Abstract:
An expandable subtalar implant device suitable for addressing a flatfoot condition. The device is a sinus tarsi implant that blocks excessive motion between the talus and calcaneus bones in the foot while permitting normal motion and alignment. The device comprises a generally cylindrical metal structure having a first proximal expandable end and a second distal adjustment end. A first component of the implant forms each of the cylindrical end sections and a second component comprises a movable internal rod that serves to progressively expand the outer cylinder of the implant. The internal rod is externally threaded and mates with internal threading on the cylindrical outer component. Progressive turning of the internal rod engages a number of radially arranged inclined surfaces that form the interior walls of the cylindrical shell component. Progressive engagement of the rod with these inclined surfaces forces the end of the cylindrical shell component outward; expanding the overall diameter of the implant once it has been positioned within the sinus tarsi.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to medical implant devices, especially those associated with modifying skeletal structure and motion. The present invention relates more specifically to an implant device for the correction of skeletal alignment deformities in the feet. 
         [0003]    2. Description of the Related Art 
         [0004]    A skeletal deformity in humans commonly referred to as “flatfoot” is an excessive pronation of the foot caused by abnormal motion between two bones of the foot. The result of this abnormality is that the inside arch of the foot becomes flattened, generally as a result of the calcaneus or heel bone turning outward. This abnormal motion of the ankle bone (talus) with respect to the calcaneus can eventually cause anatomical misalignment and therefore requires some correction. 
         [0005]    It is also known that in the above described motion between the talus and calcaneus, a naturally occurring opening (sinus) formed between the talus and calcaneus is closed. Efforts in the past therefore to correct the abnormal flatfoot condition have focused on the insertion of a prosthetic type implant into this opening, generally referred to as the sinus tarsi. The joint involved is generally referred to as the subtalar joint and is formed by the posterior talar facet of the calcaneus and the posterior calcaneal facet of the talus. 
         [0006]    The primary technique, therefore, to correct a flatfoot condition is to use a subtalar implant that is inserted into the sinus tarsi to reposition the talus relative to the calcaneus. Various structural configurations have been offered for such an implant, although none have been found fully satisfactory in solving the problems associated with placing the implant, maintaining it in place, and of course, having the implant function properly during normal motion of the foot. The initial critical aspect of the design of the implant is the ease with which the implant may be placed, both in terms of surgical invasiveness and proper placement or alignment. Many designs previously offered have unfortunately focused almost entirely on the ease with which the implant is placed and its proper alignment within the sinus tarsi. 
         [0007]    A second issue with subtalar implants is the ability to maintain the implant in place and to prevent misalignment or dislodgement of the implant over time. Many structural features of implants developed in previous efforts, therefore, have been directed to what has been perceived to be the necessity of maintaining the implant fixed in place within the sinus tarsi. Additionally, although most subtalar implants proposed to date achieve the function of obstructing the space formed between the talus and the calcaneus, and thereby achieve some level of functionality, various structural configurations detract from that functionality either through the aforementioned misalignment or through pain and discomfort associated with the individual receiving the implant. Efforts in the past to provide solutions to the above mentioned issues and problems have included a number reflected in the following U.S. Patents: 
         [0008]    U.S. Pat. No. 6,168,631 issued to Maxwell et al. on Jan. 2, 2001 entitled  Subtalar Implant System and Method for Insertion and Removal.  This patent describes an implant in the nature of a metal screw having a number of crossing slots formed in the threads. The screw configuration is intended to secure the implant within the joint although the resultant lack of a smooth surface can lead to irritation of the bone surfaces and the surrounding tissue. Maxwell et al. further describes the use of a coaxial guide element and a sizer for placement of the implant. 
         [0009]    U.S. Pat. No. 4,450,591 issued to Rappaport on May 29, 1984 entitled  Internal Anti - Proratory Plug Assembly and Process of Installing the Same.  This patent describes a plastic cone shaped plug provided with a tie line that may be inserted into the sinus tarsi or opening of the subtalar joint to correct pes plano valgus (flatfoot). The tie line is connected around the deltoid ligament after insertion of the plug. 
