusion device itself.  
           [0014]    Several alternative embodiments of the osteogenic fusion device are presented, all retaining the capability of supporting bone growth-inducing material so that it is in direct contact with the adjacent vertebrae. In some embodiments, additional elements of the central element are provided, while in another embodiment, an intermediate piece is provided for further support across the disc space. In one embodiment, osteogenic fusion devices are provided wherein at least one of the opposite end pieces includes a truncated surface. In yet another embodiment, the truncated surface advantageously includes opposite faces, such as opposite edges, that define an entrance to a cutout region. The cutout region is typically defined by the truncated surface and the truncated surface is preferably concave. Such implants are advantageously configured to nest within another fusion device, such as the fusion device of the present invention.  
           [0015]    Another embodiment of the present invention provides an implant system including at least two load bearing members as described above adapted to be bilaterally placed between adjacent vertebrae, wherein at least one of the load bearing members has a truncated surface configured to nest within the other load bearing member.  
           [0016]    Yet another embodiment of the invention provides an implant system for promoting fusion bone growth in the space between adjacent vertebrae which includes at least first and second load bearing members adapted to be bilaterally placed between adjacent vertebrae, wherein the load bearing members are connected to one another so as to resist lateral separation. In particular, a preferred embodiment provides a first of the load bearing members including a male member, and a second of the load bearing members including a female member. The male and female members cooperate to resist lateral separation of said devices. In another preferred embodiment, the load bearing members can be connected by a connecting member such as a plate spanning the two load bearing members.  
           [0017]    In other embodiments of the invention, methods of promoting fusion bone growth in the space between adjacent vertebrae are provided. The methods include providing load bearing members or implant systems as described above, preparing adjacent vertebrae to receive the load bearing members or implant systems in an intervertebral space between adjacent vertebrae and placing the load bearing members or implant systems into the intervertebral space after the preparing step.  
           [0018]    The present invention also contemplates an insertion tool and certain modifications to the osteogenic fusion device to accommodate the tool. In one preferred embodiment, the tool is essentially an elongated shank having opposite prongs extending therefrom. The prongs can engage truncated side walls of one of the end pieces. In addition, the opposite end piece can be formed with notches to receive the tips of the two prongs. With this design, the osteogenic fusion device can be a push-in or a threaded type osteogenic fusion device.  
           [0019]    It is one object of the present invention to provide an interbody fusion device that allows the greatest possible contact between the adjacent vertebrae and the bone growth-inducing material supported by the osteogenic fusion device. It is a further object to provide such a osteogenic fusion device that is capable of supporting the loads generated throughout the spine without stress-shielding developing bone within the osteogenic fusion device.  
           [0020]    Another object of the invention is achieved by features that minimize the radiopacity of the device. This results in a benefit to the surgeon of being able to more readily assess the progress of a spinal fusion.  
           [0021]    Yet another object of the invention is to provide an interbody fusion device whereby enough lateral exposure is present to place two large devices side-by-side to distract the disc space and facilitate fusion.  
           [0022]    It is yet another object of the invention to provide an interbody fusion device which can be placed closer to another interbody fusion device and which will require no or minimal resection of facet joints.  
           [0023]    Yet a further object of the invention is to provide an implant system which is placed in the intervertebral space with minimal retraction of the spinal cord to lessen the chance of neurological complications or damage.  
           [0024]    Other objects and benefits of the present invention can be discerned from the following written description and accompanying figures.  
       
    
    
     DESCRIPTION OF THE FIGURES  
       [0025]    [0025]FIG. 1 is a top elevational view of a osteogenic fusion device in accordance with one embodiment of the present invention.  
         [0026]    [0026]FIG. 2 is an end elevational view of one end of the osteogenic fusion device shown in FIG.1.  
         [0027]    [0027]FIG. 3 is a top elevational view of an alternative embodiment of the osteogenic fusion device utilizing exterior threads.  
         [0028]    [0028]FIG. 4 is a top cross-sectional view of a osteogenic fusion device as shown in FIG. 1 with a bone growth-inducing material supported by the osteogenic fusion device.  
         [0029]    [0029]FIG. 5 is an cross-sectional view of the osteogenic fusion device and bone growth material shown in FIG. 4 taken along line  5 - 5  as viewed in the direction of the arrows.  
         [0030]    [0030]FIG. 6 is a plan view of a sheet for a bone growth-inducing material used with the osteogenic fusion device shown in FIG. 4.  
         [0031]    [0031]FIG. 7 is an end elevational view of one end of a osteogenic fusion device, such as the osteogenic fusion device of FIG. 1, modified to include apertures.  
         [0032]    [0032]FIG. 8 is an end elevational view of one end of a osteogenic fusion device, such as the osteogenic fusion device of FIG. 1, modified to include apertures.  
         [0033]    [0033]FIG. 9 is a side, partially cross-sectional view of an intervertebral disc space with a osteogenic fusion device according to FIG. 1 implanted between adjacent vertebrae.  
         [0034]    [0034]FIG. 10 is a top elevational view of the superior aspect of the instrumented vertebral level shown in FIG. 9, depicting bilateral placement of osteogenic fusion devices according to the present invention.  
         [0035]    [0035]FIG. 11 is a cross-sectional view of the instrumented vertebral segment shown in FIG. 10, taken along line  11 - 11  as viewed in the direction of the arrows.  
