Patent Publication Number: US-6214050-B1

Title: Expandable implant for inter-bone stabilization and adapted to extrude osteogenic material, and a method of stabilizing bones while extruding osteogenic material

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an expandable implant capable of providing inter-bone stabilization (e.g., intervertebral spinal stabilization) and also capable of extruding materials, such as osteogenic material, upon expansion. The present invention also relates to a method of stabilizing bones (e.g., spinal vertebrae) with respect to one another while extruding osteogenic material. 
     There are several situations where it becomes desirable to stabilize one bone with respect to another. One exemplary situation arises in patient&#39;s who suffer from chronic low back pain. Chronic low back pain is one of the most common and perplexing problems facing the field of orthopedic surgery. In addition to patient discomfort, chronic low back pain has several adverse societal impacts, including lost income and possible chronic dependence on drugs, alcohol and public relief programs. 
     In many cases, low back pain can be avoided by preventing relative motion between spinal vertebrae. This treatment is commonly referred to as intervertebral stabilization. To abate low back pain, stabilization is directed to stabilizing contiguous vertebrae in the lumbar region of the spine. 
     Surgical techniques are known for use in spinal stabilization. These techniques seek to rigidly join vertebrae which are separated by a degenerative disk. Ideally, the surgery effectively replaces the vertebra-disk-vertebra combination with a single rigid vertebra. Various surgical techniques have been developed which attempt to approach or approximate this ideal. 
     One technique known in the art is to partially remove a degenerated disk and insert a bone graft into the void formed by the removed disk. Other techniques involve use of a surgical prosthetic implant which, acting alone or in combination with bone fragments, replaces the use of bone grafts. Such implants have been provided in the form of an implant that is placed between two adjacent vertebrae. The implant may contain bone fragments to facilitate bone growth. The implant contacts adjacent vertebral plates and achieves vertebral fusion after a sufficient amount of bone growth occurs, thus treating or preventing back pain in patients that have discogenic pain. 
     While conventional implants can be filled with bone fragments to expedite bone growth, it is believed that the mere presence of the bone fragments is not enough to achieve the rate of bone growth that would be provided if the bone fragments or other osteogenic material were extruded from the implant. There is consequently a need for an expandable implant which is adapted to extrude osteogenic material during its expansion and which thereby is adapted to expedite the bone growth and fusion process. By expediting the bone growth and fusion process, it is possible to reduce the amount of time between surgery and the patient&#39;s ability to return to work or perform physically demanding activities. 
     There also is a need for an expandable implant which can be repositioned in the event that the stabilization provided by the initial positioning of the implant creates an undesirable vertebral alignment. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to satisfy the foregoing needs by providing an expandable implant capable of extruding osteogenic material during expansion, to thereby expedite bone growth. 
     Another object of the present invention is to provide a method of stabilizing vertebrae with respect to one another while extruding osteogenic material. 
     Still another object of the present invention is to provide an expandable implant which can be contracted after expansion in order to facilitate repositioning of the implant. 
     To achieve these and other objects, the present invention provides an expandable implant comprising an anchor structure and a movable structure. The anchor structure is adapted to be inserted at least partially into a bone or between two bones, and is adapted to be secured thereto by expansion. The movable structure is operatively connected to the anchor structure so that movement of the movable structure with respect to the anchor structure causes expansion of the implant and at least partial extrusion of material contained within the implant. 
     Also provided is an expandable implant comprising an anchor structure, a movable structure, and an actuation device. The anchor structure is adapted to be inserted at least partially into a bone or between two bones, and is adapted to be secured thereto by expansion. The anchor structure has a cavity which contains osteogenic material. The movable structure delimits aspects of the cavity and is operatively connected to the anchor structure so that movement of the movable structure with respect to the anchor structure causes expansion of the implant and at least partial extrusion of the osteogenic material. The actuation device connects the anchor structure to the movable structure and is adapted to move, in response to mechanical manipulation, the movable structure with respect to the anchor structure in a first predetermined direction which causes expansion of the implant and reduces the volume of the cavity. As a result of the reduction in volume, the osteogenic material is extruded at least partially from the cavity. 
     The present invention also provides a method of stabilizing first and second bones with respect to one another. The method comprising the steps of inserting at least one expandable implant between the bones, expanding the implant, and extruding an osteogenic material from the implant during expansion thereof. 
