Abstract:
Embodiments of the invention include expandable, implantable devices and methods having internally contained expansion mechanisms. Devices expand linearly to provide secure fixation between or among anatomical structures. 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.  
       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. Excision of at least the 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.  
         [0003]     Many implants are known in the art for use in a corpectomy 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 body in a collapsed state and then expanded once properly positioned. Expandable implants may be advantageous because they allow for a smaller incision when properly positioning an implant. Additionally, expandable implants may assist with restoring proper loading to the anatomy and achieving more secure fixation of the implant. Implants that include insertion and expansion mechanisms that are narrowly configured may also provide clinical advantages. In some circumstances, it is desirable to have vertebral endplate contacting surfaces that effectively spread loading across the vertebral endplates. Effective implants should also include a mechanism for maintaining the desired positions, and in some situations, being capable of collapsing. Fusion implants with an opening may also be advantageous because they allow for vascularization and bone growth through all or a portion of 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 including a body and a sprocket disposed within the body having gears configured to receive a turning mechanism to rotate the sprocket, the sprocket including a first end with a threaded portion and a second end with a threaded portion. The embodiment may also include a first end component sized to engage with the threaded portion of the sprocket first end and a second end component sized to engage with the threaded portion of the sprocket second end.  
         [0006]     A further embodiment of the invention is an expandable medical implant for supporting skeletal structures including a body and an expansion means disposed within the body for converting rotational movement substantially transverse to the longitudinal axis into linear expansion of the implant along the axis. The embodiment may also include a first end component sized to engage with the expansion means and be moved away from the body by operation of the expansion means and a second end component sized to engage with the expansion means and be moved away from the body by operation of the expansion means. The expansion means is captured within the body for protection of tissue adjacent to the skeletal structures.  
         [0007]     Another embodiment of the invention is a method of placing an expandable medical implant within a spinal column. The method embodiment may include making an incision adjacent to a vertebral body and removing at least a portion of the vertebral body. Included in the embodiment are the expandable medical implant with a body, an expansion means disposed within the body for converting rotational movement into linear expansion of the implant, a first end component sized to engage with the expansion means and be moved away from the body by operation of the expansion means, and a second end component sized to engage with the expansion means and be moved away from the body by operation of the expansion means; and a surgical instrument comprising a cannula and an inner shaft disposed at least in part within the cannula, the inner shaft configured to couple with the expansion means. The method may also include releasably attaching the cannula to the body, inserting the expandable medical implant at least in part into a volume left open after removal of the portion of the vertebral body, rotating the inner shaft to activate the expansion means, detaching the cannula from the body, and removing the surgical instrument through the incision.  
         [0008]     Further aspects, forms, embodiments, objects, features, benefits, and advantages of the present invention shall become apparent from the detailed drawings and descriptions provided herein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is an elevation view of a segment of a lumbar spine.  
         [0010]      FIG. 2  is a perspective view of an expandable implant embodiment.  
         [0011]      FIG. 3  is a side cross-sectional view of the implant of  FIG. 2 .  
         [0012]      FIG. 4  is a partial longitudinal cross-sectional view of the implant of  FIG. 3 .  
         [0013]      FIG. 5  is a partial cut-away view of the implant of  FIG. 2 .  
         [0014]      FIG. 6  is a perspective view of an end component of the implant of  FIG. 2 .  
         [0015]      FIG. 7  is a perspective view of a sprocket of the implant of  FIG. 2 .  
         [0016]      FIG. 8  is a perspective view of a worm gear or the implant of  FIG. 2 .  
         [0017]      FIG. 9  is a partial longitudinal cross-sectional view of a further embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0018]     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.  
         [0019]     Referring now to  FIG. 2 , there is shown an expandable implant  100  according to one aspect of the present invention. Expandable implant  100  is sized and shaped for insertion into the space remaining in the vertebral column after removal of vertebra V 3  and discs D 2  and D 3  in  FIG. 1 . Expandable implant  100  includes an outer housing  110  having a number of openings  111  extending from the outer surface to an interior cavity  112 . In the illustrated embodiment best seen in  FIG. 3 , housing  110  is a substantially cylindrical tube having a relatively thin wall thickness in comparison to its diameter. The housing  110  extends along a longitudinal axis L 1  ( FIG. 3 ). Housing  110  defines an external protrusion  130  having a cylindrical aperture  132  oriented transverse to and spaced laterally from longitudinal axis L 1 . The protrusion  130  has a proximal opening  132  and an opposite distal opening spaced along axis L 2 .  