         [0036]    [0036]FIG. 12 is a top elevational view of a osteogenic fusion device, such as shown in FIG. 1, with features to permit insertion of the osteogenic fusion device.  
         [0037]    [0037]FIG. 13 is an end elevational view of the osteogenic fusion device shown in FIG. 12.  
         [0038]    [0038]FIG. 14 is a side elevational view of an insertion tool according to one embodiment of the present invention.  
         [0039]    [0039]FIG. 15 is a top elevational view of the insertion tool shown in FIG. 14.  
         [0040]    [0040]FIG. 16 is a top elevational view of a osteogenic fusion device for restoring the lordotic angle between adjacent vertebrae according to a further embodiment of the present invention.  
         [0041]    [0041]FIG. 17 is a top elevational view of a osteogenic fusion device according to a further embodiment of the present invention.  
         [0042]    [0042]FIG. 18 is a top elevational view of a osteogenic fusion device according to a still further embodiment of the present invention.  
         [0043]    [0043]FIG. 19 is an end elevational view of the osteogenic fusion device shown in FIG. 18.  
         [0044]    [0044]FIG. 20 is a top elevational view of a osteogenic fusion device according to another embodiment of the present invention.  
         [0045]    [0045]FIG. 21 is an end elevational view of the osteogenic fusion device shown in FIG. 20.  
         [0046]    [0046]FIG. 22 is a top elevational view of a osteogenic fusion device according to yet another embodiment of the present invention.  
         [0047]    [0047]FIG. 23 is an end elevational view of the osteogenic fusion device shown in FIG. 22.  
         [0048]    [0048]FIG. 24 is a top elevational view of a osteogenic fusion device according to a further embodiment of the present invention.  
         [0049]    [0049]FIG. 25 is an end elevational view of the osteogenic fusion device shown in FIG. 25.  
         [0050]    [0050]FIG. 26 is a top elevational view of a pair of fusion devices according to FIGS.  24 - 25  disposed in a bilateral configuration in the lumbar spine.  
         [0051]    [0051]FIG. 27 is a top elevational view of a fusion device according to FIGS.  24 - 25  disposed in the cervical spine.  
         [0052]    [0052]FIG. 28 is an end elevational view of osteogenic fusion devices of the present invention within a surgical window showing how such fusion devices of particular sizes may not fit entirely within the surgical window.  
         [0053]    [0053]FIG. 29 is an end elevational view similar to that of FIG. 28 and depicting one embodiment of the implant system of the present invention.  
         [0054]    [0054]FIG. 30 is a side elevational view of a osteogenic fusion device in accordance with an alternative embodiment of the present invention.  
         [0055]    [0055]FIG. 31 is an end elevational view of one end of the osteogenic fusion device shown in FIG. 30.  
         [0056]    [0056]FIG. 32 is an end elevational view of the other end of the osteogenic fusion device depicted in FIG. 31.  
         [0057]    [0057]FIG. 33 is a perspective view of an alternative embodiment of the osteogenic fusion device of the present invention.  
         [0058]    [0058]FIG. 34 is a top elevational view of an alternative embodiment of the implant system of the present invention.  
         [0059]    [0059]FIG. 35 is an end elevational view of one end of the implant system depicted in FIG. 34.  
         [0060]    [0060]FIG. 36 is an end elevational view of the other end of the implant system depicted in FIG. 35.  
         [0061]    [0061]FIG. 37 is an end elevational view of an alternative embodiment of the implant system of the present invention.  
         [0062]    [0062]FIG. 38 is a perspective view of an alternative embodiment of the implant system of the present invention.  
         [0063]    [0063]FIG. 39 is a perspective view of yet a further alternative embodiment of the implant system of the present invention.  
         [0064]    [0064]FIG. 40 is an end elevational view of mated osteogenic fusion devices of the invention.  
         [0065]    [0065]FIG. 41 is a perspective view of one of the osteogenic fusion devices depicted in FIG. 40.  
         [0066]    [0066]FIG. 42 is a perspective view of another of the osteogenic fusion devices depicted in FIG. 40.  
         [0067]    [0067]FIG. 43 is a perspective view of an osteogenic fusion device of the invention including a stop member.  
         [0068]    [0068]FIG. 44 is an end elevational view of mated osteogenic fusion devices connected by a connecting plate in accordance with the invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0069]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0070]    The present invention contemplates osteogenic fusion devices for use as interbody fusion devices. The osteogenic fusion devices include opposite end pieces that are configured to span the intervertebral disc space and engage the adjacent vertebral bodies. The inventive osteogenic fusion devices include a central element separating the two end pieces and substantially spanning the anterior-posterior length of the disc space. The invention further contemplates that a bone growth-inducing material be disposed about the central element and between the opposite end pieces. When the inventive osteogenic fusion device is implanted within a patient, the bone growth-inducing material is in direct contact with the adjacent vertebral bodies. The end pieces are formed of a material sufficient to withstand the spinal loads generated at the instrumented vertebral level.  
         [0071]    In accordance with one embodiment of the invention, a osteogenic fusion device  10 , depicted in FIGS.  1 - 2 , includes a first end piece  11  and a second end piece  12 . The end pieces are separated by a central element  13 . The first end piece  11  could be substantially cylindrical or any geometrical shape and includes an outer bone contacting surface  15 . The end piece  11  also defines an inwardly facing retaining surface  17 . The central element  13  integrally extends from the retaining surface  17  of the first end piece  11 .  
