for the purposes of promoting an understanding of the principles of the invention , reference will now be made to the embodiments , or examples , 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 . any alterations and further modifications in the described embodiments , and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates . referring now to fig2 a and 2b , there is shown an expandable implant 100 in accordance with one aspect of the present invention . the expandable implant 100 includes a tubular main body 110 with an upper portion 111 . upper portion 111 includes a first helically threaded area 112 extending over approximately one - quarter of the outer circumference of the device and an opposing helically threaded portion 113 extending around substantially the same portion of the device . threaded portions 112 and 113 are spaced from each other by unthreaded channels 114 and 115 . in the illustrated embodiment , the helically threaded portions 112 and 113 have a thread crest diameter of a first dimension and the unthreaded areas 114 and 115 have a diameter slightly less than the root diameter in the threaded portions and significantly less than the first dimension of the thread crests . at the intersection between threaded portion 112 and unthreaded portion 115 , there is a dovetail projection 118 formed along the longitudinal axis l 1 . similarly , extending along the junction between unthreaded channel 115 and threaded portion 113 is a dovetail projection 120 . on the alternate side of the device , a dovetail projection 122 extends between threaded portion 113 and unthreaded channel 114 . in a similar manner , a dovetail projection 124 is formed along the longitudinal axis extending between the junction of unthreaded channel 114 and threaded portion 112 . the main body 110 includes an upper end surface 116 . the term “ dovetail projection ” as used herein includes at least any extension of material capable of capturing or being captured within an adjacent segment of material or separate component , and is not limited to walls of a specific shape . lower portion 127 of the main body 110 has substantially the same features as described above with respect to upper portion 111 and will not be further described . in one aspect , main body 110 includes a pair of locking tabs or wedge structures 140 and 150 centrally located near the mid - section of the main body . a similar pair of locking tabs , partially shown in fig2 a , are formed on the opposite side of the device . the height of the locking tabs is smaller adjacent to the midpoint of the main body and extends to a greater height from the surface of the unthreaded channel 114 as it extends distally away from the middle of the main body . tab 140 includes an abutting shoulder 142 having a height extending above the outer surface of the main body and tapers to a decreasing height at central portion 144 . similarly , wedge 150 includes an abutting shoulder 152 as it extends away from the midpoint of main body 110 . as shown more fully in fig4 b , main body 110 includes a pair of longitudinally extending , semi - circular grooves 119 and 121 defined within the outer surface . grooves 119 and 121 are formed at the juncture between the threaded portions and the unthreaded portions . grooves similar to 119 and 121 are formed on the opposing exterior surface of the device . an interference clip 130 is received on main body 110 . interference clip 130 includes a tool engagement opening 132 and a pair of retaining rails 134 and 136 . retaining rails 134 and 136 have an exterior convex surface substantially matching the diameter of semi - circular grooves 119 and 121 . retaining rails 134 and 136 are slidingly received along grooves 121 and 119 , respectively . it will be appreciated that interference clip 130 may slide longitudinally along longitudinal axis l 1 and is retained on main body by engagement of the rails 134 and 136 within groves 121 and 119 . a second retaining clip 160 is also positioned on the device adjacent to wedge 150 . the second retaining clip 160 also engages grooves 119 and 121 , and may move longitudinally along the device . as shown in fig4 b , clip 137 is positioned on the opposite side on the device to be slidingly received within the longitudinal grooves . while only retaining clip 137 is shown , it will be appreciated that a lower clip similar to clip 160 is also present . referring now to fig5 b , clip 130 is shown in cross - section . clip 130 has a leading end 131 of a first thickness and a trailing end 133 of a second thickness , the clip tapers between the smaller first thickness and the larger second thickness . it will be appreciated that the tapering wedge shape of clip 130 acts as a shim when received between unthreaded channel 114 and the unthreaded channel 190 of the bone engaging end . clip 160 has a similar tapered shape with the leading tapered tip oriented toward the lower distal end of the device . in a further embodiment , clips 130 and 160 have a greater longitudinal height and a substantially uniform thickness allowing the clips to slide freely in the space between unthreaded sections . the longitudinal height is sufficient such that when positioned past the end 142 of locking tab 140 , the clip will extend longitudinally adjacent to at least a majority , if not all , of the threaded projections . it will be appreciated , that the upper interference clip 130 may positioned on one side of the main body , and the lower interference clip 160 may positioned on the opposite side of the main body to allow sufficient room for the clips in the central area of the main body in the unlocked condition . a pair of bone engaging sleeves are also provided for engagement with main body 110 . upper body engaging sleeve 180 includes a tubular barrel portion 182 and an enlarged flange area 184 . enlarged flange portion 184 includes a bone engaging surface 186 which in the illustrated embodiment