Abstract:
An improved interbody fusion device incorporating anti-rotation features to inhibit accidental screwing or unscrewing of the fusion device in an intervertebral disc space. One such feature is a thread pattern having a crest diameter that increasingly tapers from a smaller diameter at the insertion end to a larger diameter at the opposite end making further advancement of the thread into the bone more difficult. Another feature is a thread cut out on the trailing edge of the thread, creating a barb which has a tendency to inhibit unscrewing. These features may be used alone or in combination for an improved interbody fusion device.

Description:
[0001]    The present application claims the benefit of the filing date of Provisional application Serial No. 60/118,939, filed Feb. 4, 1999, entitled IMPROVED INTERBODY FUSION DEVICE WITH ANTI-ROTATION FEATURES. The referenced application is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to interbody fusion devices disposed between two bony structures to maintain spacing therebetween and promote bony fusion. More specifically, the present invention relates to interbody fusion devices having threads or other structures defined on the outer surface to limit movement of the device between the bony structures. Most often, such devices find application in the spine for fusing adjacent vertebrae.  
           [0003]    It is well known to utilize an artificial spinal fusion implant to be inserted in the space between two adjacent vertebra after removal of a damaged spinal disc or portion thereof. Common forms of such devices may be configured in a substantially cylindrical configuration. These cylindrical configurations can include truncated sidewalls or a tapering body portion. However, for bone fusion to occur, the invasion of new delicate blood vessels from the adjacent healthy bone is necessary for the creation of new living interconnecting bone. Motion around the implant can restrict or even prevent bone healing. Therefore, it is important to stabilize the implant upon insertion. In most applications, the outer body of the fusion device is provided with one or more structures to resist repulsion from the disc space when a load is applied to the spinal column. For example, U.S. Pat. No. 5,015,247 issued to Michelson discloses substantially cylindrical interbody fusion devices with an external thread disposed on the outer surface. The thread is interrupted at various locations. Further, the trailing portion of the thread may be twisted slightly. This twisted portion of the trailing edge acts as a locking thread to resist subsequent unscrewing of the fusion device. While such interrupted locking threads may be satisfactory in preventing unscrewing, the manufacturing process of twisting each of the individual teeth to create a locking thread may be costly and difficult to control from a quality aspect.  
           [0004]    Thus, there remains a need for an improved interbody fusion device that incorporates features to resist undesired rotation after implantation while at the same time simplifying the insertion of the implant into a human body and minimizing manufacturing complexity.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides an interbody fusion device having structures to limit rotation in at least one direction after the device is implanted between two bony structures. In one aspect of the present invention, an improved interbody fusion device is provided with an anti-rotation thread cut out. In this aspect, the fusion device includes a body portion with a thread pattern defined thereon. The thread pattern extends at least partially from a first end to an opposite second end. At least one thread is interrupted by a thread cut out on a trailing edge of the thread. The thread cut out includes an undercut portion extending beneath the outer surface of the interrupted thread to create a barb. When force is applied, attempting to unscrew the device, material from the thread path may be urged into the undercut area and retained there to resist accidental unscrewing. Preferably, the thread cut out also creates a relatively sharp pointed barb. The pointed barb may tend to impale bone tissue, further increasing resistance to unscrewing.  
           [0006]    In a further aspect, the invention provides an interbody fusion device with an outer surface extending between a first end an opposite second end. A thread pattern is defined on the outer surface and extends at least partially between the first and second ends. A first thread adjacent the first end has a first height and a second thread adjacent the second end has a second larger height. The height of intervening threads between the first and second threads includes a substantially continuously tapering height increasing from said first thread to said second thread. In a preferred aspect, the outer surface of the fusion device has a substantially continuous outer diameter. When inserted into the disc space with the first end leading the insertion, the first thread travels through a thread path and each subsequent larger thread expands the thread path by engaging bone not contacted by the preceding thread. In this manner, resistance to further screwing of the device into the disc space is increased. Thus, the expanding thread pattern resists accidental movement of the device into the disc space as a result of further rotation.  