         [0072]    The second end piece  12  also defines a bone contacting surface  20  that, in this embodiment, does not extend entirely around the end piece. The bone contacting surface  20  could be any geometrical shape, preferably circular and is defined at a radius equal to the radius of the outer surface  15  of the first end piece. Thus, as depicted in FIG. 2, the bone contacting surface  20  of the second end piece  12  is generally coincident with portions of the outer surface  15  of the first end piece  11  when the osteogenic fusion device is viewed along the longitudinal axis of its central element  13 . The second end piece  12  also includes opposite truncated surfaces  21  that are disposed between the circular bone contacting surfaces  20 . Preferably, the truncated surfaces  21  are generally flat and can be configured to be engaged by an insertion tool. The insertion tool preferably has arms that contact the flat truncated surfaces  21 , yet still fall within the envelope defined by the outer surface  15  of the first end piece  11 .  
         [0073]    The second end piece  12  also defines a second retaining surface  22  that faces the first retaining surface  17  of the first end piece  11 . Again, the central element  13  is preferably integral with and projects outwardly from the second retaining surface  22 .  
         [0074]    Alternatively, the central element can be in the form of a central rod that is engaged within colinear bores formed in the two end pieces. In this variation. the engagement between the central rod and the end pieces can be threaded.  
         [0075]    The central element  13  includes an outer central surface  23 . Preferably, the central element  13  is substantially cylindrical along its length. In one aspect of the invention, the first end piece  11  defines a diameter D 1 , while the central element  13  defines a diameter D 2 . The diameter D 1  is at least equal to the height of the intervertebral space within which the osteogenic fusion device  10  is to be interposed. Most preferably, the diameter D 1  corresponds to the diameter of a cylindrical channel cut into the endplates of the adjacent vertebrae. In this instance, the diameter D 1  will be somewhat larger than the intervertebral disc space height. Moreover, the diameter D 1  is significantly larger than the diameter D 2  of the central element  13 . This diameter differential creates an annular pocket  24  surrounding the central element  13 .  
         [0076]    The osteogenic fusion device  10  has a length L 1  between the opposite ends of the osteogenic fusion device. This length L 1  is preferably selected to be slightly less than the anterior-posterior length of the intervertebral disc space, although the length can be calibrated to the lateral dimension of the space. Most preferably, the length L 1  is sized so that the first and second end pieces  11 ,  12  can contact at least a portion of the apophysis or harder cortical bone at the perimeter of the vertebral endplates. The osteogenic fusion device  10  further defines a length L 2  which is essentially the length of the central element  13 . The length L 2  is calibrated so that the end pieces  11  and  12  are sufficiently wide to provide adequate support between the adjacent vertebrae. Conversely, the length L 2  is sufficiently long so that the annular pocket  24  has the capacity for retaining a substantial quantity of bone growth-inducing material.  
         [0077]    In a modification of the osteogenic fusion device  10 , the second end piece can be configured with threads. For example, as depicted in FIG. 3 an end piece  25  includes external bone engaging threads  26  extending from the outer surface  27 . In accordance with this embodiment, the second end piece  25  can be cylindrical, like the first end piece  11 , or the threads can be formed between truncated surfaces, such as truncated surfaces  21  in the prior embodiment. At any rate, the threaded end piece  25  is configured to be threadedly advanced into a drilled and tapped channel within the adjacent vertebral bodies. The first end piece  11  can also be threaded to facilitate insertion and to reduce the chance of expulsion.  
         [0078]    In a further aspect of the invention, a bone growth-inducing material  30  is provided for support by the osteogenic fusion device  10 . Preferably the material  30  is in the form of a sheet. In a specific example, the carrier sheet  30  can be a collagen sheet that is soaked with a solution containing a bone growth-inducing substance, or a bone morphogenetic protein (BMP). In accordance with the invention, the carrier sheet  30  can be formed of a variety of materials other than collagen, provided the materials are capable of containing a therapeutically effective quantity of a bone growth-inducing substance or BMP. Moreover, the material  30 , whether in sheet form or not, is most preferably susceptible to manipulation to be disposed within the annular pocket  24  of the fusion device  10 .  
         [0079]    In accordance with the specific embodiment, the carrier sheet  30  is wound around the outer surface  23  of the central element  13  (see FIG. 5). The carrier sheet  30  is held between the retaining surface  17  of the first end piece  11  and the retaining surface  22  of the second end piece  12 . In accordance with one specific embodiment, the retaining surface  22  is curved or convex. In this way, the carrier sheet  30  can project into the convexity to serve as a sort of anchor to hold the carrier sheet  30  within the annular pocket  24  of the osteogenic fusion device  10 . In addition, the convex surface  22  conforms better with the anterior portion of the vertebral body profile when the fusion device is implanted.  
