Abstract:
Embodiments of the invention include expandable, implantable devices and methods. Devices expand linearly to provide secure fixation between or among anatomical structures. The expanded height of some embodiments is greater that twice the unexpanded height of the device. In some embodiments, an implant replaces one or more vertebral bodies of the spine.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to the field of replacing portions of the human structural anatomy with medical implants, and more particularly relates to an expandable implant and method for replacing skeletal structures such as one or more vertebrae or long bones or portions thereof. 
       BACKGROUND 
       [0002]    It is sometimes necessary to remove one or more vertebrae, or a portion of the vertebrae, from the human spine in response to various pathologies. For example, one or more of the vertebrae may become damaged as a result of tumor growth, or may become damaged by a traumatic or other event. Removal, or excision, of a vertebra may be referred to as a vertebrectomy. Excision of a generally anterior portion, or vertebral body, of the vertebra may be referred to as a corpectomy. An implant is usually placed between the remaining vertebrae to provide structural support for the spine as a part of a corpectomy.  FIG. 1  illustrates four vertebrae, V 1 -V 4  of a typical lumbar spine and three spinal discs, D 1 -D 3 . As illustrated, V 3  is a damaged vertebra and all or a part of V 3  could be removed to help stabilize the spine. If removed along with spinal discs D 2  and D 3 , an implant may be placed between vertebrae V 2  and V 4 . Most commonly, the implant inserted between the vertebrae is designed to facilitate fusion between remaining vertebrae. Sometimes the implant is designed to replace the function of the excised vertebra and discs. All or part of more than one vertebrae may be damaged and require removal and replacement in some circumstances. It may also be clinically appropriate in some circumstances to remove only one or a part of one of the discs, D 1 -D 3  for example, and replace the disc or a portion of the disc with an expandable implant in a fusion procedure. 
         [0003]    Many implants are known in the art for use in a corpectomy or fusion procedure. One class of implants is sized to directly replace the vertebra or vertebrae that are being replaced. Another class of implants is inserted into the human body in a collapsed state and then expanded once properly positioned. Expandable implants may assist with restoring proper loading to the anatomy and achieving more secure fixation of the implant. Expandable implants may be advantageous because they allow for a smaller incision when properly positioning an implant. Implants that expand from a relatively small height to a relatively tall height while providing good structural strength may be more particularly advantageous to minimize incision size. Implants that included insertion and expansion mechanisms that are narrowly configured may also provide clinical advantages. Effective implants should also include a mechanism for securely locking in desired positions, and in some situations, be capable of collapsing. Implants with openings at or near their ends may also be advantageous in some embodiments because they allow for vascularization and bone growth into or through the implant. 
         [0004]    Expandable implants may also be useful in replacing long bones or portions of appendages such as the legs and arms, or a rib or other bone that is generally longer than it is wide. Examples include, but are not limited to, a femur, tibia, fibula, humerus, radius, ulna, phalanges, clavicle, and any of the ribs. 
       SUMMARY 
       [0005]    One embodiment of the invention is an expandable medical implant for supporting skeletal structures. The expandable medical implant may include a base with a first end, a second end, and a cannula extending between the first and second ends along a length of the base. The expandable medical implant may also include a first post having a proximal end that travels within the cannula and a distal end that extends beyond the first end of the base, and a second post having a proximal end that travels within the cannula and a distal end that extends beyond the second end of the base. The proximal end of the first post of some embodiments is configured to interdigitate with the proximal end of the second post within the cannula along at least a portion of the length of the base. 
         [0006]    An embodiment of the invention is a method of spacing apart vertebral bodies. The method embodiment includes providing an expandable medical implant. The expandable medical implant may have a base with a first end, a second end, and a cannula extending between the first and second ends along a length of the base. The provided expandable medical implant may also include a first post having a proximal end that travels within the cannula and a distal end that extends beyond the first end of the base, and a second post having a proximal end that travels within the cannula and a distal end that extends beyond the second end of the base. The provided expandable medical implant is capable of having a first height between the distal end of the first post and the distal end of the second post, and is capable of expanding to a second height between the distal end of the first post and the distal end of the second post that is greater than the first height. The method includes expanding the first post relative to the base and the second post relative to the base by translating the first and second posts in opposite directions in the cannula so that the medical implant has a second height greater than two times the first height. 
         [0007]    Another embodiment of the invention is an expandable medical implant means for spacing apart vertebral structures. The embodiment includes a base having a first end, a second end, and a cannula extending between the first and second ends along a length of the base. The embodiment also includes a first post having a proximal end that travels within the cannula and a distal end that extends beyond the first end of the base, and a second post having a proximal end that travels within the cannula and a distal end that extends beyond the second end of the base. The embodiment additionally includes an expansion means for extending the first post relative to the base and the second post relative to the base in opposite directions in the cannula so that a second height defined by a distance between the distal end of the first post and the distal end of the second post is greater than two times a first height defined by a distance between the distal end of the first post and the distal end of the second post. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an elevation view of a segment of a lumbar spine. 
