Abstract:
A fusion implant apparatus for facilitating fusion of adjacent bone structures includes n implant member for positioning between adjacent opposed bone structures. The implant member defines a longitudinal axis and first and second longitudinal ends and has an outer wall dimensioned to engage the opposed bone structures upon positioning therebetween in supporting relation therewith. The outer wall includes at least one thread for facilitating positioning between the opposed bone structures. The implant member has an intermediate portion which defines a cross-sectional dimension transverse to the longitudinal axis which is greater than respective cross-sectional dimensions of the first and second longitudinal ends of the implant member.

Description:
BACKGROUND  
         [0001]    The present disclosure generally relates to a surgical apparatus for fusing adjacent bone structures, and, more particularly, to a threaded, barrel-shaped apparatus and method for fusing adjacent vertebrae.  
           [0002]    The fusion of adjacent bone structures is commonly performed to provide for long-term replacement to compensate for vertebral subluxation typically caused by severe trauma to the spine, degenerative or deteriorated bone disorders, e.g., osteoporosis, abnormal curvature of the spine (scoliosis or kyphosis) and/or weak or unstable spine conditions typically caused by infections or tumors. In addition, an intervertebral disc, which is a ligamentous cushion disposed between adjacent vertebrae, may also undergo deterioration or degeneration as a result of injury, disease, tumor or other disorders. The disk shrinks or flattens leading to mechanical instability and painful disc translocations, commonly referred to as a “slipped disc” or “herniated disc”.  
           [0003]    Conventional procedures for disc surgery include partial or total excision of the injured disc portion, e.g., discectomy, and replacement of the excised disc with biologically acceptable plugs or bone wedges. The plugs are driven between adjacent vertebrae to maintain normal intervertebral spacing and to achieve, over a period of time, bony ingrowth or “fusion” with the plug and opposed vertebrae.  
           [0004]    Alternatively, at least one metallic fusion cage may be is inserted within a tapped bore or channel formed in the intervertebral space thereby stabilizing the vertebrae and maintaining a pre-defined intervertebral space. A pair of fusion cages may also be implanted within the intervertebral space. After a period of time, the soft cancellous bone of the surrounding vertebral bone structures infiltrates the cage through a series of apertures disposed within its external wall and unites with bone growth inducing substances disposed within an internal cavity to eventually form a solid fusion of the adjacent vertebrae.  
         SUMMARY  
         [0005]    The present disclosure relates to a barrel-like fusion implant apparatus for facilitating fusion of adjacent bone structures. The apparatus includes an implant member which is positioned between adjacent opposed bone structures and which defines a longitudinal axis and first and second longitudinal ends. The implant member includes an outer wall dimensioned to engage the opposed bone structures upon positioning therebetween in supporting relation therewith. The outer wall includes at least one thread for facilitating positioning of the implant member between opposing bone structures. Preferably, the implant member also includes an intermediate portion which defines a cross-sectional dimension transverse to the longitudinal axis which is greater than respective cross-sectional dimensions of the first and second longitudinal ends of the implant member.  
           [0006]    The present disclosure also relates to a method of fusing adjacent vertebrae utilizing a barrel-like fusion cage. The method includes the steps of: 1) accessing a space defined between the vertebrae; 2) providing a barrel-like fusion cage as described above; 3) advancing one of the first and second ends of the fusion cage into the space between adjacent vertebrae and positioning the cage in contact with the adjacent vertebrae; and 4) permitting bone ingrowth into contacting surfaces of the fusion cage to facilitate fusion of the adjacent vertebrae. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    Preferred embodiments of the disclosure are described herein with reference to the drawings wherein:  
         [0008]    [0008]FIG. 1 is a perspective view of a fusion cage according to the present disclosure;  
         [0009]    [0009]FIG. 2 is a side view of the fusion cage shown in FIG. 1;  
         [0010]    [0010]FIG. 3 is a cross-sectional view of the fusion cage taken along section line  3 - 3  of FIG. 2;  
         [0011]    [0011]FIG. 4 is an axial view of the fusion cage of FIG. 1;  
         [0012]    [0012]FIG. 5 is a cross-sectional view of the fusion cage taken along section line  5 - 5  of FIG. 4;  
         [0013]    [0013]FIG. 6A is a lateral view illustrating a pair of cylindrically-shaped prior art fusion implants positioned within the intervertebral space for fusion of adjacent vertebrae;  
         [0014]    [0014]FIG. 6B is a top view showing a side-by-side orientation of two prior art cylindrically-shaped fusion cages between two adjacent vertebrae;  
         [0015]    [0015]FIG. 7A is a lateral view showing the placement of the fusion cage of FIG. 1 between two adjacent vertebrae;  
         [0016]    [0016]FIG. 7B is a top view showing a pair of fusion implants according to the present disclosure positioned within the intervertebral space for fusion of adjacent vertebrae; and  
         [0017]    [0017]FIG. 8 is an axial view detailing insertion of a pair of the implants within the vertebrae. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring now to the drawings in which like reference numerals identify similar or identical elements throughout the several views, FIGS.  1 - 5  illustrate the fusion cage implant according to the present disclosure. Fusion cage  10  is contemplated to be a self-tapping implant, i.e., the implant is intended to be inserted within a preformed bore in adjacent bone structures, e.g., adjacent vertebrae, without necessitating tapping of an internal thread within the bone structures prior to insertion. Alternatively, the implant may be inserted within a tapped bore formed in adjacent vertebral bodies as is conventional in the art.  