         [0080]    In the illustrated embodiment, the carrier sheet  30  can be provided as a single sheet, as shown in FIG. 6. The inner end  31  of the sheet is disposed against the central outer surface  23  of the central element  13 . The sheet can be wound in a spiral fashion about the central element  13  until its outer end  32  is disposed adjacent the outer surface  15  of the first end piece  11 . The carrier sheet  30  has width W that is preferably slightly larger than the length L 2  between the first and second end pieces to allow a portion of the carrier sheet  30  to project into the concave retaining surface  22  of the second end piece  12 . The overall length of the sheet  30  between ends  31  and  32  depends upon its thickness and the difference in diameters D 1  and D 2 . For example, in one embodiment the diameter D 2  is about one-fourth (¼) the diameter D 1 . Preferably, the length is sufficient so that the carrier sheet  30  can be tightly wound about the central element  13  and fill the annular pocket  24 . One important object of the present invention is that the carrier sheet  30  or bone growth-inducing material reside in direct contact with the adjacent vertebral bone. Consequently, the sheet  30  is preferably wound so that its outer end  32  is at least slightly outside the envelope of the outer surface  15  of the first end piece  11 .  
         [0081]    The carrier sheet  30  of FIGS.  4 - 6  illustrates one specific embodiment of bone growth-inducing material usable with the osteogenic fusion device of the present invention. It is also contemplated that the carrier can be in the form of a sponge, paste, gel or a settable osteogenic material. The osteogenic material must be provided in some form that can be generally retained about the central element  13  and within the annular pocket  24  of the osteogenic fusion device  10 . Put differently, the present invention contemplates an osteogenic material that does not need to be contained in the traditional manner of the hollow cylindrical cages of the prior art. In these prior art devices, cancellous bone chips have been contained within a hollow cage. The present invention does not contemplate the use of bone chips alone. However, bone chips contained within a bone paste or a gel may be utilized with the osteogenic fusion device  10 , provided that the paste or gel have a consistency sufficient to hold the bone, growth-inducing material on and within the osteogenic fusion device  10 .  
         [0082]    In accordance with one specific embodiment, the end pieces  11  and  12  are solid and circular in configuration. Alternative end piece configurations are shown in FIGS. 7 and 8. For example, end piece  11 ′ can have a plurality of generally circular apertures  34  disposed circumferentially about the end piece, as shown in FIG. 7. The end piece  11 ″ shown in FIG. 8 includes a plurality of pie-shaped apertures  35  so that the end piece gives the appearance of a spoked wheel. The second end piece  12  of the osteogenic fusion device  10  can have similar apertures defined therethrough. The apertures  34  and  35  in the end pieces  11 ″,  11 ″ provide a further avenue for facilitating fusion bone growth. The apertures themselves can be filled with a osteogenic material, such as a gel or a paste. Moreover, once the osteogenic fusion device  10  is implanted within an intervertebral disc space, osteogenic material can be packed around the osteogenic fusion device within the disc space. These additional apertures in the end pieces  11 ,  12  provide further avenues for the formation of a bony bridge between adjacent vertebrae.  
         [0083]    The end pieces  11 , 12 , etc. can also have non-circular shapes. For instance, the end pieces can be rectangular or other multi-sided shapes. If the osteogenic fusion device resides within a channel prepared in the endplates, the channel shape can be modified to conform to the bone engaging surfaces  15 ,  20  of the end pieces.  
         [0084]    FIGS.  9 - 11  depict a pair of osteogenic fusion devices  10  implanted in a bi-lateral configuration between adjacent vertebral bodies V 1  and V 2 . As depicted, the disc annulus A is retained but at least one portal must be defined in the annulus A to permit insertion of the osteogenic fusion devices  10 . The present invention also contemplates insertion of each osteogenic fusion device  10  through its own portal formed in the disc annulus A. Alternatively, in conformance with other known procedures, a single portal can be provided through which each osteogenic fusion device  10  is successively inserted. Further in accordance with the present invention, the osteogenic fusion devices  10  can be positioned within the intervertebral disc space according to known posterior or postero-lateral techniques.  
         [0085]    According to the present invention, the osteogenic fusion device  10  is inserted into the disc space S with the first end piece  11  proceeding first into the space. Preferably, a channel C is bored into the vertebral endplates E to a preferred depth of insertion of the osteogenic fusion device  10 , in accordance with known techniques. If the osteogenic fusion device to be implanted is of the type shown in FIG. 3 with the threaded second end piece  25 , the channels C can be appropriately drilled and tapped to accommodate the bone engaging threads  26 . In a modification of this embodiment, the first end piece  11  can also carry external threads.  
         [0086]    The preferred embodiment contemplates a generally cylindrical osteogenic fusion device placed within circular channels. Alternatively, the osteogenic fusion devices can operate as spacers that directly contact the endplates, without a prepared channel. In this instance, the bone engaging surfaces of the end pieces can be modified to conform to the vertebral endplate geometry.  
         [0087]    As depicted in FIGS.  9 - 11 , the osteogenic material  30  is disposed in direct contact with the adjacent vertebral endplates E. Moreover, the placement of osteogenic fusion devices  10  can present a medial space  37  between the two osteogenic fusion devices. Osteogenic material can then be placed within the medial space  37 , again in direct contact with the osteogenic material  30  situated around the central elements  13  of each of the osteogenic fusion devices  10 . Once complete fusion occurs, new bone growth will substitute the carrier material  30  to form a solid bony bridge spanning the adjacent vertebrae V 1 , V 2 . As can be seen from FIGS.  9 - 11 , the region of continuous bone growth is very substantial and is not interrupted by the structure of the fusion device itself.  