     The above and other objects and advantages will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation of an expandable implant according to a preferred embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of an actuation device according to a preferred embodiment of the present invention. 
     FIG. 3 is a plan view of a preferred embodiment of a clip for use in the actuation device illustrated in FIG.  2 . 
     FIG. 4 is partial cross-sectional view of an exemplary tool that can be used in manipulating the expandable implant of the present invention. 
     FIG. 5 is a cross-section view of an exemplary extension device that can be used to interconnect the implant and a spinal stabilization rod. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As best shown in FIG. 1, an expandable implant  10  according to a preferred embodiment of the present invention comprises an anchor structure  12  and a movable structure  14 . The anchor structure  12  is adapted for insertion at least partially into a bone or between two bones and can be secured to the bone(s) by expansion. The expansion is achieved by moving the movable structure  14  with respect to the anchor structure  12 . Such movement also advantageously causes at least partial extrusion of material contained within the implant. 
     In the exemplary embodiment, the anchor structure  12  and the movable structure  14  have shapes which cooperate to form a cavity  16  within the implant  10 . The cavity  16  has a volume which varies depending upon the position of the movable structure  14  with respect to the anchor structure  12 . The cavity  16  preferably is filled with an osteogenic material. The osteogenic material, for example, may include actual bone matter or any other substance capable of expediting or facilitating bone growth. 
     Preferably, the movable structure  14  and/or the anchor structure  12  has (or have) at least one extrusion opening  18 . The extrusion opening(s)  18  provide access from the cavity  16  to an exterior of the implant  10 . Preferably, both the movable structure  14  and the anchor structure  12  have a plurality of such extrusion openings  18 , as illustrated in FIG.  1 . 
     The anchor structure  12  also preferably includes a cap  20  and a plurality of fingers  22  extending axially from the cap  20 . The plurality of fingers  22  preferably are located at the circumferential edge  24  of the cap  20 . The plurality of fingers  22  are arranged with respect to one another and with respect to the cap  20  such that the cavity  16  is delimited by inside surfaces of the cap  20  and of the plurality of fingers  22 . The cavity  16  also is delimited by an inside surface of the movable structure  14 . 
     The movable structure  14  is movable at least partially through the cavity  16  to effect increases or reductions in the volume of the cavity  16  depending on the direction of movement. Arrow I in FIG. 1 denotes the direction of movement that increases the volume of the cavity  16 , whereas arrow R denotes the direction of movement that causes the volume to be reduced. Preferably, as illustrated in FIG. 1, the movable structure  14  also includes fingers  26  (only one of which is visible in FIG. 1) which are interposed between the fingers  22  of the anchor structure  12 . The fingers  26  of the movable structure  14  extend in the opposite axial direction as the fingers  22  of the anchor structure  12 . 
     The exemplary implementation shown in FIG. 1 includes a total of four fingers  22 , 26  located symmetrically about the circumference of the implant  10 . Each of the anchor structure  12  and movable structure  14  is associated with two of the fingers  22 , 26 . The fingers  22 , 26  are centered about 90 degrees apart from one another along the circumference of the implant. The symmetrical arrangement, while not a limitation of the invention, is preferred regardless of the number of total fingers  22 , 26 . Thus, six-finger embodiments preferably have fingers which are centered about 60 degrees from one another. Preferably, adjacent fingers  22 , 26  are spaced apart from one another to provide additional space through which the osteogenic material can be extruded. 
     Preferably, the movable structure  14  and the anchor structure  12  include bearing surfaces  30 . The bearing surfaces  30  engage respective ones of the fingers  22 , 26  and urge the fingers  22 , 26  radially outward to effect expansion of the implant  10  when the movable structure  14  is moved in the direction denoted by arrow R. 
     The implant  10  also preferably includes an actuation device  32  adapted to move the movable structure  14  with respect to the anchor structure  12 . While the illustrated exemplary embodiment uses a suitably configured bolt or screw as the actuation device  32 , it is understood that the invention is not limited to such embodiments. 