         [0020]     Referring now to  FIGS. 1-3 , and  4 , additional elements of the illustrated embodiment will be further described. Expandable implant  100  includes an upper bone engaging component  150  and a lower bone engaging component  170 . Bone engaging component  150  includes a series of projections  152  extending from the substantially planar surface  151 . The projections  152  are adapted and configured for engagement and penetration of adjacent bone structures. Bone engaging component  150  further includes a central aperture  154  and a plurality of minor apertures  156 . It will be understood that the central aperture and the minor apertures allow for the communication of bone growth promoting substances and bodily fluids to encourage and enhance bone ingrowth into expandable device  100 . As shown in  FIG. 6 , bone engaging component  150  further includes a pair of opposing channels  158  and  160  extending away from upper surface  151  and being open at the opposite end. The upper portion of the bone engaging component  150  includes an annular flange  164  defining a rim and projecting cylindrical body  162  having an external thread  163  defined on the outer surface. In one aspect, the threads  163  are a right-handed thread form. The threads  163  and cylindrical body  162  are interrupted by channels  158  and  160 . The lower bone engaging component  170  has substantially identical features in the inverse orientation to the upper bone engaging component  150  and will not be further described herein.  
         [0021]     Referring now to  FIG. 5 , there is shown an expansion assembly  200  in accordance with one aspect of the present invention. Expansion assembly  200  includes an upper sprocket assembly  210  and a lower sprocket assembly  270 . As shown in greater detail in  FIG. 7 , upper sprocket assembly  210  includes a central chamber  216  defined by an annular wall having internal threads  212  extending around the inner surface. In the illustrated embodiment, the inner surface is cylindrical and the threads are right-handed threads. Chamber  216  is further defined by a floor  214  having a plurality of holes extending there through to allow communication of bodily fluids and bone growth. The exterior of the upper sprocket assembly includes a plurality of longitudinally extending grooves  218 . A central drive shaft  220  is integrally formed in this embodiment with upper sprocket assembly  210 . A multi-toothed sprocket gear  230  is disposed on drive shaft  220 . Multi-toothed gear  230  has a plurality of longitudinally extending gear teeth oriented along the longitudinal axis L 1  of the device and having a center of rotation in substantial alignment with longitudinal axis L 1 .  
         [0022]     Referring now to  FIG. 5 , there is shown a lower sprocket assembly  270  having features substantially identical to the upper portion of upper sprocket assembly  210 . More specifically, as shown in  FIG. 3 , lower sprocket assembly  270  includes an internally left-hand threaded chamber open towards the lower portion of the device and a plurality of longitudinally extending external grooves  272  on the exterior of the device. In the illustrated embodiment, lower end of driveshaft  220  is received within a similarly sized bore on lower sprocket assembly  270 . In the illustrated embodiment, there is a tight interference fit between the driveshaft  220  and the bore of lower sprocket assembly  270 . In this manner, the upper sprocket assembly  210  is press fit to the lower sprocket assembly  270  such that they maintain their rotational relationship as gear  230  is rotated. In an alternative embodiment, the bore in lower sprocket assembly  270  includes at least one a channel or passageway and a distal end of driveshaft  220  includes at least one key or projection configured to be received within the passageway. In this manner, a greater amount of rotational force or torque may be transmitted between driveshaft  220  and lower sprocket assembly  270  without slippage there between. Although an interference fit or keyed engagement has been described and illustrated, it is contemplated that other forms may be utilized to interconnect the upper and lower sprocket assemblies. For example, in other embodiments the connection is maintained by adhesive pins, threading, and/or formation as an integral component.  
         [0023]     Associated with the expansion assembly  200  is a worm gear  250 . Worm gear  250  is best seen in  FIGS. 4 and 8 . Worm gear  250  includes a drive socket  252  on its proximal end and a bearing shoulder  254  adjacent drive socket  252 . A helical gear  256  extends between shoulder  254  and distal end  258 . In one aspect of the present embodiment, a further drive pattern (not shown) is formed adjacent distal end  258 .  
         [0024]     The expansion assembly  200  is assembled as shown in  FIG. 5 . Within the interior cavity  112 , housing  110  includes an inner annular flange  114  defining an upper bearing surface configured to engage at least a portion of upper sprocket assembly  210  and a lower bearing surface  118  configured to engage at least a portion of lower sprocket  270 . The interior  112  of housing  110  includes an open area adjacent projecting flange  114  such that gear  230  may spin freely without obstruction. Thus, sprocket  230  is completely within the perimeter of the housing  110 . The upper socket assembly  210  is positioned into the upper portion of the interior cavity  112 . In a similar manner, the lower sprocket assembly  270  is positioned in the lower portion of cavity  112 . The lower portion of drive shaft  220  is press fit into engagement with lower sprocket assembly  270  to form a unitary sprocket assembly completely within the outer perimeter of the housing  110 .  