           [0009]      FIG. 2  is a perspective view of an expandable medical implant in a partially expanded state. 
           [0010]      FIG. 3  is a cross-sectional view of the expandable medical implant of  FIG. 2 . 
           [0011]      FIG. 4  is a perspective view of the expandable medical implant of  FIG. 2  with some components removed for illustrative purposes. 
           [0012]      FIG. 5  is a cross-sectional view of the expandable medical implant of  FIG. 4 . 
           [0013]      FIG. 6  is a perspective view of the expandable medical implant of  FIG. 2  with some components removed for illustrative purposes. 
           [0014]      FIG. 7  is cross-sectional view of a perspective view of the expandable medical implant of  FIG. 2  with some components removed for illustrative purposes. 
           [0015]      FIG. 8  is a perspective view of an embodiment of a collar of some embodiments of the invention. 
           [0016]      FIG. 9  is a perspective view of an embodiment of posts of some embodiments of the invention. 
           [0017]      FIG. 10  is a cross-sectional view of a perspective view of the expandable medical implant of  FIG. 2  with some components removed for illustrative purposes. 
           [0018]      FIG. 11  is a perspective view of an embodiment of a collar of some embodiments of the invention. 
           [0019]      FIG. 12  is a plan view of the collar of  FIG. 11 . 
           [0020]      FIG. 13  is an elevation view of an instrument embodiment. 
           [0021]      FIG. 14  is an elevation view of an end of the instrument of  FIG. 13 . 
           [0022]      FIG. 15  is an elevation view of an instrument embodiment. 
           [0023]      FIG. 16  is an elevation view of an end of the instrument of  FIG. 15  with an end portion removed for illustrative purposes. 
           [0024]      FIG. 17  is a perspective view on an end of the instrument of  FIG. 15 . 
           [0025]      FIG. 18  is a perspective view of an expandable medical implant in a partially expanded state. 
           [0026]      FIG. 19  is a perspective view of the expandable medical implant of  FIG. 18  with some components removed for illustrative purposes. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    An expandable medical implant for supporting skeletal structures is illustrated in different views and with certain variations in  FIGS. 2-12 ,  18  and  19 . The expandable medical implant  1  shown in  FIG. 2  includes a base  10 , a first post  100  and a second post  200 . The base  10  is illustrated with a core  20 , a first collar  21 , and a second collar  22 . The base  10  has a first end  11  and a second end  12 . A cannula  13  extends between the first end  11  of the base  10  and second end  12  of the base  10  along a length of the base  10 . 
         [0028]    The core  20  illustrated in  FIGS. 2-7  is a cylindrical body. A core of other embodiments could have a cross-sectional shape other than round. For example, the core could be oval, triangular, rectangular, square, an other polygonal or curved shape, or a combination of the shapes noted or other functional shapes. As seen in  FIGS. 3-7 , the core  20  of the base  10  includes a separator  15  dividing a cross-section of the base  10  such that one or more portions of the first post  100  are divided from one or more portions of the second post  200 . In the illustrated embodiment, the separator  15  divides the cross-section of the base  10  into substantially quarter sections. Division into quarter sections may be advantageous in some embodiments because it allows for each of the first post  100  and the second post  200  to have a pair of symmetrical legs occupying opposite quarters of the base. The symmetrical legs are well-positioned to receive loads applied to the posts  100 ,  200  without particular eccentricities being induced in the materials of the posts  100 ,  200 . Additionally, the configuration provides relatively large amounts of material in each leg that are near the periphery of each of the posts  100 ,  200 . Materials near the peripheries contribute to the ability of the posts  100 ,  200  to resist loading as columns. In other embodiments, a cross-section of the base  10  may be divided into only two sections so that each post may include a maximum amount of material in a single element. Such an arrangement would minimize the amount of cross-sectional area occupied by a separator and therefore would allow for each post to be as large as possible in cross-section. In still other embodiments, the number of legs of each post may be increase to better distribute the load carried by the posts around a periphery of the base  10 , and as noted above, reduce eccentricities, and therefore loads, in the materials of the posts. In addition to approximate wedge sections of a circle as depicted in  FIGS. 3 and 5 , legs of the posts may be of any functional shape, and the separator  15  may be configured to intervene between all or part of the posts. For example, and without limitation, legs of the posts may be formed with a tongue on one leg or post and a groove on the other leg or post, or the posts may be formed with male and female dovetail connections along their lengths that allow the posts to interconnect. 