           [0007]    In yet a further preferred aspect of the present invention, an interbody fusion device is provided incorporating both a thread pattern having a cut out on the trailing surface of the thread to resist accidental unscrewing and a tapering thread height to resist accidental advancing. This combination provides an improved interbody fusion device resistant to accidental rotation after implantation.  
           [0008]    These and other objects of the present invention will be apparent from the following description of the preferred embodiments.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a perspective view of an improved interbody fusion device according to the present invention.  
         [0010]    [0010]FIG. 2 is a second perspective view of the interbody fusion device of FIG. 1.  
         [0011]    [0011]FIG. 3 is a third perspective view of the interbody fusion device of FIG. 1.  
         [0012]    [0012]FIG. 4A is a top view of the interbody fusion device of FIG. 1.  
         [0013]    [0013]FIG. 4B is a side view of the interbody fusion device of FIG. 1.  
         [0014]    [0014]FIG. 4C is a top view of an alternative embodiment according to the interbody fusion device of FIG. 1.  
         [0015]    [0015]FIG. 5A is an end view of the interbody fusion device of FIG. 4.  
         [0016]    [0016]FIG. 5B is an enlarged end view of a portion of FIG. 5A.  
         [0017]    [0017]FIG. 5C is an end view of the interbody fusion device of FIG. 4C.  
         [0018]    [0018]FIG. 6A is a cross-sectional view taken along line  6 A- 6 A of FIG. 5A.  
         [0019]    [0019]FIG. 6B is an enlarged view of a portion of FIG. 6A.  
         [0020]    [0020]FIG. 7 is a perspective view of a further embodiment of an improved interbody fusion device according to the present invention.  
         [0021]    [0021]FIG. 8 is a perspective view of a third embodiment of an improved interbody fusion device according to the present invention.  
         [0022]    [0022]FIG. 9 is a perspective view of a fourth embodiment of an improved interbody fusion device according to the present invention.  
         [0023]    [0023]FIG. 10 is a perspective view of a fifth embodiment of an improved interbody fusion device according to the present invention.  
         [0024]    [0024]FIG. 11 is an end view of the interbody fusion device of FIG. 10.  
         [0025]    [0025]FIG. 12 is a perspective view of a sixth embodiment of an improved interbody fusion device according to the present invention.  
         [0026]    [0026]FIG. 13 is a side view of a seventh embodiment an improved interbody fusion device according to the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    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, and 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.  
         [0028]    Referring now to FIGS. 1 through 6, there is shown an improved interbody fusion device or cage  10  according to the present invention. Device  10  comprises an elongated body having an outer surface  54  extended between a first end  34  and a second end  36  and defining a longitudinal axis  19 . End  34  is referred to as the proximal end since it is the end closest to the user as the device is being inserted into a vertebral disc space. A helical thread pattern  12  is formed on outer surface  54 , having an advancing rotation direction shown by arrow A for inserting device  10  into a disc space. Thread pattern  12  is interrupted by thread cut outs  14  and  20 , multiple bone ingrowth or fusion windows  26 ,  28 ,  30  and  32 , and opposing sidewalls  22  and  24 .  
         [0029]    Interbody fusion device or implant or spacer  10  further includes an inner surface  42  defining a hollow interior chamber  44  adapted to receive bone growth promoting material to promote fusion between adjacent vertebral bodies. As known in the art, hollow chamber  44  may be sealed by a cap (not shown) placed over the opening adjacent distal end  36 .  
         [0030]    The fusion device further includes driving slot  16  and internally threaded opening  18  adapted to receiving an insertion tool (not shown). Preferably, the insertion tool has a driving shoulder to engage driving slot  16  and an externally threaded shaft adapted for engagement with internally threaded opening  18 .  
         [0031]    The interbody fusion devices, or implant or spacer  10  are commonly sized in diameters ranging from 16 mm to 20 mm and in lengths ranging from 20 mm to 23 mm, although other sizes of the interbody fusion devices are contemplated depending on the requirements of a given medical indication. A preferred embodiment having a diameter of approximately 18 mm is being illustrated.  
