Abstract:
The present invention provides a subtalar implant as well as methods of use thereof for the purpose of correcting podiatric disorders such as various types of flat foot conditions relating to the subtalar joint. The subtalar implant is capable of threaded engagement with a positioning element used to position and manipulate the implant during surgical implantation in the sinus tarsi of the foot. The implant is cannulated to receive a guide rod to facilitate final positioning of the implant. Once implanted, the subtalar implant provides anatomical fit with the subtalar joint anatomical structure without the need for indentations to receive osseous tissue growth to anchor the subtalar implant.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to U.S. Provisional Application No. 60/710,336, filed Aug. 22, 2005, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a surgical implant in the nature of a subtalar implant and the method of use thereof for implantation to correct podiatric disorders. 
     2. Description of Related Art 
     A subtalar joint (STJ) arthroereisis is provided as well as methods of use thereof for the purpose of correcting podiatric disorders such as various types of flat foot conditions relating to the STJ. The STJ arthroereisis is surgically implanted in the sinus tarsi. The sinus tarsi is a conical-shaped cavity located between the anterosuperior surface of the calcaneus and the inferior aspect of the neck of the talus. Opening laterally, the sinus tarsi is anterior to the fibular malleolus and terminates posteromedially directly behind the sustentaculum tali. Once implanted, the STJ arthroereisis functions primarily to preserve the STJ while also to limit excessive valgus motion and calcaneal eversion and to limit anterior and plantar migration of the talus. 
     The development of STJ arthroereisis for correcting podiatric disorders including flat foot conditions dates back to 1946. The original STJ arthroereisis incorporated a theory of using a bone graft to elevate the sinus tarsi and to limit pronation of the subtalar joint. Procedures built upon the bone grafting techniques to eliminate pronation by positioning a bone graft or a custom carved silicone wedge inside the sinus tarsi. The bone grafts and silicone wedges were further developed into a stemmed polyethylene block known better as the STA-peg, which was further modified through time. The bone grafts, the silicone wedges, and the stemmed polyethylene block were later replaced with modern, threaded implants for surgical implantation within the sinus tarsi. For example, the Maxwell-Brancheau Arthroereisis (MBA) implant is a cannulated, soft-threaded screw-shaped cylindrical prothesis manufactured from a premium titanium alloy. Newer generation threaded implants developed as a conical subtalar implant (CSI), which is a cannulated, soft-threaded conical prothesis, and a domed-stemmed subtalar implant (DSI), which is a cannulated, domed and stemmed prothesis, both of which are manufactured from a premium titanium alloy. 
     While initially described as a simple bone graft to elevate the floor of the sinus tarsi, STJ arthroereisis has developed into a modern generation of implants. The outcomes anticipated from STJ arthroereisis with the modern implants include a decrease in frontal plane heel valgus, an improved medial arch height while bearing weight, a decreased pronatory motion of the STJ, a decreased mid-tarsal joint (MTJ) inversion and eversion, and a spared STJ inversion. Additionally, STJ arthroereisis with the modern implants provide a re-alignment of previously anteriorly displaced cyma line, decreased talo-navicular joint (TNJ) subluxation and talar declination, and an increased calcaneal inclination. 
     Several problems are associated with the modern implants for STJ arthroereisis. The features of modern implants lack positioning control capabilities for use during implantation surgery, which requires precise and controlled manipulation of the implant for final positioning. Additionally, the geometric shape of the modern implants is either cylindrical or conical. Modem cylindrically-shaped implants provide poor anatomical fit with the STJ structure. Modem conically-shaped implants may offer a slightly better fit with STJ anatomical structure than cylindrically-shaped implants, but require apertures along the softened thread surface for post implantation osseous tissue growth to stabilize the implant. 
     Accordingly, the subtalar implant of the present invention overcomes the problems associated with modern implants for STJ arthroereisis. In accordance with the present invention, the subtalar implant is capable of threaded engagement with a positioning element, which is used for positioning control and manipulation of the subtalar implant during surgical implantation. For assistance in final positioning for implantation, the subtalar implant is cannulated to receive a guide rod to facilitate proper positioning of the subtalar implant with a cannulated driver. Once implanted, the subtalar implant of the present invention provides a superior anatomical fit with the STJ anatomical structure, without the need for osseous growth indentations to anchor the subtalar implant. 
     SUMMARY OF THE INVENTION 
     The present invention provides a subtalar implant capable of threaded engagement with a positioning element, which is used for positioning control and manipulation of the subtalar implant during surgical implantation. For assistance in final positioning for implantation, the subtalar implant is cannulated to receive a guide rod to facilitate proper positioning of the subtalar implant with a cannulated driver. Once implanted, the subtalar implant of the present invention provides anatomical fit with the STJ anatomical structure without the need for indentations to receive osseous tissue growth to anchor the subtalar implant. 
     The conically-shaped subtalar implant comprises a low domed distal end, a threaded surface of a concentric v-shaped thread, and a flat proximal end. A cannulation is provided centrally along the longitudinal axis of the subtalar implant. