Abstract:
An apparatus for facilitating fusion of adjacent bony structures includes an implant body dimensioned for positioning between adjacent bone structures to maintain the bone structures in desired spaced relation during interbody fusion. The implant body has an outer wall and an external threaded configuration disposed on the outer wall. At least one concave surface at least partially extends along the implant body. The concave surface advantageously reduces the transverse cross-sectional dimension of the implant member thereby facilitating placement of the implant member in restricted intervertebral areas. In addition, the concave surface enables placement of a pair of implants in nested side-by-side relation. Preferably, the threaded configuration has portions removed along an arc section of the outer wall thereby defining a series of generally longitudinally aligned concave surfaces in individual turns thereof. A system and method for facilitating fusion of adjacent vertebrae is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part application of co-pending U.S. application Ser. No. 09/545,320 filed on Apr. 7, 2000. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to a surgical apparatus for fusing adjacent bone structures, and, more particularly, to an apparatus and associated method for fusing adjacent vertebrae. 
     2. Background of the Related Art 
     The fusion of adjacent bone structures is commonly performed to provide for long-term replacement to compensate for degenerative or deteriorated disorders in bone. For example, an intervertebral disc, which is a ligamentous cushion disposed between adjacent vertebrae, may undergo deterioration as a result of injury, disease, tumor or other disorders. The disk shrinks or flattens leading to mechanical instability and painful disc translocations. 
     Conventional procedure 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 fusion with the plug and opposed vertebrae. More recently, emphasis has been placed on fusing bone structures (i.e., adjoining vertebrae) with metallic or ceramic prosthetic cage implants. One fusion cage implant is disclosed in commonly assigned U.S. Pat. No. 5,026,373 to Ray et al., the contents of which are incorporated herein by reference. The Ray &#39;373 fusion cage includes a cylindrical cage body having a thread formed as part of its external surface and apertures extending through its wall which communicate with an internal cavity of the cage body. The fusion cage is inserted within a tapped bore or channel formed in the intervertebral space thereby stabilizing the vertebrae and maintaining a pre-defined intervertebral space. Preferably, a pair of fusion cages are implanted within the intervertebral space. The adjacent vertebral bone structures communicate through the apertures and with bone growth inducing substances which are within the internal cavity to unite and eventually form a solid fusion of the adjacent vertebrae. FIGS. 1-2 illustrate the insertion of a pair of the Ray &#39;373 fusion cages positioned within an intervertebral space. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to further improvements in spinal fusion procedures. In accordance with a preferred embodiment, an apparatus for facilitating fusion of adjacent bony structures includes an implant body dimensioned for positioning between adjacent bone structures to maintain the bone structures in desired spaced relation during interbody fusion. The implant body has an outer wall and an external threaded configuration disposed on the outer wall. At least one concave surface at least partially extends along the implant body. The concave surface advantageously reduces the transverse cross-sectional dimension of the implant member thereby facilitating placement of the implant member in restricted intervertebral areas. In addition, the concave surface enables placement of a pair of implants in nested side-by-side relation. Preferably, the threaded configuration has portions removed along an arc section of the outer wall thereby defining a series of generally longitudinally aligned concave surfaces in individual turns thereof. A system and method for facilitating fusion of adjacent vertebrae is also disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiment(s) of the present disclosure are described herein with reference to the drawings wherein: 
     FIG. 1 is a view illustrating a portion of the vertebral column of a patient; 
     FIG. 2 is a view taken along line  2 — 2  of FIG. 1 illustrating a pair of prior art fusion implants positioned within the intervertebral space for fusion of adjacent vertebrae; 
     FIG. 3 is a perspective view of the fusion implant apparatus in accordance with the principles of the present disclosure; 
     FIG. 4 is a side plan view of the implant apparatus; 
     FIG. 5 is an axial view of the implant apparatus; 
     FIG. 6 is a side cross-sectional view of the implant apparatus taken along the lines  6 — 6  of FIG. 5; 
     FIG. 7 is an axial cross-sectional view of the implant apparatus taken along the lines  7 — 7  of FIG. 4; 
     FIG. 8 is a view illustrating details of the threaded configuration of the implant apparatus; 
     FIG. 9 is a perspective view of an alternate embodiment of the implant apparatus of FIG.  3 : 
     FIG. 10 is an axial view of the implant apparatus of FIG.  9 . 
     FIG. 11 is a perspective view of another alternate embodiment of the implant apparatus of FIG. 3; and 
     FIGS. 12-14 are views illustrating a preferred sequence of the implant apparatus within adjacent vertebrae. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The preferred embodiment of the apparatus and method disclosed herein are discussed in terms of orthopedic spinal fusion procedures and instrumentation. It is envisioned, however, that the disclosure is applicable to a wide variety of procedures including, but, not limited to ligament repair, joint repair or replacement, non-union fractures, facial reconstruction and spinal stabilization. In addition, it is believed that the present method and instrumentation finds application in both open and minimally invasive procedures including endoscopic and arthroscopic procedures wherein access to the surgical site is achieved through a cannula or small incision. 