     As illustrated in FIG. 1, the actuation device  32  connects the anchor structure  12  to the movable structure  14 . In particular, the exemplary actuation device  32  passes through a hole  34  in the cap  20  of the anchor structure  12  and threadedly engages an internally threaded post  33  of the movable structure  14 . Mechanical manipulation (e.g., rotation) of the actuation device  32  in a first predetermined direction causes the movable structure  14  to move toward the anchor structure  12  (i.e., toward the cap  20  of the anchor structure  12 ) in the direction denoted by arrow R. This movement serves to reduce the volume of the cavity  16  and thereby causes the osteogenic material or any other material contained within the cavity  16  to be at least partially extruded from the implant  10  (e.g., through the extrusion openings  18 ). The movement in the direction denoted by arrow R also causes the bearing surfaces  30  to urge the fingers  22 , 26  radially outward, and thereby effects expansion of the implant  10 . 
     The broken lines in FIG. 1 show one of the fingers  22  in a radially outward position. Further movement of the movable structure  14  in the direction denoted by arrow R can cause the fingers  22  to extend radially out well beyond the exemplary position denoted by the broken lines. 
     Mechanical manipulation in a second predetermined direction (e.g., rotation in the opposite direction), by contrast, causes the movable structure  14  to move away from the cap  20  of the anchor structure  12  in the direction denoted by arrow I. Such movement in the direction denoted by arrow I causes radial contraction of the implant  10  as the force exerted by the bearing surfaces  30  on the fingers  22 , 26  diminishes. The resulting radial contraction of the implant  10  permits repositioning of the implant  10  should such repositioning become necessary or desirable after initial implantation. 
     As illustrated in FIGS. 2 and 3, the actuation device  32  preferably includes a retention mechanism  40 . The retention mechanism  40  includes a clip  42 . The clip  42  is partially received in a circumferential notch  44  of the actuation device  32 . When the clip  42  engages the notch  44 , enough of the clip  42  extends out of the notch  44  to prevent the notched portion of the actuation device  32  from being withdrawn through the hole  34  in the anchor structure  12 . The retention mechanism  40  thereby keeps the actuation device  32  from becoming separated from the anchor structure  12 . While the preferred retention mechanism  40  uses the combination of the clip  42  and the notch  44 , it is understood that the retention mechanism  40  can be implemented using alternative structures. 
     While various dimensions can be provided, depending on the intended use of each implant  10 , a preferred implementation of the implant  10  includes dimensions that permit use of the implant  10  as an intervertebral stabilization device. The anchor structure  12  and movable structure  14 , in this regard, are provided with dimensions that permit insertion of the expandable implant  10  between two vertebrae and expansion of the anchor structure  12  to provide stabilization of the two vertebrae with respect to one another. The exact dimensions will depend on the intervertebral spacing. In pediatric patients, the dimensions will be correspondingly smaller than the dimensions of an implant  10  that is intended for use with an adult patient. Exemplary dimensions for an adult spinal patient include an axial length of about 20 to 26 millimeters. The unexpanded diameter for the adult patient preferably is about 9 to 10 millimeters, with a diametric expansion of about 8 to 10 millimeters being provided by movement of the bearing surfaces  30  against the fingers  22 , 26 . 
     As illustrated in FIG. 1, the cap  20  preferably is provided with a tool engagement feature  50 . The exemplary tool engagement feature  50  comprises external threads  52 . It is understood, however, that the invention is not limited to the exemplary feature, and that alternative structures can provide similar results. 
     An exemplary tool  60  which is capable of engaging the threads  52  for purposes of inserting or otherwise manipulating the implant  10 , and which is also capable of turning the actuation device  32  to effect expansion or contraction of the implant  10  while remaining engaged to the threads  52 , is disclosed in my U.S. Pat. No. 5,531,792. The contents of U.S. Pat. No. 5,531,792 are incorporated herein by reference. 
     As illustrated in FIG. 4, the tool  60  has a sleeve  62  which is internally threaded to engage the threads  52 . A screw driver portion  64  passes through the sleeve  62  and engages the head  66  of the actuation device  32  to permit manual turning of the actuation device  32  while the sleeve  62  remains engaged to the threads  52 . The sleeve  62  allows the entire implant  10  to be manipulated as a unit to achieve a desired position in the implant site, while the screw driver portion  64  facilitates movement of the movable structure  14  for purposes of selectively expanding or contracting the implant  10 . The screw driver portion  64  preferably is connected to a screw driver handle  65  which provides a mechanical advantage when the screw driver portion  64  is turned. A knurled ring  67  is rigidly connected to the sleeve  62  to facilitate turning of the sleeve  62  with respect to the threads  52 , for example, when connecting or disconnecting the tool  60  from the implant  10 . 