         [0010]    U.S. Pat. No. 6,136,032 issued to Viladot Perice et al. on Oct. 24, 2000 entitled  Implant for Correcting Flat Foot Condition.  This patent describes a subtalar implant made up of a complex arrangement of multiple elements including a cylinder with serrated external protrusions and an internal expansion cone that is directed into the wedge shaped cylinder by means of a number of threaded components. 
         [0011]    U.S. Pat. No. 5,360,450 issued to Giannini on Nov. 1, 1994 entitled Prosthesis for the Correction of Flatfoot. This patent describes a prosthetic implant intended to be inserted into the tarsal sinus that is made up of a bioreabsorbable material and is club shaped or slightly conical in configuration. A pair of wings are forced outward to help secure the implant in place. 
         [0012]    U.S. Pat. No. 7,033,398 issued to Graham on Apr. 25, 2006 entitled  Sinus Tarsi Implant.  This patent describes a device that is composed of a non-metallic polymer and is structurally the combination of a frustum of a right cine (a conical formation) and an axially extending cylinder that is cannulated and additionally threaded on its exterior surface. 
         [0013]    U.S. Patent Application Publication No. US 2005/0177165 filed by Zang et al. and published on Aug. 11, 2005 entitled  Conical, Threaded Subtalar Implant.  This patent application describes a subtalar implant generally conical in configuration and including a plurality of threads formed around the exterior surface of the body of the implant in order to help secure the implant in place. Specific structural geometries for the threading and the angle of the cone exterior surfaces are described. 
         [0014]    U.S. Patent Application Publication No. US 2005/0177243 filed by Lepow et al. and published on Aug. 11, 2005 entitled  Subtalar Implant Assembly.  This application describes a subtalar implant that includes a rounded end cap region as well as a threaded region on the basic cylindrical implant structure. The threads have shapes that vary according to their position on the implant between the rounded end cap and the wider external face of the implant. A number of apertures are provided that extend from the external walls of the implant interior to the core. 
         [0015]    U.S. Patent Application Publication No. US 2006/0041315 filed by Katz et al. and published on Feb. 23, 2006 entitled  Subtalar Implant.  This application describes an implant for insertion into the tarsal sinus that includes a metal body with a plurality of threads on one section of the body as well as a generally smooth non-threaded portion intended to make contact with the articulating surface of the bones of the joint. The configuration is intended to mimic the shape of the tarsal canal when the foot is bearing weight in an effort to distribute that weight over a relatively large surface area. 
         [0016]    U.S. Patent Application Publication No. US 2005/0251264 filed by Katz et al. and published on Nov. 10, 2005 entitled  Subtalar Implant.  This application further describes the generally cylindrical metal implant having a threaded section and a separate smooth polymeric section that is designed to be positioned between the articulating bones of the joint. A pin axially disposed through the polymeric section provides the manner of attachment between the polymeric section and the metal section and thereby facilitates the rotation of the implant and engagement of the threads within the joint. 
         [0017]    While as indicated above, many efforts have been made in the past to address the primary issues associated with insertion, placement, maintenance, comfort, and function, none of the prior efforts have managed to optimally address each of these issues. There remains a need, therefore, for a subtalar implant that is easy to insert and position within the sinus tarsi, that can be maintained in position during use, and that provides both the comfort and non-damaging effects that many of the prior efforts have failed to provide. It would be desirable if the surgical procedure for placing the implant were straightforward and simple and further allowed for easy adjustment of the implant at the time of its placement or subsequent as necessary. It would therefore be desirable if the implant device had a smooth outer surface so as to prevent discomfort to the individual and to further prevent damage to the bone surfaces and surrounding tissue during both placement and use of the device. It would be desirable if despite the smooth outer surface of the implant it remained fixed in place without becoming misaligned or slipping out from its placement during use. 
       SUMMARY OF THE INVENTION 
       [0018]    In fulfillment of the above and further objectives, the present invention provides an improved expandable subtalar implant device suitable for addressing a flatfoot condition. The device is a sinus tarsi implant that serves to block excessive motion between the talus and calcaneus bones in the foot while still permitting normal motion and alignment. The device comprises a generally cylindrical metal structure having a first cylindrical (proximal) expandable end and a second slightly tapered (distal) adjustment end. A first outer component of the implant forms the external cylindrical end sections mentioned above and a second inner component comprises a movable (rotatable) rod that serves to expand a portion of the outer cylinder component of the implant. The internal rod component is externally threaded and mates with internal threading on the cylindrical outer component. Progressive turning of the internal rod engages a number of radially arranged inclined surfaces that form the interior walls of one end of the cylindrical shell component. Progressive engagement of the rod with these inclined surfaces forces the end of the outer cylindrical component outward, expanding the overall diameter of the implant once it has been placed into position within the sinus tarsi. Basic tools may be provided to assist with the placement and/or removal of the implant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a perspective view of the subtalar implant device of the present invention in its fully assembled but unexpanded condition. 