includes a series of knurled projections for engagement with a boney surface . in the embodiments illustrated in fig2 a and 3 , the bone engaging end extends at a non - perpendicular angle with respect to the longitudinal axis l 1 . the angle may be any desired angle to substantially match the natural or restored angulation of lordosis or kyphosis between the remaining vertebral endplates . for example , each end plate may have 0 , 3 , or 6 degrees of angulation allowing the device to achieve 0 to 12 degrees of angulation between the remaining vertebrae . while a knurled surface has been shown for the purposes of illustration , in alternative embodiments the projections take the form of cones , blades , keels , fins , ridges , pegs or any other surface projection . further , the surface 151 may be formed such that recesses in the surface create projections in a bone ingrowth type surface allowing bone to grow into the surface or to interdigitate with native bone on the endplates . bone engaging sleeve 180 is substantially tubular and includes a central passage 188 . in a manner corresponding to the threaded and unthreaded portions of the main body 110 , the bone engaging member 180 includes unthreaded channels 190 and 192 and threaded portions 194 and 196 each spaced at approximately 90 degree intervals around the internal surface . disposed between the respective threaded and unthreaded portions , are a series of dovetail recesses 197 , 198 , 200 and 202 . it will be appreciated that the size and shape of the dovetail recesses are formed to substantially mate with the dovetail projections of the main body 110 previously described . moreover , the extent of the threaded portions 194 and 196 and the unthreaded portions 190 and 192 around the internal circumference of internal passage 188 substantially match the corresponding external structures disposed on main body 110 . the expandable device 100 is shown in a disassembled condition in fig2 a . in the illustrated embodiment , interference clips 130 and 160 have already been installed on the main body 110 . it will be appreciated that upper bone engaging member 180 will be substantially aligned with threaded portions 194 and 196 aligned with the unthreaded channels 114 and 115 of main body 110 . in this rotational alignment , upper bone engaging member 180 may be advanced toward the middle or center of the main body 110 along the longitudinal axis . in a similar manner , lower bone engaging member 220 is rotationally aligned such that the threaded portions are in substantial alignment with the unthreaded portions of main body 110 . in this alignment , the lower bone engaging portion is longitudinally advanced towards the middle of main body 110 . each of the upper and lower bone engaging members may be rotated several degrees such that the internal threads of the bone engaging members engage the external threads of the main body . interference clips 130 and 160 are held near the midline by wedges 140 and 150 , respectively . in this provisionally retained condition , the expandable implant 100 is positioned into the spinal segment in the space previously occupied by vertebral body v 3 and the adjacent disks d 2 and d 3 . once the implant 100 has been properly positioned , the upper and lower bone engaging members are rotated about the longitudinal axis such that their angled faces are properly positioned against the adjacent vertebral bodies to obtain the desired amount of lordosis or kyphosis . it will be understood , that while the illustrated embodiments disclose angulation on the bone engaging surfaces , modified bone engaging end pieces may be provided having zero degrees of angulation up to or exceeding about 6 degrees of angulation . within this range of end pieces , the total expanded construct may provide zero degrees of angulation ( i . e . parallel orientation ) between the endplates of the intact vertebra and up to or exceeding 12 degrees of angulation . once the bone engaging portions have been properly oriented , a spreader or other type of distraction device is inserted to engage the large flange on each of the bone engaging members to urge them apart and into engagement with the intact vertebra . the central member is engaged by a tool ( not shown ) or by hand to substantially align the unthreaded channels with the threads of the bone engaging portions . in one aspect , the spreader engages each end piece to prevent rotation of the end pieces with respect to the central member . with the threads of the end pieces aligned with the unthreaded channels of the main body , the spreader is actuated to push the end pieces to slide along the longitudinal axis l 1 to enlarge the height of the implant from its initial reduced height insertion configuration to its increased height spacing configuration , shown in fig2 b , that engages bone on opposing ends of the device . once the desired amount of expansion of the expandable device 100 has been obtained , and with the spreader still in place to hold the expansion , the main body 110 is engaged with a tool or by hand and rotated approximately 90 degrees to engage the threaded portions of the main body with the corresponding threaded portions of the respective upper and low bone engaging portions . in one embodiment , the spreader instrument includes a central cable and a winch spool to retract the cable . the end of the cable is removably coupled to the main body and at least partially wound around the main body . this attachment and winding can be performed before implantation into the body . after implantation and expansion of the implant , the cable winch is operated to retract the cable resulting in rotational movement of the main body . after this rotation , the spreading device and cable are removed and the expandable spacer 100 will maintain the height that was previously achieved during expansion of the spacer . in order to inhibit rotation of the main body 110 and end