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 , fig3 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 fig3 - 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 fig4 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 fig6 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 ( fig3 ) 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 . fig9 - 10 illustrate an alternate embodiment of the implant apparatus of fig3 . this implant apparatus is substantially similar to the apparatus disclosed in fig3 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 ”. fig1 illustrates an alternate embodiment of the implant apparatus of fig3 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 . 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 fig1 . 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 fig1 , 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 fig1 , 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 fig9 - 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 fig1 , 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 .
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 .