           [0020]      FIG. 2A  is an end view of the adjustment end section of the subtalar implant device of the present invention. 
           [0021]      FIG. 2B  is a partial sectional side view of the subtalar implant device of the present invention showing the internal structures of the device in dashed outline form. 
           [0022]      FIG. 2C  is an end view of the expandable end section of the subtalar implant of the present invention. 
           [0023]      FIG. 3A  is a side view of the skeletal components of a foot showing placement of the implant device of the present invention. 
           [0024]      FIG. 3B  is a top view of the skeletal components of a foot showing placement of the implant device of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Reference is made first to  FIG. 1  which discloses the overall structure of the subtalar implant device of the present invention. In  FIG. 1 , subtalar implant device  10  is seen in a perspective view which exhibits each of the components of the device as well as its external structural features. The tapered head section  12  of implant device  10  is generally cylindrical in structure and tapers from an approximate midpoint on the implant to what ends up being the exposed distal face  16  of the implant device. The balance of subtalar implant device  10  comprises the expansion cylinder section  14  of the device described in more detail below. 
         [0026]    Hex screw  18  is centrally positioned within tapered head section  12  of the implant device, coaxially with the cylindrical structure of the implant device  10 . As described in more detail below, hex screw  18  may, by way of engagement of a tool with internal hex socket  20 , be threaded into, or threaded out from, engagement with tapered head section  12  of implant  10 . The process of turning hex screw  18  within implant device  10  comprises use of a hex drive tool device (not shown) sized to appropriately engage internal hex socket  20 . At the same time the hex head tool device engages hex screw  18  a separate tool, or a separate portion of a unitary tool, oppositely engages grip notches  22   a  and  22   b.  In this manner the external cylindrical structure of implant device  10  may be held stationary while hex screw  18  is turned to fix the implant device in place, again as described in more detail below. 
         [0027]    Expansion cylinder section  14  of implant device  10  comprises a cylindrical structure divided into a number of radial segments  28 . These segments  28  comprise six segments in a preferred embodiment shown in  FIG. 1 . Segments  28  are established by a number of flex holes drilled radially into the cylindrical walls of expansion cylinder  14 , shown in  FIG. 1  by flex holes  24   a,    24   b  and  24   c  as examples. Joined to each of these flex holes  24   a,    24   b  and  24   c  are flex slots  26   a,    26   b  and  26   c  that extend from a proximal end of expandable cylinder section  14  of implant device  10  (the end not seen in the view of  FIG. 1 ) through the walls of expansion cylinder  14  to flex holes  24   a,    24   b  and  24   c  respectively. This manner of drilling apertures and cutting slots in the walls of expansion cylinder  14  creates the radially arranged expansion head sections  28  that expand outward under the influence of the internal rod to expand the overall diameter of the implant device and thereby facilitate the retention of the device in position within the sinus tarsi once the implant has been surgically positioned and placed. 
         [0028]    Reference is now made to  FIGS. 2A through 2C  for more detailed description of the structural elements of subtalar implant device  10  and the manner in which the internal threaded rod serves to expand the expansion cylinder as described above.  FIG. 2A  is an end view of implant device  10  showing in greater detail the tapered head section  12  of implant device  10 . The slight taper extends from the overall diameter of implant device  10  (the larger external circle shown) down to the diameter of exterior (distal) face  16  (the smaller circle shown in  FIG. 2A ). Grip notches  22   a  and  22   b  are shown positioned at opposing points on the perimeter of the edge of exterior (distal) face  16 . Internal hex socket  20  is shown positioned centrally in the exterior face of hex screw  18  which is itself positioned centrally within the exterior face  16  of subtalar implant device  10 . 