         [0032]    Referring to FIGS. 1, 3,  5 A, and  5 B, two substantially identical thread cut outs  14  and  20  interrupt thread pattern  12  on diametrically opposing sides along transverse axis  68  of fusion device  10 . While two thread cut outs are shown in a preferred embodiment, it is contemplated that a greater or lesser number of thread cut outs may be utilized without deviating from the spirit and scope of the invention. Further, while thread cut outs are shown extending along substantially the entire length of device  10  from a first end  34  to a second end  36  and parallel to longitudinal axis  19 , it is contemplated that the thread cut outs  14 ,  20  may be formed in a single thread turn or form or in alternating positions. As shown in FIG. 1, the thread cut outs form a series of barbs  15  and  21  on the trailing edge of the thread form. The trailing edge of the thread refers to the back side of the thread as it threadedly advances into the vertebral bodies. Due to the elastic nature of cancellous bone, a portion of the cancellous bone often springs back into the open thread path left in the area of the thread cut outs. This also occurs to a much greater extent during the healing process following insertion. Thus, after insertion the thread cut outs will be at least partially filled with bone.  
         [0033]    Thread cutouts  14  and  20  are formed by cutting into the extending thread forms of thread pattern  12  at a predetermined angle to a predetermined depth with a cutting instrument having specified width or diameter. Preferably, each of the thread cut outs  14  and  20  includes a base portion  62  and  66 , a barb or protrusion  60  and  64 , leading wall  65  and trailing wall  67 . Barbs or overhanging thread portions  60  and  64  resemble a shark&#39;s dorsal fin. As shown more clearly in FIG. 5B, base portion  66  and overhanging thread portion  64  define a recess or undercut  72 . In a preferred embodiment, undercut  72  is substantially concave. Further, extending thread crest of barb  64  forms a relatively sharp point with trailing wall  67  of cut out  20 . Barbs  60  and  64 , in combination with recess  72  and a corresponding recess  73  in cut out  14 , respectively, encourage material from the thread path to be trapped within the concave area thereby inhibiting unscrewing of the device through the previous thread path. Further, the sharp points of fin-shaped barbs  60  and  64  face away from the threadedly advancing rotation direction. In the forward insertion direction, barbs  60  and  64  have little impact on the rotational motion. In the reverse, backward direction, barbs  60  and  64  tend to impale the adjacent bones or matters which collected in the thread path, thus resisting reverse rotation of the device  10 .  
         [0034]    Referring again more specifically to FIG. 5B, thread cut out  20  is formed in a preferred aspect of the invention by forming a cut into thread pattern  12  at an angular orientation A1 with respect to axis  70  along the rotation direction A. It is contemplated that this angle may range from 0° to less than 90°. In a preferred embodiment, angle A1 is approximately 45°. The leading wall  65  of cut out  20  is spaced from imaginary line  74  by a distance L2 and extends at angle A1 with respect to axis  70 . In a preferred embodiment of a fusion device having a maximum diameter of 18 mm, L2 is equal to approximately 4.6 mm. Thread cut out  20  has a width extending between leading wall  65  and trailing wall  67  of L3. In a preferred embodiment, L3 is approximately 1.75 mm. Thus, the ratio of the width or diameter of the thread cut out  20  to the diameter of device  10  is approximately 10%. However, it will be understood that this ratio may be as high as 50% or as low as single digit percentages. Further, the thread cut out extends to a depth of L4 from imaginary line  76 , which is perpendicular to imaginary line  74 . In a preferred embodiment, L4 is approximately 4.7 mm. In further explanation of the features disclosed in FIGS. 5A and 5B, the centerline  80  of the thread cut out  20  is disposed at angle A2 with respect to a ray  78  extending from the center to the intersection  82  of thread cut out  20  and thread form. It is contemplated that this angle may be less than 90° but greater than 0° to achieve a thread cut out according to the present invention. In the embodiment of FIG. 5B, angle A2 is approximately 30°. Preferably, as shown in FIG. 5B, a ray  68  connects thread overhang  64  with the center of the device  10 . Trailing wall  67  is offset with respect to ray  68  to thereby define recess  72 .  