     The following discussion includes a description of the fusion implant utilized in performing a spinal fusion followed by a description of the preferred method for spinal fusion in accordance with the present disclosure. 
     In the discussion which follows, the term “proximal”, as is traditional, will refer to the portion of the structure which is closer to the operator while the term “distal” will refer to the portion which is further from the operator. 
     Referring now to the drawings in which like reference numerals identify similar or identical elements throughout the several views, FIG. 3 illustrates, in perspective, the fusion implant apparatus of the present disclosure. Fusion implant  100  is intended to be inserted within a preformed bore in adjacent bone structures, e.g., adjacent vertebrae, with the bore spanning the intervertebral space and penetrating the vertebral end plates. 
     Fusion implant  100  includes elongated implant body  102  which is preferably fabricated from a suitable biocompatible rigid material such as titanium and/or alloys of titanium, stainless steel, ceramic materials or rigid polymeric materials. Implant body  102  is preferably 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. 
     With reference to FIGS. 3-7, implant body  102  includes exterior or outer wall  104  concentrically arranged about longitudinal axis “a” of the implant body  102  and inner cavity  106  within the exterior wall  104 . Implant body  102  is preferably substantially cylindrical in configuration defining a constant diameter along its length. Inner cavity  106  is intended to accommodate bone growth inducing substances such as bone chips taken from allograft or autograft, etc . . . which facilitate the fusion process. Implant body  102  is preferably provided in various lengths ranging from about 18 mm-24 mm and in corresponding various diameters ranging from about 14 mm-18 mm. Other dimensions are also contemplated and may vary depending on the intended use of the implant in the cervical, thoracic or lumbar regions of the spine. 
     Outer wall  104  has an external threaded configuration  108  formed thereon. External threaded configuration  108  includes a uniform helical thread  110  which assists in advancing implant body  102  into a preformed channel provided in the adjacent vertebrae. In the preferred embodiment, thread  110  cooperates with an internally threaded bore within the adjacent vertebrae to advance implant. body  102  within the threaded bore. Alternatively, thread  110  may be a self-tapping cutting thread, i.e., the thread is capable of deburring bone material during advancement into the performed channel thereby precluding the requirement of tapping the internal bore in the vertebrae. 
     A plurality of apertures  112  extend through outer wall  104  of implant body  102 . Apertures  112  are preferably formed by broaching grooves in the internal surface of the internal cavity  108 . The effect of such broaching is to remove material from the valleys between the individual turn of the thread  110 , thus defining the apertures  112 . The advantages of such an arrangement are disclosed in U.S. Pat. No. 4,961,740, the contents of which are incorporated herein by reference, and include immediate bone to bone contact between the vertebral bodies or bone structures and the bone inducing substances packed within the internal cavity  108  of the implant body  102 . Apertures  112  are preferably substantially the same in dimension although it is envisioned that the dimensions of the apertures may vary to provide for more or less bone to bone contact as desired. 
     As best depicted in FIGS. 4 and 7, apertures  112  are clustered about a transverse axis “t 1 ”, both at the upper and lower end of the axis. Consequently, apertures  112  come into contact with the upper and lower vertebral bone structures to encourage bone growth through implant body  102  from the vertebral bone structures when appropriately positioned within the vertebrae. The lateral sections of implant body  102  formed along transverse axis “t 2 ” do not have apertures in order to prevent growth of disk material which might interfere with the bone. fusion process. 
     Outer wall  104  has a plurality of independent arcuate surfaces  114  defined in the outer wall and extending along the length of implant body  102 . The arcuate surfaces  114  are preferably concave in configuration and may be formed by grinding, blasting applications, etc. Preferably, concave surfaces  114  extend radially inwardly within each thread turn without penetrating or extending into the outer wall surface thereby defining removed portions of the thread as shown. 
     The concave surface arrangement provides two specific advantages. First, such arrangement increases the pull out or expulsion force necessary to remove the implant from the adjacent vertebrae. Secondly, the. concave surface arrangement permits a pair of implants to be positioned in side by side relation within the adjacent vertebrae in a nested contacting relation. Moreover, the concave surface arrangement provides a reduced cross-sectional dimension along second transverse axis “t 2 ” relative to the cross-sectional dimension along first transverse axis “t 1 ” thereby facilitating placement of the implant body  102  within restricted vertebral locations. 
     Implant body  102  defines entry and trailing end faces  116 ,  118 . End faces  116 ,  118  are preferably open, i.e, having apertures  120 ,  122  therein in communication with the inner cavity  106 . As best depicted in FIG. 6, implant body  102  has internal annular recesses  124  adjacent each end face  116 ,  118 . Annular recesses  124  are intended to receive plastic end caps  126  (FIG. 3) which are received within the recesses in snap-fit relation therewith to enclose internal cavity  108  thereby retaining the bone.growth inducing substances therein. Implant body  102  further includes tool receiving structure in the form of longitudinal extending internal rails  128  extending the length of the implant body  102  in diametrically opposed relation. Rails  128  receive correspondingly dimensioned prongs of an insertion instrument such that the insertion instrument may be rotated to cause corresponding rotation and entry of implant body  102  into the intervertebral space. 