         [0029]    In  FIG. 2B  the internal structures of subtalar implant device  10  are disclosed in greater detail. The depth dimension of grip notches  22   a  and  22   b  is shown in dashed outline form on the top and bottom edge of the perimeter of exterior face  16  of tapered head  12 . Expansion push rod  30  is shown as an extension of hex screw  18 . External threads  36   a  on hex screw  18  are shown to engage with internal threads  36   b  on the interior wall of tapered head  12  of implant device  10 . 
         [0030]    The balance of the interior structures of subtalar implant device  10  are shown within the interior configuration of expansion cylinder  14 . Extending from the threaded section of the interior wall described above, the otherwise solid construction of expansion cylinder  14  is hollowed out to form a number of different regions in the walls of the expansion cylinder  14 . A first region extends from the area of threads  36   b  on the interior wall of tapered head  12 , generally to a point where flex holes  24   a,    24   b  and  24   c  (as examples) have been drilled through the walls of expansion cylinder  14 . A relatively thin-walled portion of expansion cylinder  14  then extends somewhat more than half-way along its length toward the opposing end of expansion cylinder  14 . At approximately the midpoint along the length of expansion cylinder  14  the interior walls increase in thickness and continue to increase forming inclined internal surfaces  34  to a point near the end of expansion cylinder  14  where the walls stop increasing in thickness to form the wall of aperture  38  that finally extends out from the end of expansion cylinder  14 . 
         [0031]    This internal cylindrical structure, combined with the establishment of the radially arranged expansion head sections  28 , allows expansion push rod  30 , under the influence of the hex tool, to be threaded into the cylindrical implant body and engage the inclined surfaces of expansion head faces  34  that form one side of expansion head wedges  32 . Forcing these wedges outward results in the bending of the thin-walled sections of expansion cylinder  14  as described above. This results in an overall expansion of the external diameter of expansion cylinder  14  and therefore of the subtalar implant device  10  as a whole. 
         [0032]      FIG. 2C  is a detailed view of the opposing end of the implant device  10  from that shown in  FIG. 2A . In this view expansion head sections  28  are more clearly seen as is the interior end aperture  38 . The end of expansion push rod  30  is shown in dotted outline form (although it does appear through end aperture  38 ). The thickness of the thin-walled sections of expansion cylinder  14  is also show in dotted outline form in  FIG. 2C , extending from the beginning of the thin-walled section to the centrally positioned interior end aperture  38 . The additional flex slots (not numbered in this view) are likewise radially arranged with flex slots  26   a,    26   b,  and  26   c  to divide expansion cylinder  14  into the six equally sized and radially arranged expansion head sections  28 . 
         [0033]    Those skilled in the art will recognize that the number and placement of the expansion sections shown in  FIGS. 2A through 2C  may be varied and still implement the basic concept of the present invention. Varying the number of sections along with the thickness of the bendable wall portions of the expansion cylinder will provide implant devices that are more or less easy to expand, thereby providing more or less secure placement. Implementation of the basic concepts of the present invention would generally require at least four expansion head sections be established and as many as twelve. The six expansion head sections shown in  FIGS. 2A through 2C  provide an optimal configuration that balances strength with flexibility in allowing for the expansion of the implant device without significantly degrading its stability once in place. 
         [0034]    Reference is now made to  FIGS. 3A and 3B  for a more detailed description of the actual placement of the implant device within the skeletal structure of the foot.  FIG. 3A  is a side view of the skeletal structure of the foot while  FIG. 3B  is a top view of the same. Bones of the foot  40  in each case are shown as they are typically arranged in conjunction with the right foot of an individual. The view in  FIG. 3A  therefore is that of the outside portion of the right foot of an individual while the view in  FIG. 3B  is, as indicated, a top view of the right foot. 
         [0035]    This orientation positions the sinus tarsi approximately one-third of the way along the length of the foot measured from the heel oriented to the front and outside portion of the foot. Implant device  10  is shown generally placed within the sinus tarsi as described above. This placement is between the talus bone  42  and the calcaneus bone  44  and generally serves to modify the motion of these two bones one against the other. The view in  FIG. 3A  provides perhaps the best indication of the manner in which the flatfoot condition is prevented by maintaining the “elevated” distance between the talus bone  42  and the calcaneus bone  44 . Absent placement of the subtalar implant device  10  the bones described could collapse to close the tarsal sinus normally defined between these bones. 