         [0088]    It is understood, of course, that the end pieces  11  and  12  provide sufficient support for the vertebral loads passing between the adjacent vertebrae. At the same time, this load bearing capacity is concentrated outside the middle regions of the vertebral endplates E. It is known that the central region of the endplates is very rich in blood flow and has a high capacity for new bone growth. Thus, the elimination of structural material of the osteogenic fusion device  10  from that region provides for a faster and more complete arthrodesis than may have been possible with prior fusion cages.  
         [0089]    Referring next to FIGS. 14, 15, an insertion tool  50  is depicted for inserting a osteogenic fusion device  10  according to the present invention. The insertion tool  50  includes a solid shank  51  to which a knob or handle  52  is affixed. The knob  52  is configured for manual grasping and manipulation during insertion of the osteogenic fusion device. In the case where the osteogenic fusion device is not threaded, the insertion tool  50  simply acts as a pushing device. On the other hand, in the instance where the osteogenic fusion device includes threaded end pieces such as shown in FIG. 3, the insertion tool  50  must be rotated as the end piece is threaded into the prepared channel between the adjacent endplates.  
         [0090]    The insertion tool  50  includes a pair of prongs  53  that are disposed apart to define an end piece recess  54 . For insertion of the osteogenic fusion device  10  shown in FIG. 1, the end piece recess  54  is configured so that the prongs  53  are in tight contact with the truncated surfaces  21  of the second end piece  12 . The outer surface of the prongs  53  can conform to a portion of the outer surface  15  of the first end piece  11 .  
         [0091]    The insertion tool  50  depicted in FIGS.  14 - 15  also includes tapered tips  55  at the ends of each of the prongs  53 . These tapered tips are configured to be received within driving notches  41  in a modified first end piece  40 , as depicted in FIGS.  12 - 13 . The osteogenic fusion device depicted in FIGS.  12 - 13  is substantially similar to the osteogenic fusion device  10  shown in FIG. 1, with the exception of the added driving notches. The insertion tool  50  is configured so that the tips  55  project into the notches  41  while the prongs  53  directly contact the truncated surfaces  21  of the second end piece  12 . This particular configuration of the insertion tool is particularly useful for threaded insertion of the osteogenic fusion device. Preferably, the prongs  53  have an effective outer diameter that is approximately equal to the diameter D 1 . Moreover, the prongs  53  can have an arc segment configuration to complement the truncated surfaces  21 . If the end piece  12  is threaded (see FIG. 3), the prongs  53  can include complementary threads along their length.  
         [0092]    The present invention also contemplates a osteogenic fusion device for restoring the normal lordotic angle of an intervertebral segment. Specifically, a lordotic osteogenic fusion device  60  includes a first end piece  61  and a second end piece  62  as shown in FIG. 16. As with the prior embodiments, a central element  63  is provided to connect the two end pieces. The outer surface  65  of the first end piece  61  is in the form of a frusto-conical surface. The outer surface  65  tapers toward the second end piece  62  at a preferred lordotic angle. Similarly, the outer surface  66  of the second end piece  62  is also tapered at a similar lordotic angle. Alternatively, the second end piece  62  can include threads formed on the outer surface  66 . While the threads  66  at the smaller second end piece  62  may not contact the vertebral endplates at the larger insertion end, the threads will contact the endplates at the anterior end of the intradiscal space and will act as an anchor to resist expulsion of the lordotic osteogenic fusion device  60 .  
         [0093]    The present invention contemplates several modifications to the basic osteogenic fusion device  10 . For example, the osteogenic fusion device  70  shown in FIG. 17 includes first and second end pieces  71 ,  72  and a center piece  73  disposed between the two end pieces. First and second central elements  74  and  75  connect each of the end pieces  71 ,  72  to the center piece  73 . In this instance, the center piece  73  will contact the interior of the disc endplates E. Osteogenic material, such as carrier sheets  30 , can be disposed or wound around each of the central elements  74 ,  75  until the end of the bone growth-inducing material is exposed at the outer surface of the osteogenic fusion device  70 .  
         [0094]    In a further modification, a osteogenic fusion device  80  depicted in FIG. 18 includes first and second end pieces  81  and  82  that are connected by a plurality of central beams  83 . In the illustrated embodiment as shown in FIG. 19, four such beams  83  are provided; however, other arrangements and numbers of beams are contemplated. Important aspects of the present invention are retained by the osteogenic fusion device  80  because osteogenic material can be supported by the several beams  83  between the first and second end pieces  81 ,  82 , with the bone growth-inducing material in direct contact with the adjacent vertebral bodies.  
         [0095]    The two embodiments of FIGS.  20 - 21  and FIGS.  22 - 23  pose a slight deviation from the general concept of the osteogenic fusion device  10 . In these two embodiments, the smaller diameter central element  13  is replaced by a wall. In the embodiment of FIGS.  20 - 21 , a osteogenic fusion device  85  includes first and second ends  86 ,  87  separated by a central element  88 . The first and second ends  86  and  87  can be in the form of short cylindrical sections, such as the first end piece  11  of the osteogenic fusion device  10  in FIG. 1. While the central element  88  can be in the form of a solid wall, the osteogenic fusion device  85  preferably includes a number of slots  89  defined through the central element  88 . In accordance with the specific embodiment, the slots extend along substantially the entire length of the central element  88 . While the osteogenic fusion device  85  deviates somewhat from the concept of the osteogenic fusion device  10 , this latter osteogenic fusion device  85  retains the broad beneficial feature of the present invention, namely provision for direct contact between osteogenic material supported by the osteogenic fusion device  85  and the vertebral endplates. In the present instance, the osteogenic material can be situated on opposite sides of the central element  88 . In addition, the material can be passed through the slots  89 .  