piece 180 with respect to each other , the interference clip 130 is moved longitudinally away from the center of the device to a locked position as shown in fig2 b . in a similar manner , clip 160 is moved longitudinally towards the opposite end of the implant to maintain the end piece 220 in the engaged position with respect to main body 110 . more specifically , a locking tool ( not shown ) such as a spreader is engaged with slot 132 on interference clip 130 and the corresponding slot 162 on interference clip 160 . the locking tool then moves each of the retaining clips away from the middle of the main body and towards the distal ends , such that each clip passes over wedges 140 and 150 and ultimately passes beyond engagement shoulders 142 and 152 , respectively . the clips continue to advance along grooves 119 and 121 distally until they are wedged at least in part between the unthreaded channel of the main body and the unthreaded channel of each of the bone engaging members . in the illustrated embodiment , the interference clips have a tapered thickness such that under longitudinal force the reduced thickness leading ends are pressed into the space between the unthreaded channels . the engagement of the interference clip outer surface with the inner surface of the bone engaging end piece and opposing engagement of the clip inner surface with the outer surface of the main body firmly holds the clip in position . it is anticipated that the longitudinal force applied to the clips is such that once moved to the interference position , they may not be dislodged without a tool . moreover , inadvertent movement of the clips out of the unthreaded channels is prevented by engagement with wedges 140 and 150 . in the interference position between the unthreaded channel of the bone engaging portion and the unthreaded channel of the main body , the interference clips interrupt the threaded path of the threaded components to prevent the rotation of the main body with respect to the end portions and thereby maintain the threaded portions of the main body engaged with the threaded portions of the bone engaging end portions . thus , the interference clips lock the device in its expanded condition . in an alternative embodiment , the main body is substantially solid such that while it is telescopically received within the end portions , no material may be received within the main body . moreover , while the main body has been shown as telescopically received within tubular upper and lower bone engaging members , it will be appreciated that in a further embodiment the respective configuration is inverted such that a shaft portion of the upper and lower bone engaging members is received within a hollow portion of a tubular main body . moreover , while a substantially cylindrical structure has been shown for the purposes of illustration , in an alternative embodiment the tubular shapes may take the form of a rectangle , square , ellipse , diamond , oval , d - shape or any shape desired to conform and substantially match the adjacent bone or the bone structure that is being replaced . as a result , the definition of tubular is not intended to be limited to cylindrical but is instead intended to cover all components that may be utilized to reduce the present invention . in an alternative embodiment , rather than have a separate retaining clip sliding along the longitudinal axis , at least one of the inner member or end pieces include one or more tabs that are moveable into the unthreaded channels to interfere with the rotation of the end piece with respect to the inner member once the tab is in the retained position . in one aspect , a plurality of locking tabs are formed to extend as ratchet members along the longitudinal axis . in one embodiment , the tabs are formed to resiliently snap into a locked position to prevent angular movement between the tubular end piece and the tubular main body . in an alternative embodiment , the tabs are formed such that a tool is used to deform the tab material to move it to a locked position . in still a further embodiment , the tubular endpieces are advanced by threaded engagement with the central tubular portion rather than the sliding longitudinal advancement discussed above . in this aspect , the interference clips are advanced along the longitudinal axis to retain the relative angular position of the components in a plurality of different configurations . in still a further embodiment , an elongated locking element is having a plurality of locking protrusions is provided . the locking element has a length that longitudinally extends through a plurality of support protrusions . in a first longitudinal position , the locking protrusions of the locking element are aligned with the support protrusions to allow movement between the main body and the end piece . in a second longitudinal position , the locking protrusions are aligned with spaces between the support protrusions to prevent movement between the main body and the end piece . the locking member is configured to move longitudinally between the first longitudinal position and the second longitudinal position . while a portion of a helical threaded has been shown for the purpose of illustrating a type of locking protrusion , the disclosure encompasses any interlocking engagement structure that resists compressive forces applied along the longitudinal axis . for example , in one embodiment , the protrusions are ratchetings extending substantially perpendicular to the longitudinal axis l 1 of the implant . in still further examples , but without limitation to alternative structures , alternative embodiments have protrusions taking the form of grooves , ridges , splines , tabs , flanges , fingers or any other type of projections . while the present device has been described with respect to insertion between two vertebrae after removal of the intervening vertebrae and intervertebral disc , it is contemplated that