         [0036]    Placement of the implant device  10  initially follows a fairly well established surgical procedure for implanting any of the prior art subtalar implant devices in terms of incisions, positioning of the device, and orientation of the same. Once in position, however, the unexpanded device of the present invention is then set in place by the use of a pair of tools (or two components of a single tool) positioned on the exterior or distal face of the implant as described above. A first tool element serves to hold and prevent the rotation of the external cylindrical component of the implanted device, while a second tool element (a hex key) serves to turn or rotate the internal expansion rod component of the implant in a manner that expands the outer expansion cylinder, again as described above. Therefore, after an incision has been made and the device has been positioned and placed within the sinus tarsi, the tools are placed on the distal end face that remains exposed in the manner described. The internal push rod is rotated which advances the rod proximally into the implant device and expands the exterior expansion cylinder to increase the diameter and effectively retain the device within the cavity defined by the sinus tarsi. 
         [0037]    Adjustment of the device once placed as described above may involve slightly loosening the hex screw, thereby slightly decreasing the expansion cylinder diameter to allow for the repositioning or realignment of the implant device as necessary. Such action could also be carried out at a point subsequent in time after surgery with relatively little invasion, again by slightly loosening the expansion cylinder and then retightening or reexpanding it when the preferred position and alignment has been established. 
         [0038]    A number of common tools or specifically designed tools may be used for positioning, orientation and placement of the implant device of the present invention. The use of a hex socket to drive the turning of the threaded push rod in the implant device in order to initiate the expansion of the device may in part facilitate the orientation of the implant device once it is positioned in the sinus tarsi. A hex drive tool positioned on the end of a screwdriver or the like may allow the physician to orient the device and position it appropriately by manipulating the handle of the tool. Thereafter, once the external cylinder is fixed in place, the screwdriver handle of the hex drive tool may be turned to expand the implant and set it in place. In the simplest form, a hex drive screwdriver in combination with right angled needlenose pliers are all that is required to position, orient and set the implant device in place. The present invention envisions, however, the use of a specialized tool that incorporates both the hex drive component, a grasping component, and a pair of fixed plier tips into a single tool sized to fit directly over the top of the distal end of the implant device as it is manipulated by the physician. Any of these various tool embodiments would be appropriate for use in conjunction with the basic structure of the implant device of the present invention. 
         [0039]    In addition to the above described tools that may be used to facilitate the process of fixing the implant in place and/or loosening the implant for removal, a simple threaded rod tool may be utilized to facilitate the process of placing the implant, and perhaps more importantly, the process of removing the implant. Hex screw  18  (as shown in  FIG. 2B , for example) may be removed entirely from tapered head section  12  of the implant device  10  to allow access to the internal threading  36   b  on the interior wall of tapered head  12 . A tool constructed simply of a rod with a threaded end and a hand grip opposite the threaded end, may be used to hold the implant device  10  for either placement or removal. Partially threading the threaded rod tool into the place of the hex screw  18  in the implant provides a stable means for manipulating the implant by hand. For placement of the implant, the threaded rod tool would be removed once the implant is in position and the hex screw  18  (retained loosely on the end of a hex head tool) may be threaded into place. For removal of the implant, the hex screw  18  may be removed from the implant device  10  using a hex head tool and the threaded rod tool may be threaded into its place. Gentle retraction force is then all that is necessary to remove the implant, even an implant that has been in place for some time. 
         [0040]    Although the present invention has been described in terms of the foregoing preferred embodiments, this description has been provided by way of explanation only, and is not intended to be construed as a limitation of the invention. Those skilled in the art will recognize modifications of the present invention that might accommodate specific patients and skeletal structures. As is known in the art, it is necessary to provide various sizes of a similarly structured implant device in order to accommodate patients of different ages and different skeletal structures. Such modifications as to components, size, and even configuration where such modifications are merely coincidental to the size of the patient, do no necessarily depart from the spirit and scope of the invention. It is further anticipated that some variation may occur, for example, in the configuration and number of the expansion head sections of the implant device to allow variations in the expansion force required and experienced when placing the device. Again, all of these various modifications and variations do not necessarily depart from the spirit and scope of the invention.