     The movable structure  14 , anchor structure  12 , and actuation device  32  preferably are made of biocompatible material, such as surgical grade titanium or stainless steel. The osteogenic material preferably comprises bone material. 
     The implant  10  is particularly well-suited for use in stabilizing first and second bones with respect to one another. The implant  10  may be inserted into and/or between the bones after drilling or other appropriate preparation of the implantation site. Preferably, the insertion process includes threadedly connecting the threads  52  of the implant  10  to the internal threads  68  of the tool&#39;s sleeve  62 , and using the tool  60  to insert the expandable implant  10  into the implantation site. 
     Once a desired position is achieved between the bones, the implant  10  can be expanded to tighten its engagement between the bones and/or prevent relative movement of the bones. This expansion preferably is accomplished by rotating the screw driver portion  64  of the tool  60  so that the actuation device  32  also rotates. During expansion, the osteogenic material is extruded from the implant  10 , preferably through the extrusion openings  18  of the expandable implant  10 . In particular, the extrusion is achieved by reducing the volume of the cavity  16  which holds the osteogenic material in each implant  10 . The volume reduction is achieved by turning the actuation device  32  in the direction that causes the movable structure  14  to move in the direction of arrow R. The resulting extrusion during expansion advantageously expedites bone growth through the implant  10  and hastens the stabilizing effect provided thereby. 
     When stabilizing two vertebrae with respect to one another, for example, the stabilization method can be performed using two expandable implants  10  of the type described above. In particular, the steps of inserting, expanding, and extruding are performed between the two vertebrae. 
     After expansion of each implant  10 , a determination can be made as to whether a desired alignment of the bones has been achieved and/or whether the relative position of the implant is appropriate. If this determination indicates that the desired relative alignment was not achieved, then one or both of the expandable implants  10  can be radially compressing (e.g., by turning the actuation device  32  in the direction that causes the movable structure  14  to move in the direction denoted by arrow I). The implant(s)  10  then can be repositioned and reexpanded. After reexpansion, the surgeon can verifying whether the desired relative alignment has been achieved. If this verification indicates that the desired relative alignment has not been achieved, the steps of radially compressing, repositioning, reexpanding, and verifying are repeated until the desired relative alignment is achieved. 
     Once the desired relative alignment is achieved, the knurled ring  67  can be rotated manually to disconnect the tool  60  from the threads  52  of the implant  10 . It will be appreciated that the radial expansion provided by the implant  10  increases the frictional forces between the implant  10  and the implantation site and thereby serves to more positively retain the implant  10  in the implantation site. The additional frictional forces also advantageously keep the implant  10  from turning during removal of the tool  60  from the threads  52 . 
     In treating some patients (e.g., scoleosis patients, patients with spinal curvature, and/or trauma patients), it may be desirable to connect the implant  10  to a spinal stabilization rod. Such rods are well known in the art. 
     As shown in FIG. 5, the implant  10  can be provided with an implant extension  80 . The exemplary implant extension  80  includes internal threads  82 . The internal threads  82  are adapted to engage the threads  52  of the implant  10  and thus can be used to secure the extension  80  to the implant  10 . The extension  80  further includes a neck portion  84  and a ball  86 . The ball  86  and neck portion  84  can be engaged and locked to a suitable spinal stabilization rod in a trailer hitch-like manner. Alternatively, other engagement and locking means can be used to interconnect the rod and the implant  10 . 
     While the expandable implant  10  and method are particularly well-suited for use in stabilizing vertebrae, it is understood that the invention is not limited to such use. The implant  10  can be modified and/or used to stabilize other bones, as one having ordinary skill in the art would readily appreciate from the instant disclosure. 
     Moreover, while this invention has been described as having a preferred design, it is understood that the invention is not limited to the illustrated and described features. To the contrary, the invention is capable of further modifications, usages, and/or adaptations following the general principles of the invention and therefore includes such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the central features set forth above, and which fall within the scope of the appended claims.