the length of the device may be sized appropriate to span multiple vertebrae . further , in an alternative embodiment , only a single endpiece is moveable along the main body such that the device telescopes in only one direction . additionally , the device may find application in other orthopedic areas and the size and shape of the device may be made to substantially match the implantation site . for example , while the present embodiment has been illustrated as a substantially cylindrical device , it is contemplated that in certain spinal applications it is desirable that the device have a substantially d shaped cross - section as viewed from top to bottom such that the anterior portion of the device has an exterior convexly curved surface matching the anterior of the vertebral body while the posterior portion of the device is substantially flat or concave allowing it to be positioned closer to the spinal canal without protruding into the spinal canal . embodiments of the implant in whole or in part may be constructed of biocompatible materials of various types . examples of implant materials include , but are not limited to , non - reinforced polymers , carbon - reinforced polymer composites , peek and peek composites , shape - memory alloys , titanium , titanium alloys , cobalt chrome alloys , stainless steel , ceramics and combinations thereof . if the trial instrument or implant is made from radiolucent material , radiographic markers can be located on the trial instrument or implant to provide the ability to monitor and determine radiographically or fluoroscopically the location of the body in the spinal disc space . in some embodiments , the implant or individual components of the implant are constructed of solid sections of bone or other tissues . in other embodiments , the implant is constructed of planks of bone that are assembled into a final configuration . the implant may be constructed of planks of bone that are assembled along horizontal or vertical planes through one or more longitudinal axes of the implant . in some embodiments , a cavity is cut or constructed through the implant . the cavity may be useful to contain grafting materials . tissue materials include , but are not limited to , synthetic or natural autograft , allograft or xenograft , and may be resorbable or non - resorbable in nature . examples of other tissue materials include , but are not limited to , hard tissues , connective tissues , demineralized bone matrix and combinations thereof . examples of resorbable materials that may be used include , but are not limited to , polylactide , polyglycolide , tyrosine - derived polycarbonate , polyanhydride , polyorthoester , polyphosphazene , calcium phosphate , hydroxyapatite , bioactive glass , and combinations thereof . implant may be solid , porous , spongy , perforated , drilled , and / or open . in some circumstances , it is advantageous to pack all or a portion of the interior and / or periphery of the implant with a suitable osteogenetic material or therapeutic composition . osteogenic materials include , without limitation , autograft , allograft , xenograft , demineralized bone , synthetic and natural bone graft substitutes , such as bioceramics and polymers , and osteoinductive factors . a separate carrier to hold materials within the device can also be used . these carriers can include collagen - based carriers , bioceramic materials , such as bioglass ®, hydroxyapatite and calcium phosphate compositions . the carrier material may be provided in the form of a sponge , a block , folded sheet , putty , paste , graft material or other suitable form . the osteogenetic compositions may include an effective amount of a bone morphogenetic protein , transforming growth factor β1 , insulin - like growth factor 1 , platelet - derived growth factor , fibroblast growth factor , lim mineralization protein ( lmp ), and combinations thereof or other therapeutic or infection resistant agents , separately or held within a suitable carrier material . a technique of an embodiment of the invention is to first pack the interior of an unexpanded implant with material and then place one or both end members if desired . upon expanding the device to an expanded state such as is shown in fig2 b , material may additionally be placed through the opening 125 . placement may be accomplished directly or with the aid of an injection or transfer device of any effective type . access to the surgical site may be through any surgical approach that will allow adequate visualization and / or manipulation of the skeletal structures . example surgical approaches include , but are not limited to , any one or combination of anterior , antero - lateral , posterior , postero - lateral , transforaminal , and / or far lateral approaches . implant insertion can occur through a single pathway or through multiple pathways , or through multiple pathways to multiple levels of the spinal column . minimally invasive techniques employing instruments and implants are also contemplated . it is understood that all spatial references , such as “ top ,” “ inner ,” “ outer ,” “ bottom ,” “ left ,” “ right ,” “ anterior ,” “ posterior ,” “ superior ,” “ inferior ,” “ medial ,” “ lateral ,” “ upper ,” and “ lower ” are for illustrative purposes only and can be varied within the scope of the disclosure . fig1 illustrates four vertebrae , v 1 - v 4 , of a typical lumbar spine and three spinal discs , d 1 - d 3 . while embodiments of the invention may be applied to the lumbar spinal region , embodiments may also be applied to the cervical or thoracic spine or between other skeletal structures . while embodiments of the invention have been illustrated and described in detail in the disclosure , the disclosure is to be considered as illustrative and not restrictive in character . all changes and modifications that come within the spirit of the invention are to be considered within the scope of the disclosure .