         [0029]    In some embodiments, the separator may not extend fully across an interior of the base  10 , but may merely be a portion of the separator  15  closest to a wall of the base  10 . These embodiments may include components that fit between portions of two or more posts, or the components may fit in one or more notches along a side of one or more posts to guide the posts relative to the base. 
         [0030]    In some embodiments, the separator  15  is in close tolerance with one or both of the posts  100 ,  200  to stabilize the posts  100 ,  200  relative to the base  10 . A tolerance “T” is noted in  FIGS. 3 and 5 . This close tolerance may provide one or both of guidance in dynamic operation of the posts  100 ,  200  and lateral or other structural stability to the posts  100 ,  200  in the cannula  13  of the base  10 . For example, and without limitation, the separator  15  may be sized to provide gaps or tolerances from 0.02 mm and 1 mm between the posts  100 ,  200  and the separator  15 . Tolerances may be specified between the posts  100 ,  200  and the separator  15 , between portions of the posts  100 ,  200 , or between the posts  100 ,  200  and other components of the device in various embodiments. 
         [0031]    The base 10  shown in  FIGS. 2 ,  4 ,  6 , and  7  has an opening  17  for receiving an actuating instrument. In some embodiments, the opening  17  includes threads for receiving threads from a portion of the actuating instrument. As illustrated, the opening  17  is a single hole. A single hole opening would function, with the actuating instrument of  FIGS. 13 and 14 . However, in other embodiments, the opening  17  may include two or more holes. For example, an opening for receiving the actuating instrument depicted in  FIGS. 15-17  includes a set of three holes. The base  10  may include holes in one or more sides of the base  10 . As shown in  FIG. 7 , the opening  17  passes through the center of the base  10  to provide the opening  17  in opposite sides of the base  10 . In other embodiments, the base  10  may include a protrusion rather than a hole. An instrument may fit with the protrusion to align an actuating instrument on the expandable medical implant  1 . 
         [0032]    In some embodiments, the base  10  includes the first collar  21  and the second collar  22 . The first collar  21  and the second collar  22  in the illustrated embodiments are configured to regulate motion of the first post  100  and the second post  200  respectively along the length of the base  10 . The first collar  21  of the illustrated embodiment is coupled to the core  20  of the base  10  by pins (not shown) that are fixed in holes  23  ( FIGS. 2 and 10 ) and pass through a groove  27  ( FIGS. 4 ,  6 , and  7 ). The first collar  21  may be coupled to the core  20  by any effective device, including but not limited to, through retaining rings, set screws, or staking in the groove  27 . The first collar  21  is allowed to rotate about the core  20  as the pins travel in the groove  27 . The first collar  21  illustrated includes teeth  25  around its perimeter that are configured to receive an instrument for rotating the first collar  21  relative to a portion of the base  10 , such as the core  20 . For example and without limitation, the teeth  25  may receive one of the actuating instruments shown in  FIGS. 13-17 . 
         [0033]    The second collar  22  of the illustrated embodiment is coupled to the core  20  of the base  10  by pins (not shown) that are fixed in holes  24  ( FIGS. 2 ,  8 , and  10 ) and pass through a groove  28  ( FIGS. 4 ,  6 , and  7 ). The second collar  22  is allowed to rotate about the core  20  as the pins travel in the groove  28 . The second collar  22  may be coupled to the core  20  by any effective device, including but not limited to, through retaining rings, set screws, or staking in the groove  28 . The second collar  22  illustrated includes teeth  26  around its perimeter that are configured to receive an instrument for rotating the second collar  22  relative to a portion of the base  10 , such as the core  20 . For example and without limitation, the teeth  26  may receive one of the actuating instruments shown in  FIGS. 13-17 . 
         [0034]    For the illustrated embodiments, the first collar  21  and the second collar  22  may be turned simultaneously with an instrument. For example, the instrument of  FIGS. 13 and 14  or the instrument of  FIGS. 15-17  may be connected to the core  20  to simultaneously turn the first collar  21  and the second collar  22 . In the illustrated configuration, the first collar  21  and the second collar  22  would be turned in opposite directions by either of the instruments of  FIGS. 13-17 . However, because the first collar  21  and the second collar  22  have identical thread patterns, but are turned proximal end to proximal end, rotating them in opposite directions results in the first post  100  and the second post  200  moving together simultaneously and moving apart simultaneously. Any other operable combination of thread patterns and gears is also contemplated under embodiments of the invention. For example and without limitation, an actuating instrument may only have two intermeshing gears so that collars above and below would be moved in the same, rather than opposite, rotational directions. This configuration could be operable in conjunction with collars that have opposite thread patterns, for example, one with right-hand threads and one with left-hand threads. 