         [0096]    Preferably, the osteogenic fusion device  85  will be oriented within the intervertebral disc space with the central element  88 , or wall, spanning between the adjacent vertebrae. This central element  88 , then, will provide additional structure and load bearing capability for sustaining the spinal loads at the instrumented level.  
         [0097]    The osteogenic fusion device  90  of FIGS.  22 - 23  operates on a similar concept to the osteogenic fusion device  85 . However, in this instance, the first and second end pieces are in the form of arc segments, rather than shortened cylinders. Specifically, the osteogenic fusion device  90  includes upper and lower first arc segments  91   U  and  91   L , and upper and lower second arc segments  92   U  and  92   L . The osteogenic fusion device  90  also includes a central element  93  that is again in the form of a wall connecting the first and second end pieces. As can be seen most clearly in FIG. 23, the arc segments  91 ,  92  and central element  93  define a pair of cavities  96  for containing osteogenic material. In this embodiment, the osteogenic material can be contained completely from end to end of the osteogenic fusion device  90 . In the prior embodiments, the osteogenic material is contained within retaining surfaces of the opposite end pieces. In accordance with a specific embodiment, the osteogenic fusion device  90  includes a plurality of apertures  94  defined in each of the upper and lower first and second arc segments  91   U ,  91   L ,  92   U  and  92   L . Similarly, a plurality of apertures  95  can be defined through the central element  93 . In this manner, the apertures provide the maximum capacity for bone ingrowth not only around, but also through the osteogenic fusion device  90 .  
         [0098]    A osteogenic fusion device  100  shown in FIGS.  24 - 25  again presents a slightly different concept. This osteogenic fusion device  100  includes a first end plate  101 , a second end plate  102  and a central element  103  that are similar to the like-named components of the osteogenic fusion device  10 . However, the osteogenic fusion device  100  also includes a side piece  104  spanning between the first and second end pieces  101 ,  102 . Moreover, unlike the osteogenic fusion device  10 , the first and second end pieces  101 ,  102  are not generally circular in configuration, but are—WO 99/20271 19 PCTIUS98/26254 generally rectangular in configuration. In one specific embodiment, the end pieces  101 ,  102  can include cutouts  105  at opposite sides of the end pieces to provide further avenues for the formation of a bony bridge between adjacent vertebrae. As with the prior embodiments, the osteogenic fusion device  100  provides means for adequately containing osteogenic material, such as in the form of the carrier sheet  30 . In this embodiment, the carrier sheet  30  can be wound around the central element  103 , in the manner described above. This particular embodiment of the invention, namely osteogenic fusion device  100 , is preferably adapted for use in the lumbar spine as illustrated in FIG. 26 and in the cervical spine illustrated in FIG. 27, and is consequently sized accordingly.  
         [0099]    In many situations, it is preferable to use two fusion devices in a posterior lumbar interbody fusion technique (PLIF) but there is not enough lateral exposure to place two devices side-by-side. This problem can be visualized, for example, by reference to FIG. 28. Two osteogenic fusion devices, such as osteogenic fusion device  10 , may be placed side-by-side within a surgical window depicted by the dotted line. As seen in FIG. 28, the two devices do not fit within the surgical window presented. In many such cases, the facet joints must be removed to make the surgical window larger, which may lead to spinal instability.  
         [0100]    In order to address this problem, at least one end piece of an osteogenic fusion device may have a truncated surface, such as a circular cutout, as depicted in FIG. 29. As seen in FIG. 29, two fusion devices placed together thereby nest or interleave and reside within the operative window, and thus require no or minimal resection of the facet joints.  
         [0101]    As more fully shown in FIGS.  30 - 32 , osteogenic fusion device  110  is in many respects similar to osteogenic fusion device  10  depicted in FIGS. 1 and 2 and includes, for example, opposite end pieces including first end piece  111  and second end piece  112  and central element  113 . Each end piece defines two opposing surfaces as similarly described for osteogenic fusion device  10 . For example, first end piece  111  defines a bone contacting surface  114  and second end piece  112  defines a bone contacting surface  115 . Bone contacting surface  115 , in this embodiment, does not extend entirely around end piece  112 . Moreover, the bone contacting surface of second end piece  112  is generally coincident with portions of the outer surface  114  of first end piece  111  when the device is viewed along the longitudinal axis of its central element  13 . Second end piece  112  also includes two opposite truncated surfaces  117  that are disposed between bone contacting surfaces  115 . Additionally, first end piece  111  includes external face  118  and internal face  119  whereas second end piece  112  includes external face  120  and internal face  121 . Osteogenic fusion device  110  is configured to nest with another osteogenic fusion device, including other devices of the present invention. In the embodiment depicted in FIGS.  30 - 32 , the configuration of the osteogenic fusion device  110  includes a first end piece  11 I having opposite faces, including opposite edges  123 , that define an entrance  124  to a cutout region  122 . Cutout region  122  is defined by truncated surface  116 . Truncated surface  116 , in this embodiment, is concave. As best seen in FIG. 31, first end piece  111  has a minimum lateral dimension D 3  transverse to a maximum vertical dimension D 4  between the two opposite surfaces  114 . In the illustrated device, maximum vertical dimension D 4  is generally larger than minimum lateral dimension D 3 . Vertical dimension D 4  has a height approximating the desired separation of the adjacent vertebrae.  