         [0035]    In one embodiment of the present invention, thread cut outs  14  and  20  extend into, but not through, the body of fusion device  10 . In another embodiment, the cut out may extend only through the threads without disrupting the root diameter of the device. Yet in still another embodiment, referring to FIGS. 4C and 5C, the cut outs extend through the body wall to form bone ingrowth windows or openings  46  and  48 . The bone ingrowth windows  46  and  48  permit bone ingrowth between bone growth promoting material placed in hollow interior  44  of the device and the adjacent bone of the vertebral bodies. The windows may also allow bone material, which may collect in the cutout as the device or implant  10  is being advanced, to fall into interior chamber  44 . Further, while dimensions have been given for a specific and preferred embodiment having a maximum diameter of 18 mm, it is contemplated and understood that it will be readily apparent to those skilled in the art that other dimensions, angles, radii, etc., may be utilized without deviating from the spirit, scope and content of the present invention.  
         [0036]    Thread pattern  12  is further interrupted by upper bone ingrowth windows  26  and  28  and opposing lower bone ingrowth windows  30  and  32 . It will be understood that these bone ingrowth windows are disposed on the body of fusion device  10  such that, when implanted in a disc space, they are positioned adjacent the upper and lower vertebral bodies, respectively. The windows permit bone ingrowth between bone growth promoting material placed in hollow interior  44  of the device and the adjacent bone of the vertebral bodies.  
         [0037]    The fusion device further includes concave sidewall  22  and opposing concave sidewall  24 . The concave sidewall permits placement of two interbody fusion devices in a disc space that would otherwise not be able to receive two cylindrical interbody fusion devices due to their greater width. In a preferred embodiment, the fusion device  10  includes concave sidewall  22  and opposing concave sidewall  24 , each extending along the length of the device.  
         [0038]    Referring now to FIGS. 5A and 5B, concave sidewalls  22  and  24  are more clearly shown on diametrically opposed sides of device  10  along transverse axis  70 . Concave sidewalls  22  and  24  are identically formed in mirror image. Concave sidewall  24  has a radius of curvature RI approximating the external diameter of the interbody fusion device. Thus, with a fusion device  10  placed in the disc space with side walls  22  and  24  disposed laterally, an identically sized fusion device  10  may then be rotated into position with thread pattern  12  rotating within the concave area of the adjacent fusion device  10 . For instance, for an 18 mm diameter cage, R1 equals 9.25 mm.  
         [0039]    In the illustrated embodiment, each of the concave sidewall  22  and  24  is interrupted by lateral bone ingrowth windows  38  and  40 , respectively. These lateral bone ingrowth windows  38  and  40  also permit communication between bone growth material placed within the interior of the device and fusion material placed around the exterior of the device in the disc space.  
         [0040]    Referring now to FIG. 6A, there is shown a cross-sectional view of device  10  taken along line  6 A- 6 A of FIG. 5A. Inner surface  42  has a diameter D1 which defines interior chamber  44 . First thread revolution or turn or form  50  adjacent distal end  36  has a maximum thread crest diameter of D2. Outer surface  54  has a constant diameter D3 extending from distal end  36  to proximal end  34 . The last thread turn  52  adjacent proximal end  34  has a maximum thread crest diameter D4 which is preferably slightly larger than D2. While these dimensions may vary depending on the size of the device and the specific design criteria, in a preferred embodiment for an 18 mm maximum diameter interbody fusion device, D1 is equal to approximately 11.5 mm, D2 is equal to approximately 16 mm, D3 is equal to approximately 15.5 mm, and D4 is equal to approximately 18 mm. Thus, it will be understood that the outer surface  54  has a constant cylindrical diameter of approximately 15.5 mm, thereby giving the thread pattern  12  a constant root diameter along outer surface  54 . Thread pattern  12  increasingly tapers from a height of approximately 0.5 mm adjacent distal end  36  by substantially continuously increasing the height of each successive thread turn until it reaches its maximum height at last thread turn  52  adjacent proximal end  34 . At this point, thread turn  52  has a height of 2.5 mm above surface  54 . In a preferred embodiment, referring to FIG. 6B, thread turn  52  is separated from thread turn  56  by a distance of L9. Preferably distance L9 represents a 1.8 pitch for the thread pattern. The continuously expanding tapered thread pattern insures that upon each device revolution, the next greater height thread is engaging bone that has not been previously engaged or compressed by the preceding threads. The tapering thread pattern thereby resists further screwing of the device. While tapering the height of each successive thread turn is shown, the width of each successive thread may alternatively, or in combination with thread height, be increased to generate resistance to further screwing.  
         [0041]    Referring to FIG. 7, there is shown a second embodiment of the fusion device  110  of the present invention. Fusion device  110  includes a thread pattern  112  interrupted by thread cut outs  114  and  120 . Thread cut outs  114  and  120  are configured substantially as shown with respect to the embodiment of FIG. 1 to create barbs  115  and  121 . The longitudinally extending cut outs create a longitudinally extending series of such barbs substantially parallel to axis  119 . Further, fusion device  110  includes opposing concave sidewalls  122  and  124 . Moreover, the thread pattern is substantially tapering from a smaller thread form opposite end  134  to a larger thread form adjacent end  134 . Fusion device  110  differs from fusion device  10  primarily in the fact that concave sidewalls  122  and  124  are substantially solid.  
         [0042]    Referring now to FIG. 8, there is shown yet a further embodiment of an improved interbody fusion device  210  according to the present invention. Fusion device  210  includes an external thread pattern  212  having a substantially continuous thread crest diameter. Thread pattern  212  is interrupted by thread cut outs  214  and  220  configured substantially as shown in FIG. 5A to form barbs  215  and  221 , respectively. However, thread cut outs  214  and  220  differ in that the angle of the thread cut out with respect to the axis to  270  is substantially parallel rather than set at a 45° angle as shown in FIG. 5B. Thus, walls  265  and  266  are substantially parallel to axis  270 . The thread cut out continues to have the undercut area to constrain material in the thread path. Further, the embodiment of FIG. 8 includes concave sidewalls  222  and  224 . The depth of concave side walls  222  and  224  extends to a lesser extent into the body of fusion device  212  thereby retaining a greater amount of the thread form for purposes of insuring alignment of the threads during insertion into the disc space. However, a trade off of the smaller amount of concavity into side walls  222  and  224  is that, in comparison to the embodiment of FIG. 1, a pair of fusion devices according to the embodiment of FIG. 8 will have an increased width when placed within the disc space.  
         [0043]    Referring now to FIG. 9, there is shown still a further embodiment of an improved interbody fusion device according to the present invention. Interbody fusion device  310  includes a thread form  312  tapering from a smaller diameter thread form adjacent distal end  336  to a larger diameter thread form opposite end  336 . However, the outer surface of fusion device  310  also includes a tapering outer surface substantially matching the taper of threads  312 . Thus, the thread height is substantially constant from the distal end  336  to the opposite second end. Thread form  312  is interrupted by thread cut outs  314  and  320  to form barbs  315  and  321 , respectively. Thread cut outs  314  and  320  do not extend parallel to longitudinal axis  319 , but instead follow the outer taper of the thread crest diameter. In contrast to the previous embodiments, opposing sidewalls  322  and  324  are substantially flat and lack concavity necessary to rotate one adjacent device within the maximum root diameter of the other. However, it is contemplated that in some applications substantially flat sidewalls  322  and  324  may be positioned adjacent the vertebral end plates and the device urged into the disc space to the desired depth without rotation. Once the desired depth has been reached, fusion device  310  may then be rotated to engage thread pattern  312  with the bone of the adjacent vertebral bodies and thereby anchor the device. Moreover, while not shown, it is contemplated that fusion device  310  may include one or more fusion windows formed through the threads or in the valleys between adjacent threads to communicate with a hollow interior.  