         [0035]    Embodiments of the first post  100  and the second post  200  are illustrated in  FIGS. 2 ,  4 ,  9 , and  10 . The first post  100  has a proximal end  101  and a distal end  102 . The proximal end  101  travels within the cannula  13 , and the distal end  102  extends beyond the first end  11  of the base  10 . As illustrated, the first collar  21  is coupled to the first post  100  by threads  121  ( FIGS. 7 and 10 ) such that motion of the first post  100  along the length of the base  10  is induced by rotation of the first collar  21 . 
         [0036]    The second post  200  has a proximal end  201  and a distal end  202 . The proximal end  201  travels within the cannula  13 , and the distal end  202  extends beyond the second end  12  of the base  10 . As illustrated, the second collar  22  is coupled to the second post  200  by threads  122  ( FIGS. 7 ,  8 , and  10 ) such that motion of the second post  200  along the length of the base  10  is induced by rotation of the second collar  22 . 
         [0037]    The separator  15  of some embodiments also contributes toward preventing the threads  121 ,  122  of the collars from disengaging with the respective posts  100 ,  200 . Turning forces that create work along the threads may generate resulting forces that tend to push the posts  100 ,  200  toward the center of the base  10  as forces are applied to the threads. The separator  15  may contact generally opposite sides of the respective posts  100 ,  200  to counteract the resulting forces and allow the threads  121 ,  122  of the collars to remain engaged with the posts  100 ,  200 . 
         [0038]      FIGS. 9 and 10  illustrate the proximal end  101  of the first post  100  interdigitated with the proximal end  201  of the second post  200 . The proximal end  101  of the first post  100  interdigitates with the proximal end  201  of the second post  200  within the cannula  13  along at least a portion of the length of the base  10  in some embodiments. The term interdigitate and variations of the term used herein refer to components that mesh together, intermingle, or overlap along their lengths. Components are considered to be configured to interdigitate when the components have a shape that will allow portions of the components to pass by one another and occupy a common cross-sectional plane along a length of the components. As applied to the embodiment illustrated in  FIGS. 9 and 10 , the first post  100  is configured to interdigitate with the second post  200  because legs  111 ,  112  of the first post  100  are shaped to fit in a common cross-sectional plane with legs  211 ,  212  of the second post  200  along a length of the first and second posts  100 ,  200 . More particularly for the illustrated embodiment, the first post  100  and the second post  200  each include two opposing, interdigitating legs  111 ,  112 , and  211 ,  212 . Each illustrated leg  111 ,  112 ,  211 ,  212  is located substantially at a quarter point of a cross-section of the base  10 . In other embodiments, there may only be two legs such that each post  100 ,  200  includes a maximum amount of material to the exclusion of separator components, as noted above. Some embodiments of the posts may include more than two pairs of interdigitating legs that distribute the load carried by the posts around a periphery of the base to reduce loading eccentricities, and therefore stresses in the materials of the posts. 
         [0039]    One or both distal ends  102 ,  202  of the posts  100 ,  200  may include end pieces or shoes that connect with respective adjacent vertebral bodies. The shoes may include features that add an angled configuration to the distal ends  102 ,  202  so that an implant will match or alter a lordotic, kyphotic spinal curvature or a curvature resulting from scoliosis. Alternatively or in addition, the shoes may include features that enhance connection to a vertebra. For example and without limitation, the shoes may include teeth, spikes, fasteners, openings, knurling, roughened surfaces, or any surface treatments or additional materials that are effective to enhance connection to a vertebra. 
         [0040]    Another embodiment of a collar is illustrated in  FIGS. 11 and 12 . A collar  222  is shown that may be substituted on either or both ends of the core  20 . The collar  222  may by coupled to the core  20  of the base  10  by pins (not shown) that are fixed in holes  224  and pass through either of the grooves  27 ,  28  ( FIGS. 4 ,  6 , and  7 ). The collar  222  is allowed to rotate about the core  20  as the pins travel in the grooves  27 ,  28 . The collar  222  may be coupled to the core  20  by any effective device, including but not limited to, through retaining rings, set screws, or staking in the grooves  27 ,  28 . The collar  222  illustrated includes teeth  226  around its perimeter that are configured to receive an instrument for rotating the collar  222  relative to a portion of the base  10 . For example and without limitation, the teeth  226  may receive one of the actuating instruments shown in  FIGS. 13-17 . 