         [0102]    [0102]FIG. 33 depicts another embodiment, in which load bearing member  130  has a first end piece  131  with a truncation adapted for nesting and a second generally cylindrical end piece  132  having no cutout regions.  
         [0103]    The above-described osteogenic fusion devices configured to nest may also bear modifications similar to those shown in FIGS.  3 - 13  and  16 - 21 , and their accompanying descriptions in the text above. For example, osteogenic fusion devices having threaded end pieces, end pieces with apertures, and such devices having either center pieces, a plurality of central elements and a central element defining a wall may also be incorporated into osteogenic fusion devices such as those described in conjunction with FIGS.  30 - 33 . In devices with center pieces, the center pieces may be substantially cylindrical with no cutout regions or may be shaped with a cutout region as described above. Moreover, the device can also include a bone growth-inducing material as described above which may be wound around the central elements of the devices, and if desired also shaped to allow for or facilitate the nesting arrangement.  
         [0104]    It is to be noted that the shapes of the opposing end pieces of the load bearing members described above are preferably cylindrical and may include a concave truncated surface. However, opposite end pieces and truncated surfaces having any suitable geometrical shape are contemplated as forming a part of the present invention.  
         [0105]    The present invention also contemplates an implant system including at least two load bearing members as described above and wherein at least one load bearing member is configured to nest within the other load bearing member. FIGS.  34 - 36  depict one embodiment of the implant system including load bearing member  110  and load bearing member  10  (as shown in FIGS. 1 and 2) having a substantially cylindrical first end piece  11 . First end piece  11  of load bearing member  10  is nested within first end piece  111  of load bearing member  110 . In this particular embodiment as best seen in FIG. 36, width w 1  of second end piece  112  of load bearing member  110  and width w 2  of second end piece  12  of load bearing member  10 , when added together, must be such that will not prevent first end piece  11  of load bearing member  10  from nesting within first end piece  111  of first load bearing member  110 .  
         [0106]    In yet a further embodiment, the load bearing members in a nesting implant system may have an identical shape. For example, FIG. 37 depicts a perspective view of two load bearing members  110  wherein first end piece  111  of one of the load bearing members is nested within an identical end piece  111  of the other load bearing member  110 .  
         [0107]    [0107]FIG. 38 shows implant system  150  of the invention which includes load bearing member  160  and load bearing member  170 . Load bearing member  160  is similar to load bearing member  130  except that second end piece  162  of load bearing member  160  is substantially cylindrical with a cutout portion (i.e., it has the shape of first end piece  131  of load bearing member  130 ). Load bearing member  170  is similar to load bearing member  130  except that first end piece  171  of load bearing member  170  is substantially cylindrical, with no cutout regions. FIG. 38 further depicts first end piece  171  of load bearing member  170  nested within first end piece  161  of load bearing member  160  and second end piece  172  of load bearing member  170  is nested within second end piece  162  of load bearing member  160 .  
         [0108]    It is to be appreciated that the implant system may include first and second load bearing members with end pieces arranged in a variety of ways to achieve the nesting arrangement. For example, the first and second load bearing members may each include one truncated and one non-truncated end piece, such as that illustrated in FIG. 33. In such an embodiment, the two devices can be used in inverted relationship with respect to one another to achieve a nesting relationship. For example, in implant system  180  shown in FIG. 39, first end piece  191  of first load bearing member  190  and second end piece  202  of second load bearing member  200  are truncated. Nontruncated first end piece  201  of load bearing member  200  is nested within first end piece  191  of load bearing member  190  and non-truncated second end piece  192  of load bearing member  190  is nested within second end piece  202  of second load bearing member  200 .  
         [0109]    With reference now to FIGS.  40 - 42 , shown is an implant system of the invention including mated fusion devices and wherein the devices are configured to connect to one another so as to resist lateral separation of the devices. In preferred systems, such connection may also provide increased resistance to expulsion due to the cooperation of the two devices. In particular, the system  210  includes a first fusion device  211  and a second fusion device  212 . First fusion device  211  includes end pieces  213  having openings  214  serving as female members. Second fusion device  212  includes end pieces  215  having mating members  216  sized correspondingly to fit within openings  214  of device  212  and serve as male members. In this fashion, when devices  211  and  212  are assembled as depicted in FIG. 40, the two devices are connectedly mated so as to resist lateral separation from one another and/or expulsion, desirably acting more as a single unit when implanted in a patient. In this regard, devices  211  and  212  may be mated prior to implantation and implanted as a single unit: however, it is contemplated as preferred to implant a first of the devices, e.g. device  211 , and then to implant the second device, e.g.  212 , by pushing or sliding the second device in next to the first implanted device along the long axis, such that mating members  216  are received within openings  214  thus connecting the two devices to one another. As illustrated, devices  211  and  212  are also configured to nest to present a reduced lateral profile generally as described above for certain devices. Thus, device  212  includes concave shoulder portions  217 , with mating member  216  located therebetween with its outward end  218  extending radially outward to a distance which allows the nesting relationship. In device  212 , outward end  218  extends radially outward no further than the radius r of the predominant cylindrical shape of end piece  215 . Devices  211  and  212  can optionally have outer surfaces configured to resist expulsion from the space between adjacent vertebrae, for example threads, ratchets, grooves or other like features. In one mode, one of the fusion devices may include threads that facilitate controlled insertion, and that device may be implanted first. The other fusion device of the system can be of the push-in type, having no expulsion-resisting features or those features commonly used for push-in devices, for example ratchets or similar proturbances, or grooves. Still further, at least one of the fusion devices can include a stop member to controllably stop insertion of the second device by contact between the two devices. For example, illustrated in FIG. 43 is a device  220  similar to device  211 , except including a stop member  221  positioned to be contacted by mating member  216  of device  212 , for example in a procedure in which device  220  is implanted first with end piece  222  occurring distally, and device  212  is thereafter pushed in and mated with device  220 .  