         [0044]    [0044]FIG. 10 shows yet a further embodiment of an improved interbody fusion device  410  according to the present invention. Interbody fusion device  410  includes a thread pattern  412  decreasing in thread height from proximal end  434  to the opposite end of the fusion device  410 . Thread form  412  is interrupted by thread cut outs  414  and  420 . The thread cut outs  414  and  420  extend between tallest thread  452  and shortest thread  450  substantially parallel to the longitudinal axis  419  and along the entire length of device  410 . Thread cut outs  414  and  420  create undercuts  462  and  466 , respectively. Undercuts  462  and  466  create barbs  460  and  464 , respectively, adjacent proximal end  434  where the thread form is taller. At the opposite end of fusion device  410 , it can be seen that cut out  414  does not result in a substantial undercut  462  since the height of the thread  450  is substantially smaller than the height of thread  452 .  
         [0045]    Referring now to FIG. 11, a view of end  434  of the device of FIG. 10 is shown. Side walls  463  and  465  of cut outs  414  and  420 , respectively, are in substantially parallel alignment with transverse axis  470 . At thread  452 , thread cut outs  414  and  420  are formed with a radius R 2  resulting in thread undercut  462  and  466 , respectively. While the radius R 2  results in a diameter approximating 20% of the thread crest diameter. In the illustrated embodiment, radius R2 is approximately 2 mm. A smaller radius R2 will result in less material removed from the body of the device, shallower barbs, and the potential for more pronounced barbs on smaller height threads. Further, thread cut outs  414  and  420  create barbs  460  and  464  with relatively sharp points  461  and  463  where the cut out walls  463  and  465  meet the periphery of the thread form.  
         [0046]    Thread cut out  420  has a width extending between leading wall  465  and trailing wall  467  of L5. In a preferred embodiment, L5 is approximately 4 mm. Thus, the ratio of the width or diameter of the thread cut out  420  to the diameter of device  410  is approximately 20%. However, it will be understood that this ratio may be as high as 50% or as low as single digit percentages. In further explanation of the features disclosed in FIG. 11, the centerline  480  of the thread cut out  420  is disposed at angle A2 with respect to a ray  478  extending from the center to the intersection  482  of thread cut out  420  and the thread form. It is contemplated that this angle may be less than 90° but greater than 0° to achieve a thread cut out according to the present invention. In the embodiment of FIG. 11, angle A2 is approximately 30°. Preferably, as shown in FIG. 11, a ray  468  connects thread overhang  464  with the center of the device  410 . Trailing wall  467  is offset with respect to ray  468  to thereby define recess  472 .  
         [0047]    A further embodiment of the present invention is shown in FIG. 12. Fusion device  510  includes a thread form  512  having a substantially constant thread crest height. In this embodiment, the overall diameter of the thread increases from a smaller threaded diameter adjacent distal end  536  to a larger maximum thread diameter adjacent proximal end  534 . The root diameter of the device also substantially increases between the distal and proximal ends. Thread  512  is interrupted by thread cut outs  514  and  520  forming barbs  515  and  521 , respectively. Thread cut outs  514  and  520  are formed along line  570 , which runs substantially parallel to the tapering root diameter of the device and out of alignment with longitudinal axis  519 . On proximal end  534  a portion of tool engaging slot  516  is shown.  
         [0048]    Referring now to FIG. 13, there is shown still another embodiment of an improved interbody fusion device  610  according to the present invention. Fusion device  610  includes an external thread pattern  612  tapering from a relatively small diameter thread form  650  to a larger diameter thread form  652 . The outer surface  654  of device  610  has a diameter D6 which remains substantially constant along the entire length of the device  610 . Thread form  650  has a diameter D 5  that is greater than diameter D6 but less than the diameter D 7  of thread form  652 . The intervening thread forms substantially and continuously taper between diameters D5 and D7.  
         [0049]    Devices according to the present invention may be formed of any suitable biocompatible material. Such materials may include, for example but without limitation, steel, titanium, tantalum, bone, composites, ceramics, plastics and the like. Furthermore, devices incorporating the present invention may be hollow, solid or formed of porous material. Moreover, while thread cut outs have been shown extending along the length of the device to create a longitudinally extending series of barbs, it is contemplated that cut outs may be placed in one or more threads to create one or more barbs positioned at various locations on the device. It will further be understood that while cutouts have been shown in a substantially linear form along the length of the device, cut outs may be created in the threads at alternating intervals.  
         [0050]    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.