         [0041]    The first post  100  with distal end  102  and proximal end  101  that travels within the cannula  13  may be configured to couple with the collar  222  through engagement mechanisms  250 . The engagement mechanisms  250  are disposed around one or more segments of an interior diameter of the collar  222 . When the collar  222  is in a first rotational position where the engagement mechanisms  250  do not interact with threads on the first post  100 , movement of the first post  100  along the base  10  is allowed. When the collar  222  is in a second rotational position where the engagement mechanisms  250  couple with threads on the first post  100 , movement of the first post  100  along the base  10  is restricted. For the illustrated collar  222 , the rotational movement between the first rotational position and the second rotational position is approximately 90 degrees. The collar  222  and engagement mechanism  250  may be similarly employed at the second end  12  of the base  10  in combination with the second post  200 . In other embodiments where one or both of the threads of the post or the segments of engagement mechanisms  250  are sized differently, the rotational movement between the first and second rotational positions would be changed proportionally. The engagement mechanisms  250  of the illustrated embodiment are threads that engage with threads of the respective posts. In other embodiments, expansion and collapse of the medical implant may be regulated by ratchet teeth or any other functional mechanism. Embodiments of the collar  222  with appropriate engagement mechanisms  250  would be equally effective with these other mechanisms and are contemplated under embodiments of the invention. 
         [0042]    Another embodiment of the expandable medical implant is illustrated in  FIGS. 18 and 19 . The expandable medical implant  1001  shown includes a base  1010 , a first post  1100  and a second post  1200 . The base  1010  is illustrated with a core  1020 , a first collar  1021 , and a second collar  1022 . The base  1010  has a first end  1011  and a second end  1012 . The illustrated core  1020  is generally cylindrical and includes openings for one or both of the first and second posts  1100 ,  1200  to pass through the core  1020 . A cannula  1013  extends between the first end  1011  of the base  1010  and second end  1012  of the base  1010  along a length of the base  1010 . In  FIG. 18 , the cannula  1013  is occupied by the first and second posts  1100 ,  1200 . The base  1010  shown has an opening  1017  for receiving an actuating instrument. Configurations and functions of the opening  1017  and related structures are essentially similar to those describe in association with the opening  17  above. 
         [0043]    The first collar  1021  and the second collar  1022  in the illustrated embodiment are configured to regulate motion of the first post  1100  and the second post  1200  respectively along the length of the base  1010 . The first collar  1021  is coupled to the core  1020  of the base  1010  by pins (not shown) that are fixed in holes  1023  ( FIG. 18 ) and pass through a groove (not shown) in the core  1020 . The first collar  1021  is allowed to rotate about the core  1020  as the pins travel in the groove. The first collar  1021  may be coupled to the core  1020  by any effective device, including but not limited to, through retaining rings, set screws, or staking in the groove. The first collar  1021  illustrated includes teeth  1025  around its perimeter that are configured to receive an instrument for rotating the first collar  1021  relative to a portion of the base  1010 , such as the core  1020 . For example and without limitation, the teeth  1025  may receive one of the actuating instruments shown in  FIGS. 13-17 . 
         [0044]    The second collar  1022  is coupled to the core  1020  of the base  1010  by pins (not shown) that are fixed in holes  1024  ( FIGS. 18 and 19 ) and pass through a groove (not shown) in the core  1020 . The second collar  1022  is allowed to rotate about the core  1020  as the pins travel in the groove. The second collar  1022  may be coupled to the core  1020  by any effective device, including but not limited to, through retaining rings, set screws, or staking in the groove. The second collar  1022  illustrated includes teeth  1026  around its perimeter that are configured to receive an instrument for rotating the second collar  1022  relative to a portion of the base  1010 , such as the core  1020 . For example and without limitation, the teeth  1026  may receive one of the actuating instruments shown in  FIGS. 13-17 . Operation of the first and second collars  1021  and  1022  with actuating instruments is essentially similar to the operation of the first and second collars  21 ,  22  described above. 
         [0045]    Embodiments of the first post  1100  and the second post  1200  are illustrated in  FIGS. 18 and 19 . The first post  1100  has a proximal end  1101  and a distal end  1102 . The proximal end  1101  travels within the cannula  1013 , and the distal end  1102  extends beyond the first end  1011  of the base  1010 . As illustrated, the first collar  1021  is coupled to the first post  1100  by threads such that motion of the first post  1100  along the length of the base  1010  is induced by rotation of the first collar  1021 . 