         [0110]    With reference now to FIG. 44, illustrated in another implant system of the invention in which two adjacent fusion devices are connected to one another. In particular, system  230  includes a first fusion device  10  and a second fusion device  110  as described above. In addition, system  230  includes a relatively thin connecting plate  231  spanning the end pieces of devices  10  and  110 . Connectors  232 , for example screws, pins or the like, extend through plate  231  and into the end pieces of devices  10  and  110 . In this case, such end pieces can include corresponding means for receiving connectors  232 . for example a threaded hole in the case where connectors  232  are screws. Implant system  230  having devices  10  and  110  connected in this fashion at one or both ends will thus also desirably act more as a single unit within the patient, desirably adding torsional resistance. It is contemplated that the devices  10  and  110  may be connected prior to or after implant. In one mode, for example, devices  10  and  110  may be implanted separately in the nested relationship, and only a single plate  231  used to connect the proximal (more accessible) end pieces.  
         [0111]    Use of two large devices side-by-side in accordance with the invention facilitates engagement of the devices into the vertebral body endplates to distract the disc space and facilitate fusion. The larger diameter devices provide other advantages over the use of two small diameter devices. For example, the deeper the devices are placed into the endplates, the more bleeding bone is exposed and the better the chance for new bone formation. Moreover, the smaller diameter devices do not get adequate distraction or stabilization in the end plate bone allowing for motion which inhibits new bone formation. The larger diameter devices are advantageously used in situations requiring less lateral exposure to implant two devices side-by-side (i.e., bilaterally).  
         [0112]    The design of the above-described devices that have cylindrical end pieces with cutout regions can be used in current fusion cages that act as containers, or baskets, for holding autograft chips and in allograft bone dowels. Such a design allows for threading-in of the devices much closer together as desired for a PLIF procedure. Moreover, the instruments that indicate the correct vertical orientation of the cage for bone thru-growth can also assist in orienting the cage cutout on the medial side for mating with a second cage.  
         [0113]    The present invention contemplates osteogenic fusion devices that are formed of a material that is sufficiently strong to support the adjacent vertebrae and to maintain the disc height of the instrumented intervertebral space. For example, the osteogenic fusion devices, such as osteogenic fusion device  10 , can be formed of a biocompatible sterilizable metal, such as stainless steel or titanium. Of course, other medical grade materials are contemplated, such as certain ceramics, polymers, etc., as well as allograft and xenograft bone, provided the materials are sufficiently strong. The overall dimensions of each of the osteogenic fusion devices described above depends upon the instrumented level. For example, a osteogenic fusion device for use in the cervical spine must necessarily be smaller than a osteogenic fusion device used in the lumbar spine. Moreover, the relative dimensions of the components of the osteogenic fusion devices may be altered depending upon the vertebral level to be instrumented. For example, a osteogenic fusion device, such as osteogenic fusion device  10 , for use in the lumbar spine, may require a central element  13  having a diameter D 2  that is more than one fourth of the outer diameter D 1  of the outer surface  15  of the first end piece  11 . In some instances, the lumbar spine may generate bending moments across a osteogenic fusion device, such as osteogenic fusion device  10 , that would require a stronger central element  13 .  
         [0114]    In accordance with the present invention, the illustrated osteogenic fusion devices can be of the push-in or threaded-in type. Of course, the end pieces, such as end pieces  11 ,  12  of osteogenic fusion device  10 , and end pieces  111 ,  112  of osteogenic fusion device  110 , can include various surface characteristics known in the art for enhancing the degree of fixation of the osteogenic fusion device between the adjacent vertebrae. For example, the end pieces can include certain macro surface features for penetrating the vertebral endplates to resist expulsion of the osteogenic fusion devices. Likewise, the surfaces, such as outer surface  15  and  114  and bone contacting surface  20  and  115  can be provided with bone ingrowth coatings so that a certain amount of bone ingrowth occurs even between the end pieces and the adjacent vertebral bodies.  
         [0115]    The present invention also provides a method of promoting fusion bone growth in the space between adjacent vertebrae. The method advantageously includes providing the load bearing members or implant systems described above, preparing adjacent vertebrae to receive the load bearing member or implant system and placing the load bearing member or implant system into the intervertebral space after the preparing step. The load bearing members and implant system may also include an osteogenic material within the pocket of the devices that is arranged to contact the adjacent vertebrae when the vertebrae are supported by the opposite end pieces of the device as described more fully above.  
         [0116]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.