     Alternate Embodiment(s) 
     FIGS. 9-10 illustrate an alternate embodiment of the implant apparatus of FIG.  3 . This implant apparatus is substantially similar to the apparatus disclosed in FIG. 3, but, however incorporates a second series of concave surfaces  114  disposed in diametrically opposed relation to the first series. The second series provides flexibility to the user in terms of placement of the implant within the desired orientation within the intervertebral disc space. The second series also significantly reduces the cross-sectional dimension of the implant body along the second transverse axis “t 2 ”. 
     FIG. 11 illustrates an alternate embodiment of the implant apparatus of FIG. 3 where the concave surface extends through threaded configuration  110  and into exterior wall  104  thereby defining a single concave surface  114 ′ which extends along the length of implant body  102 . 
     Insertion of Fusion Implant 
     The insertion of the fusion implant  100  into an intervertebral space defined between adjacent lumbar vertebrae will now be described. The subsequent description will be particularly discussed in conjunction with an open posterior approach for spinal fusion implant insertion. However, it is to be appreciated that other approaches, e.g., anterior, lateral, posterior lateral, anterior lateral etc . . . could be utilized. Laparoscopic approaches are also envisioned. 
     Initially, a first lateral side of the intervertebral space “i” is accessed utilizing appropriate retractors to expose the posterior vertebral surface. A drilling instrument is selected to prepare the disc space and vertebral end plates for insertion of the fusion implant. The cutting depth of drilling instrument may be adjusted as desired. The drilling instrument is advanced into the intervertebral space adjacent to the first lateral side to shear the soft tissue and cut the bone of the adjacent vertebrae thereby forming a bore which extends into the adjacent vertebrae adjacent the first lateral side as depicted in FIG.  12 . With the first bore “b 1 ” drilled in the first lateral side, attention is directed to forming the bore in the second lateral side. With continued reference to FIG. 12, the second lateral side is accessed and the center entry point for the drill is identified. Preferably, the drill is positioned such that the second bore “b 2 ” will overlap the first bore “b 1 ”. The drill is activated to form the second bore. The first and second bores “b 1 , b 2 ” may be tapped with a conventional tap instrument if desired. 
     With reference to FIG. 13, a first implant  100  is packed with bone growth inducing substances as is conventional in the art. The fusion implant  100  may then be mounted on an insertion instrument (not shown) and advanced within the intervertebral space by rotating the implant  100  whereby threaded configuration  110  of the implant body  102  cooperates with the threaded bore to advance within the intervertebral space “i”. Preferably, the implant  100  is arranged such that concave surface generally extends along the axis “s” of the spine and faces the midline of the intervertebral space. If the implant of FIGS. 9-10 is utilized, the second series of concave surfaces facilitates placement of the implant  100  with the concave surface arrangement adjacent to the midline of the intervertebral space, i.e., when positioned, the implant need only be rotated a maximum of 90° in either direction to place the concave surface arrangement adjacent the midline. With the first implant positioned within the intervertebral space, a second implant “x” is implanted within the second threaded bore in the same manner. The second implant “x” is preferably a conventional cylindrical implant such as the implant disclosed in the Ray &#39;373 patent. As appreciated, although the second bore overlaps the first bore, the clearance provided by the concave surface arrangement of the first implant  100  permits the second implant “x” to be advanced within the intervertebral space without interference. The second implant “x” is arranged such that the outer convex surface is received within the concave surface area of the first implant in nested side-by-side relation as shown. Thus, the concave surface arrangement permits two implants  100 , “x” to be placed in nested side-by-side arrangement. The concave surface arrangement also reduces the effective cross-sectional dimension of implant  100  thereby facilitating placement of the implants in a restricted vertebral location. 
     With reference to FIG. 14, it is appreciated that the second implant may be identical to implant  100 . When positioned within the adjacent vertebrae, the concave surface area may be facing the midline of the intervertebral space or alternatively adjacent the outer portion of the space as shown in phantom. 
     Implants  100  form struts across the intervertebral space “i” to maintain the adjacent vertebrae “V 1,  V 2 ” in appropriate spaced relation during the fusion process. Over a period of time, the adjacent vertebral tissue communicates through apertures  112  within implants  100  to form a solid fusion. Desirably, lateral vertebral tissue growth into the implant  100  is restricted due to the concave surface areas of the implant being devoid of apertures. Such lateral growth would inhibit the fusion process and potentially restrict subsequent spinal mobility. 
     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. For example, the fusion implant  100  could also be used for thoracic and cervical vertebrae. Those skilled in the art will envision many other possible variations that are within the scope and spirit of the disclosure as defined by the claims appended hereto.