         [0046]    The second post  1200  has a proximal end  1201  and a distal end  1202 . The proximal end  1201  travels within the cannula  1013 , and the distal end  1202  extends beyond the second end  1012  of the base  1010 . As illustrated, the second collar  1022  is coupled to the second post  1200  by threads such that motion of the second post  1200  along the length of the base  1010  is induced by rotation of the second collar  1022 . Because the second post  1200  has a smaller diameter than the first post  1100 , a threaded opening  1922  ( FIG. 19 ) in the second collar  1022  is smaller than a threaded opening  1911  ( FIG. 18 ) in the first collar  1021 . The pitch of the threads of the threaded openings  1911  and  1922  may be the same pitch, or a different pitch. Different pitches may be useful in some embodiments to cause different rates of travel for the first and second posts  1100 ,  1200  in response to a common actuation. Different rates of travel may also permit greater overall expansion capacity by making a second post longer than a first post, since the longer second post would be able to nest through the center of the first post and extend all the way to a distal end of the first post. The longer second post may be expanded at a faster rate over its entire length and thereby give the implant a greater overall expansion distance. 
         [0047]    As shown in  FIG. 19 , the proximal end  1101  of the first post  1100  is interdigitated with the proximal end  1201  of the second post  1200 . As applied to the embodiment illustrated in  FIGS. 18 and 19 , the first post  1100  is configured to interdigitate with the second post  1200  by receiving the second post  1200  within the inner diameter of the first post  1100 . The first and second posts  1100 ,  1200  of the illustrated embodiment are tubular, round in cross-section, and threaded. However, in other embodiments, the posts may be oval, triangular, rectangular, square, an other polygonal or curved shape, or a combination of the shapes noted or other functional shapes. Various embodiments may or may not have threads, and may alternatively or in addition have ratchetings, notches, or other surfaces that may be engaged or gripped to move or hold the first and second posts. 
         [0048]    A central bore  1190  is shown in  FIGS. 18 and 19 . The central bore  1190  may extend completely or partially through one or both of the first and second posts  1100 ,  1200 . The central bore  1190  may be filled in whole or in part with bone growth promoting substances, such as the substances detailed below. Bone growth promoting substance may be added to the central bore  1190  before implantation, during the expansion process of the implant, after the implant is implanted and fully expanded, or at any combination of the listed times. 
         [0049]    Expandable medical implants under some embodiments are means for spacing apart vertebral structures. An expandable medical implant may include a base having a first end, a second end, and a cannula extending between the first and second ends along a length of the base; a first post having a proximal end that travels within the cannula and a distal end that extends beyond the first end of the base; and a second post having a proximal end that travels within the cannula and a distal end that extends beyond the second end of the base. Embodiments of the expandable medical implant may further include an expansion means for extending the first post relative to the base and the second post relative to the base in opposite directions in the cannula so that a second height defined by a distance between the distal end of the first post and the distal end of the second post is greater than two times a first height defined by a distance between the distal end of the first post and the distal end of the second post. In the illustrated embodiments, the first post  100  is a single component and the second post  200  is a single component. Consequently, in order for the expanded second height between the distal ends  102 ,  202  to be greater than two times the first height, the first post  100  and the second post  200  interdigitate within the base  10 . In some embodiments, such as but not limited to the embodiment illustrated in  FIGS. 18 and 19 , the first and second posts may interdigitate as concentric cylindrical cross-sections that extend from opposite ends of a base. 
         [0050]    Actuating instruments  300 ,  400  are illustrated in  FIGS. 13-17 . The actuating instruments  300 ,  400  are configured to operate with one or more collars  21 ,  22 ,  222 ,  1021 ,  1022  of some embodiments to regulate motion of the first post  100 ,  1100  and the second post  200 ,  1200  respectively along the length of the base  10 . As shown in  FIGS. 13 and 14 , embodiments of the actuating instrument  300  include a single gear  301  with gear teeth  310  configured to engage with teeth  25 ,  26 ,  226 ,  1025 ,  1026  of one or more of the collars  21 ,  22 ,  222 ,  1021 ,  1022 . The actuating instrument  300  includes a tube  320  in which a shaft  330  is rotatably coupled. A gripping surface  325  may be applied to an exterior portion of the tube  320 . A handle  335  may be included on a proximal end of the shaft  330  for grasping to turn the shaft  330  relative to the tube  320 . The shaft  330  is coupled to the gear  301 . A tab  317  may be rotatably coupled in the opening  17  to secure the actuating instrument  300  to the base  10  while allowing the gear  301  to turn relative to the base  10 . The tab  317  of some embodiments may include a threaded portion to connect to a threaded area of the opening  17 , where the threaded portion rotates relative to the shaft  330 . In other embodiments, the tab  317  may include devices to expand or otherwise change shape to rotatably or fixedly connect with the opening  17  or other portion of the base  10 . As shown by arrow  305  in  FIG. 14 , rotation of the gear  301  results in motion relative to the gear  301  at the top of the gear in a first direction and motion at the bottom of the gear in a second direction opposite from the first direction. This configuration may be useful with a device such as expandable medical implant  1  where this action results in counter-rotation of first collar  21  and second collar  22  and simultaneous movement of posts  100 ,  200  toward a collapsed or expanded state. The diameter of the gear  301  may be altered in various embodiments to fit with implants with different distances G between gears ( FIG. 2 ). 