         [0019]    Fusion implant  10  includes a generally elongated body  12  having a proximal end  13  and a distal end  14 . In the drawings and in the description which follows, the term “proximal”, as is traditional, will refer to the end of the cage  10  which is closer to the surgeon, while the term “distal” will refer to the end which is further from the surgeon. Preferably, cage  10  is fabricated from a suitable biocompatible rigid material such as titanium and/or alloys of titanium, stainless steel, ceramic materials or rigid polymeric materials. Moreover, it is envisioned that cage  10  is sufficient in strength to at least partially replace the supporting function of an intervertebral disc, i.e., to maintain adjacent vertebrae in desired spaced relation, during healing and fusion. Cage  10  is preferably provided in various lengths ranging from about 24 mm to about 28 mm for example.  
         [0020]    As best shown in FIGS. 1 and 3, the body  12  of cage  10  includes an outer wall  15  which encloses an inner cavity  18  defined within the interior of the cage body  12 . Inner cavity  18  accommodates bone chips or bone growth inducing substances as is known in the art to induce the soft cancellous bone surrounding the vertebrae to grow inwardly towards the contact surfaces of the fusion cage  10  to stabilize the cage  10  between two adjacent vertebrae  202 ,  204  (FIG. 7A). Outer wall  15  is generally barrel-shaped along a longitudinal axis “A” which extends from proximal end  13  to distal end  14  (FIG. 2) and includes a bulge  19  generally positioned midway therebetween. As explained in more detail below, it is envisioned that the barrel-like shape of cage  10  increases the overall strength and load sharing capacity of the cage  10 , tends to reduce “stiffness” which has been associated with other prior art designs and allows more bone graft substances to be packed into the augmented internal volume of the cage  10  which will further enhance bone fusion.  
         [0021]    Outer wall  15  also includes at least one side cut-out or concave wall surface portion  20  (FIG. 3) which extends parallel to longitudinal axis “A” along outer wall  15  generally from the proximal end  13  to the distal end  17 . Preferably, two side cut-outs  20  are disposed along outer wall  15  in diametrically opposing relation to reduce the effective dimension or diameter of cage  10  transversally relative to longitudinal axis “A”. In either case, the disposition of the side cut-out(s)  20  (FIG. 3) enhance the low profile features of the present disclosure and facilitate insertion between the vertebral bodies  202 ,  204 . With reference to FIGS. 3 and 4, side cut-outs  20  of body  12  provide a generally elliptical configuration or appearance to cage  10  defining a major dimension “B” which is greater than a minor dimension “C”. It is envisioned that this configuration provides a greater surface area of the implant so as to facilitate contacting engagement and support of the implant with the adjacent vertebrae  202 ,  204  (FIG. 7A). The side cut-outs  20  are disposed along the minor axis “C” to enhance the low profile features of cage  10  and facilitate insertion.  