         [0025]     Worm gear  250  is positioned within aperture  132  as shown in  FIGS. 3 and 4 . An annular groove  134  is defined in aperture  132  and is adapted to receive bearing shoulder  254  of worm gear  250 . The helical threads  256  are positioned within cylindrical chamber  136  and the distal end  258  is positioned adjacent distal opening  138 . The worm gear  250  extends along drive axis L 2  as shown in  FIG. 4 . It will be appreciated that in the illustrated embodiment, drive axis L 2  is substantially perpendicular to extension axis L 1 . Moreover, extension axis L 1  extending in a centered manner through internal chamber  112  is laterally offset from drive axis L 2  by ½ the diameter of helical gear  256  and ½ the diameter of sprocket gear  230 . In this manner, helical gear  256  may engage and mesh with longitudinally extending gear teeth of sprocket gear  230 . As a result of this engagement, rotational movement applied to internal drive socket  252  tending to rotate helical gears  256  along axis L 2  is translated into rotational movement about expansion axis L 1 .  
         [0026]     Upper bone engaging component  150  is threadedly received within internally threaded aperture  216  and threadedly engages internal threads  212 . In a similar manner, lower bone engaging component  170  is threaded into the internally threaded aperture of lower sprocket  270 . The upper bone engaging component  150  is threadedly received within the upper sprocket assembly  210  until it has been substantially advanced to its final position. Channel  158  and channel  160  are aligned with openings through housing  110  and then pins  120  and  122  are passed through apertures formed in housing  110  and into channels  158  and  160 , respectively. In this manner, bone engaging component  150  is locked in rotational alignment with housing  110  but passages  158  and  160  allow longitudinal movement of bone engaging component  150  along the longitudinal axis L 1 . In a similar manner, lower bone engaging component  170  has a pair of opposing channels formed in its threaded cylindrical portion. When assembled in the position shown in  FIG. 3 , a pair of pins  124  and  126  is positioned within the channels to retain the lower bone engaging component  170  in a rotationally locked engagement with respect to housing  110  while allowing longitudinal movement of the lower bone engaging component  170  in response to threaded engagement with lower sprocket  270 .  
         [0027]     In use, a healthcare provider obtains surgical access to a segment of the vertebral column. A damaged vertebral body such as V 3  is at least partially removed along with the adjacent soft tissue structure such as D 2  and D 3 . An expandable device  100  is inserted into the remaining space in the substantially collapsed condition shown in  FIG. 2 . As shown in  FIG. 4 , an insertion tool  400  is engaged to the device. Insertion tool  400  includes a cannula  410  and an internal drive shaft  420  terminating in a hex drive for mating with socket  252 . A locking arm  425  extends medially and includes a locking pin  430 . The device  100  is oriented such that driving tool  400  having an external hex drive shaft is engaged with driving socket  252  of worm gear  250 . Additionally, in a preferred aspect the insertion tool  400  engagement projection  430  extends into enlarged aperture  113 . In this manner, as rotational force is applied to internal drive socket  252  the insertion member can maintain the relative position of expansion implant  100 . Rotation of worm gear  250  along drive axis L 2  is translated through the engagement with sprocket gear  230  into rotational movement along expansion axis L 1 . Rotation of sprocket gear  230  results in corresponding rotation of upper sprocket assembly  210  and lower sprocket assembly  270 .  
         [0028]     As a result of the threaded engagement between the sprocket assemblies  210  and  270  with the upper bone engaging component  150  and the lower bone engagement component  170 , respectfully, rotation of the sprocket assembly about longitudinal axis L 1  causes advancement of both the upper and lower bone engagement assemblies along the longitudinal axis L 1 . It will be appreciated that continued expansion along the longitudinal axis will bring projections  152  of the upper surface and corresponding projections of the lower bone engagement component into contact with the corresponding upper and lower bone surfaces. Further rotation applied to the worm gear will be translated into rotational movement of the sprocket assembly about longitudinal axis L 1  and will tend to lengthen the device and drive the projections into the bone. It will be appreciated that the projections may in one aspect, be driven into the bone until the surface  151  engages the upper bone surface.  
         [0029]     Alternatively in another embodiment, the bone engaging projections do not extend completely into the bone. In this implantation, it is contemplated that at least some subsidence of the bone over time will occur and the upper bone surface will receive a greater extent of the projections until the bone comes to rest on the surface  151 . While projections  152  have been illustrated as pyramid shaped spikes, 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.  