         [0051]    As shown in  FIGS. 15-17 , embodiments of the actuating instrument  400  include three gears  401 ,  402 ,  403  with gear teeth  411 ,  412 ,  413 . The gear teeth  412 ,  413  are configured to engage with teeth  25 ,  26 ,  226 ,  1025 ,  1026  of one or more of the collars  21 ,  22 ,  222 ,  1021 ,  1022 . The actuating instrument  400  includes a tube  420  in which a shaft  430  is rotatably coupled. A gripping surface  425  may be applied to an exterior portion of the tube  420 . A housing  450  is coupled to the tube  420  in the embodiment shown. A tab  417  is coupled to the housing  450 . A handle  435  may be included on a proximal end of the shaft  430  for grasping to turn the shaft  430  relative to the tube  420 . The shaft  430  is coupled to the gear  401 . The tab  417  has three prongs in the illustrated embodiment and may be inserted in an opening in a base similar to the base  10  to secure the actuating instrument  400  to the base while allowing the gear  401  to turn relative to the base. The opening in which the actuating instrument  400  is inserted in the illustrated embodiment is similar to some embodiments of the opening  17  but includes holes for at least three prongs. One or more of the prongs of the tab  417  of some embodiments may include a threaded portion to connect to a threaded area of an opening. The tab  417  may include devices to expand, change orientation, or otherwise change shape to connect with a base or another portion of an implant. In  FIG. 16 , an end portion of the housing  450  is shown removed for illustrative purposes. As depicted by arrow  405  in  FIG. 16 , rotation of the gear  401  results in motion relative to the gear  401  at the top of the gear  401  in a first direction and motion at the bottom of the gear  401  in a second direction opposite from the first direction. The gear  401  meshes with and turns a gear  402  at the top of the gear  401  in a rotational direction depicted by arrow  406 . The gear  401  meshes with and turns a gear  403  at the bottom of the gear  401  in a rotational direction depicted by arrow  407 . This configuration may be useful with a device such as expandable medical implant  1  where this action results in counter-rotation of first collar  21  and second collar  22  and simultaneous movement of posts  100 ,  200  toward a collapsed or expanded state. The diameter of the gears  401 ,  402 ,  403  may be altered in various embodiments to fit with implants with different distances G between gears ( FIG. 2 ) or to generate different amounts of mechanical advantage or relative rate of turning. For example and without limitation, the collars  21 ,  22 ,  222 ,  1021 ,  1022  may be more narrow top to bottom in some embodiments and create a greater distance G between gears of the collars. Rotation of the handle  435  of the actuating instrument  400  results in an opposite rotation of collars  21 ,  22 ,  222 ,  1021 ,  1022  compared with a like rotation of the handle  335  of the actuating instrument  300 . In still another embodiment, an actuating instrument (not shown) may include two gears so that rotation of the upper and lower parts of the instrument would be in a common direction. Such a rotation may be useful where upper and lower collars of an implant have opposite thread directions or for various other implant embodiments. 
         [0052]    An embodiment of the invention is a method of spacing apart vertebral bodies. The method may include providing an expandable medical implant and expanding the implant so that the medical implant has an expanded height greater than two times the height of the implant or a component of the implant in an unexpanded state. For example expandable medical implants of the method may be capable of having a first height between a distal end of a first post and a distal end of a second post, and may be capable of expanding to a second height between the distal end of the first post and the distal end of the second post that is greater than the first height. The method may also include expanding the first post relative to the base and the second post relative to the base by translating the first and second posts in opposite directions in the cannula so that the second height is greater than two times the first height. 
         [0053]    Referring to a non-limiting example, the expandable medical implant  1  has a base  10  with a first end  11 , a second end  12 , and a cannula  13  extending between the first end  11  and the second end  12  along a length of the base  10 . The illustrated first post  100  has a proximal end  101  that travels within the cannula  13 , and a distal end  102  that extends beyond the first end  11  of the base  10 . The illustrated second post  200  has a proximal end  201  that travels within the cannula  13 , and a distal end  202  that extends beyond the second end  12  of the base  10 . In the example, a first height is defined between the distal end  102  of the first post  100  and the distal end  202  of the second post  200 . The expandable medical implant  1  may be capable of expanding to a second height between the distal end  102  of the first post  100  and the distal end  202  of the second post  200  that is greater than the first height. The first post  100  of the illustrated embodiment is a unitary piece that does not telescope, fold out, or expand in any way with additional components beyond its distal end  102 . Likewise, the second post  200  of the illustrated embodiment is a unitary piece that does not telescope, fold out, or expand in any way with additional components beyond its distal end  202 . The diameters or lateral periphery of the proximal ends  101 ,  201  of the posts  100 ,  200  of the illustrated embodiment are approximately equal, thus giving the posts  100 ,  200  approximately the same structural characteristics and load capacity. 