         [0022]    As best shown in FIG. 5, the major dimension “B” along axis “A” varies from a minimum dimension “B min ” proximate the ends  13 ,  14  of cage body  12  to a bulge section  19  having a maximum dimension “B max ” generally disposed midway between ends  13 ,  14  along a medial axis “M”. More specifically, body  12  of cage  10  is symmetrically arranged about medial axis “M” whereby the maximum diameter or cross-sectioned dimension B max  extends along the medial axis “M” and progressively decreases to the proximal and distal ends  13 ,  14  where the cross-sectional diameters or dimensions are substantially equal. As can be appreciated, this gives cage  10  its barrel-like or bulge-like appearance. Preferably, the maximum diameter or dimension “B max ” ranges from about 12 mm to about 20 mm and the minimum diameter or dimension “B min ” ranges from about 13 mm to about 19 mm. In the preferred embodiment, the maximum diameter is 17.5 mm and the minimum diameter is 16 mm. The length is 21 mm. Other dimensions are also contemplated. It is envisioned that dimensioning the cage  10  in this fashion has several distinct advantages: 1) the barrel-like cage is an inherently stronger pressure vessel than a simple cylinder design, i.e., the barrel-like cage has a higher compressive strength, exhibits greater resistance to fatigue and possesses a higher yield load; 2) the barrel-like shape promotes a better anatomical fit between adjacent vertebrae  202 ,  204  in both the transverse plane (Compare FIG. 6B with FIG. 7B) and the sagittal plane (Compare FIG. 6A with FIG. 7A); 3) the low profile ends  13 ,  14  and the side cut-outs  20  facilitate insertion of the cage  10  and allow two cages  10  to be placed side-by-side with reduced overhang  125  outside the periphery of the vertebral bodies  202  (Compare FIG. 6B with FIG. 7B); and 4) the barrel-like shape of the cage  10  results in an increase in the internal volume of the cage  10  which enables more bone to grow into the cage  10 , thus enhancing bone-to-cage fusion. The barrel cage also exhibits a higher expulsion load, i.e., force required to eject the cage from the intervertebral space.  
         [0023]    With reference to FIGS. 1, 2 and  5 , outer wall  15  also includes an external threaded configuration formed as part of the exterior surface. Preferably, the external threaded configuration of outer wall  15  includes a generally continuous helical thread  16  which assists in advancing cage  10  into a preformed or pre-drilled cavity between adjacent vertebrae  202 ,  204 . Thread  16  provides a varying bite across the cage  10  length to facilitate insertion of the cage  10  and enhance retention of the cage  10  once positioned between the vertebral bodies  202 ,  204 . Thread  16  is generally helical in shape and includes a self-tapping cutting thread, i.e., the threads are capable of deburring bone material during advancement into the performed channel. Preferably, the thread path is curved along both the major dimension “B” and minor dimension “C” which creates a series non-linear thread segments across the cage  10 . In some cases it may be preferable to curve the thread  16  only along one of the dimensions, e.g., major dimension “B”, depending upon a particular purpose. It is envisioned that the non-linear thread path of the present disclosure will also provide a self-distracting mechanism during the insertion process which is believed to be advantageous to achieving proper disc height. Alternatively, a thread can be tapped in the bone prior to insertion of the cage  10 . As stated above, it is envisioned that cage  10  can be dimensioned such that cage  10  is generally symmetrical about axis “A”, i.e., front-to-end symmetry, which will permit insertion of the cage  10  from either the proximal or distal end  13 ,  14 , respectively. In some cases, however, threads  16  can be disposed at an angle relative to axis “A” which will also facilitate insertion of the cage  10  between the vertebral bodies  202 ,  204  and enhance retention of the cage  10  once inserted.  
         [0024]    As best shown in FIGS. 1 and 2, a plurality of apertures  22  extend through outer wall  15  of cage body  12 . Apertures  22  are preferably formed by broaching grooves in the internal surface of the internal cavity  18 . The effect of such broaching is to remove material from the valleys between the threads  16 , thus defining the apertures  22 . The advantages of such an arrangement promote immediate bone to bone contact between the vertebral bodies  202 ,  204  and the bone inducing substances packed within the internal cavity  18  of the cage body  12 . Such configuration is disclosed in commonly assigned U.S. Pat. Nos. 4,961,740 and 5,026,373, the contents of which are hereby incorporated by reference.  
         [0025]    Preferably, apertures  22  are oriented such that when the cage  10  is inserted between vertebrae, a majority of apertures  22  contact the upper and lower vertebral bone structures  202 ,  204  to encourage bony ingrowth through cage body  12  from the vertebral bone structures  202 ,  204 . Similarly, the side cut-outs  20  of cage body  102  preferably do not include apertures in order to prevent growth of disc material which might interfere with the overall bone fusing process. Apertures  22  are preferably substantially the same in dimension although it is envisioned that the dimensions of the apertures  22  may vary to provide for more or less bone-to-bone contact depending upon a particular purpose.  