         [0030]     Should the device need to be repositioned or revised, rotation of the worm gear in the opposite direction will have a tendency to collapse the expandable device such that it can be extracted from the patient or repositioned into a more desirable location.  
         [0031]     Although the above described embodiment has illustrated the worm gear as an integral part of the housing, it is contemplated that the worm gear may be an extension of an internal driveshaft of an insertion tool. Referring now to  FIG. 9 , an insertion/expansion tool  300  is shown in combination with an expandable implant  400 . Expandable implant  400  is constructed virtually identical to implant  100  except protrusion  130  has been replaced with a side opening defined by walls  412  and  414  extending through housing  410 . Instrument  300  includes a cannula  310  and an internal drive shaft  312  rotatably disposed therein. Cannula  310  includes a medial projection  320  including a locking projection  322 . Drive shaft  312  has a worm gear  314  adjacent its distal end  316 . The worm gear is disposed within a protective housing  317  of the cannula terminating in a housing engagement end  318 . Thus, the rotating worm gear is shielded from soft tissue surrounding implant  400 . It will be appreciated that the worm gear may be positioned adjacent sprocket  430  and caused to rotate such that the sprocket will rotate about the longitudinal axis of the insertion tool. Further, the tool is engaged to the expansion implant  400  on the lateral aspect of the implant such that the instrument does not block the central area of the implantation site. This provides greater visualization of the soft tissues and vertebrae for the surgeon. Once the desired height of the expandable implant  400  has been obtained, the instrument with its associated worm gear may be withdrawn from engagement with sprocket  430  and withdrawn from the patient.  
         [0032]     It will be appreciated with respect to  FIG. 1 , that the entire expansion assembly  200  is contained within housing  110 . Thus, all moving components of expansion assembly  200  are shielded from engagement with the surrounding tissues of the body. It will be appreciated that the shielding of the expansion assembly  200  within housing  110  thereby protects the surrounding tissues from potential damage by engagement with any of the expansion assembly components. As will be appreciated, this can be particularly important with respect to significant neural structures such as the spinal cord, dura, exiting nerve roots and adjacent lateral nerves. Further, significant vascular structures extend along the spine including the aorta and vena cava which respond poorly to engagement with abrasive structures due to the pulsatile nature of blood circulation. The unitary outer structure of housing  110  provides a continuous curved surface from atraumatic engagement with surrounding tissue.  
         [0033]     Once expandable implant  100  has been expanded to the desired height, a lock screw  128  may be advanced into engagement with upper sprocket  210  and in particular with one of the grooves  218  to prevent further rotation of the sprocket. Additionally, although not required, a further locking screw  129  may be advanced into engagement with lower sprocket assembly  270  and one of the grooves  272 . It will be appreciated that locking the relative rotation will insure that the height established during the surgical procedure will be maintained throughout the life of the device. While a pair of locking set screws has been shown for the purposes of illustration, it will be appreciated that other mechanisms and techniques may be utilized to inhibit the relative rotation and encourage maintenance of the established height. For example, it is contemplated that bone growth promoting material such as allograft, autograft, or bone matrix materials may be inserted into the interior  112  of the device after implantation. For example, opening  1   13  provides access to the interior sufficient to fill the interior. Additional bone growth promoting substances may be inserted through the plurality of apertures  111  extending through outer housing  110 . In addition to cooperating to inhibit rotation of the internal components, the bone growth promoting substance may participate in and encourage bone fusion through the interior  112  of the device and between the upper and lower vertebral bodies such as V 2 and V 4 .  
         [0034]     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. 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.  
         [0035]     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. In other embodiments, the implant may be solid, porous, spongy, perforated, drilled, and/or open.  
         [0036]     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 β 1 , 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 all or a portion of an unexpanded implant, as shown in  FIG. 2 , with material and then place the device in the body. Upon expanding the device to an expanded state such as is shown in  FIG. 5 , material placed in the bone engaging components will be urged against the bone. Additional material may be placed through the outer housing  110  to fill any voids created during the expansion process. Placement of additional material may be accomplished directly or with the aid of an injection or transfer device of any effective type.  
         [0037]     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 to the spine 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.  
         [0038]      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. Further, the end pieces may be angled to achieve or maintain angulation of lordosis or kyphosis between the remaining vertebral end plates. For example, each end plate may have 0°, 3°, or 6° of angulation allowing the device to achieve between 0° and 12° of angulation between remaining vertebrae.  
         [0039]     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.