         [0054]    Continuing with the present example, expanding the medical implant  1  includes expanding the first post  100  relative to the base  10  and the second post  200  relative to the base  10 . The expansion of the embodiment shown includes translating the first and second posts  100 ,  200  in opposite directions in the cannula  13  so that the medical implant  1  has a second height that is greater than two times the first height. In some embodiments, the expansion distance may also be defined as expanding to a total height greater than two times the height of the base  10 . For the device shown, the first post  100  and the second post  200  are moved relative to the base  10  by rotating the collars  21 ,  22  that are engaged with threads on the posts  100 ,  200 . In particular, the collars  21 ,  22  are rotated by inserting a tip of an instrument, such as either of the tabs  317 ,  417  of the activation instruments  300 ,  400 , into the base  10  and rotating the respective handles  335 ,  435  of the instruments to turn one or more gears that mesh with the teeth  25 ,  26  of the collars  21 ,  22 . In other embodiments, either of the collars  21 ,  22  may be independently turned to selectively control expansion from either end of the base  10 . 
         [0055]    In an alternate embodiment, an expandable medical implant is expanded by attaching an expansion instrument to a distal end of a first post and a distal end of a second post. The expansion instrument may be of any variety capable of moving the distal ends away from one another. For example and without limitation, expansion instruments with mechanical linkages that react to spreading or compressing proximal components, screw driven devices, ratcheting devices, hydraulic, electrical, or other powered devices may be used to move the distal ends of the posts apart. Method embodiments include operating such an instrument to expand the expandable medical implant. Once expanded, the expandable medical implant may be locked into place by any effective mechanism. Non-limiting examples include locking with a pin, a set screw or other fastener, a locking ring, a clamp, an interference fit, and a collar. A more specific example of a collar is embodied in the collar  222 . Method embodiments may further include positioning the collar  222  is in a first rotational position where the engagement mechanisms  250  do not interact with threads on the posts  100 ,  200  while the posts  100 ,  200  are translated relative to the base  10 . When the posts  100 ,  200  are moved to an acceptable position, the collar  222  may be moved to a second rotational position where the engagement mechanisms  250  couple with threads on the respective posts  100 ,  200 , thus restricting movement of the posts  100 ,  200  along the base  10 . As illustrated, the rotational movement between the first rotational position and the second rotational position is approximately 90 degrees. In other embodiments where one or both of the threads of the posts  100 ,  200  or the segments of engagement mechanisms  250  are sized differently, the rotational movement between the first and second rotational positions would be changed proportionally. 
         [0056]    Some embodiments may also include supplemental fixation devices as part of the expandable medical implant for further stabilizing the anatomy. For example, and without limitation, rod and screw fixation systems, anterior or lateral plating systems, facet stabilization systems, spinal process stabilization systems, and any devices that supplement stabilization may be used as a part of the expandable medical implant. 
         [0057]    In some embodiments, the expandable medical implant may also include a bone growth promoting substance as part of or in combination with the expandable medical implant. All or a portion of the interior and/or periphery of the implant may be packed with a suitable bone growth promoting substance or therapeutic composition. For example, and without limitation, one or both of an end chamber  105 ,  205  ( FIG. 2 ) may be filled with a bone growth promoting substance such as an osteogenic material to promote bone growth into the respective distal ends  102 ,  202  of the expandable medical implant  1 . In other embodiments, a chamber may be created in a shoe to be placed on the end of a post, or a chamber may extend the entire length of the expandable medical implant that may be filled during or after expansion. The central bore  1190  shown in  FIGS. 18 and 19  may be filled in whole or in part with a bone growth promoting substance. Bone growth promoting substances 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 may also be used. These carriers may 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 osteogenic compositions may include an effective amount of a bone morphogenetic protein (BMP), transforming growth factor β 1 , insulin-like growth factor, 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. Placement may be accomplished directly or with the aid of an injection or transfer device of any effective type. 
         [0058]    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, reinforced polymer composites, PEEK and PEEK composites, shape-memory alloys, titanium, titanium alloys, cobalt chrome alloys, stainless steel, ceramics and combinations thereof. Reinforcing materials may include carbon, fiberglass, metal pieces, or any other effective reinforcing material. If a 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. 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. 
         [0059]      FIG. 1  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. 
         [0060]    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.