         [0026]    The present disclosure also relates to a method of inserting the barrel-like fusion cage  10  into an intervertebral space “I” defined between adjacent vertebrae  202 ,  204 . The method discussed hereinafter will generally relate to an open antero-lateral approach for spinal fusion implant insertion. However, as can be appreciated, other spinal implant procedures are also contemplated, e.g., posterior, direct anterior, etc . . . Laparoscopic approaches are also envisioned.  
         [0027]    Initially, one lateral side of an intervertebral space “I” between the two vertebral bodies  202 ,  204  is accessed utilizing appropriate retractors (not shown) to expose the anterior vertebral surface. Thereafter, the retractor is inserted within the intervertebral space “I” from an antero-lateral or oblique position with relation to the vertebral bodies  202 ,  204 . Such an approach provides advantages with regard to avoiding vessels and ligaments.  
         [0028]    Upon insertion of the retractor, the vertebral bodies  202 ,  204  are distracted whereby the retractor becomes firmly lodged within the intervertebral space “I”. A drilling instrument is now utilized to prepare the disc space and vertebral bodies  202 ,  204  for insertion of the fusion cage  10 . Preferably, the cutting depth of drilling instrument can be readily adjusted to correspond to the length of the fusion cage  10 . As can be appreciated, as the drilling instrument is advanced into the intervertebral space “I”, the surrounding soft tissue is sheared and the bone of the adjacent vertebrae  202 ,  204  is cut thereby forming a bore which extends into the adjacent vertebrae  202 ,  204 .  
         [0029]    The fusion cage  10  is then packed with bone growth inducing substances as in conventional in the art and then mounted on an insertion instrument (not shown) and advanced to a position adjacent the vertebral bodies  202 ,  204 . As mentioned above, the non-linear thread configuration of the fusion cage  10  also provides a self-distracting feature which is believed to enhance implantation of the fusion cage  10  and aid in achieving proper disc height.  
         [0030]    Preferably, the insertion instrument includes rotational features which, in turn, cause the fusion cage  10  to rotate and bite into the vertebral bodies  202 ,  204 . As mentioned above, it is envisioned that the center thread or bulge  19  as well as the angle of the threads  16  relative to the longitudinal axis “A”, will vary the thread  16  bite during the insertion process which facilitates insertion and retention of the cage  10 . Moreover, the low profile ends  13  and  14  of cage body  12  as well as the side cut-outs  20  will also facilitate insertion and allow two cages to be placed closer together decreasing the likelihood of cage  10  overhang. Continued rotation of the insertion instrument causes cage  10  to self-tap within the preformed bore. Cage  10  is then released from the mounting instrument which is subsequently removed from the disc area.  
         [0031]    Thereafter, a second lateral side of the intervertebral space “I” is accessed and the above-described process is repeated to insert a second cage  10  in lateral side-by-side relation as shown in FIG. 7B. As appreciated, the cages  10  are arranged such that respective side cut-out portions  20  of each cage  10  are disposed in adjacent side-by-side relation. Alternatively, the cages  10  may be positioned such that the curved body  12  of one cage is received within the side cut out  20  of the other cage to further reduce the profile of the implanted cages as depicted in FIG. 8. Such arrangement permits cages  10 ,  10  to be placed in closer proximity thereby facilitating insertion of the cages  10 ,  10  within the intervertebral space “I”.  
         [0032]    Fusion cages  10  form struts across the intervertebral space “I” to maintain the adjacent vertebrae  202 ,  204  in appropriate spaced relation during the fusion process. Over a period of time, the adjacent vertebral tissue communicates through apertures  22  within cages  10 ,  10  to form a solid fusion. It is envisioned that the barrel-like shape of each fusion cage  10  is inherently stronger that a cylinder-shaped fusion cage and provides a better anatomical fit between adjacent vertebrae  202 ,  204  (Compare FIG. 6B with FIG. 7B).  
         [0033]    From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. For example, it is envisioned that a series of apertures could be drilled at one end of the cage  10  which would allow a surgeon to use a smaller tang and smaller drill thereby preserving more of the posterior elements of the spine during the operation.  
         [0034]    While the above description contains many specifics, these specifics should not be construed as limitations on the scope of the disclosure, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.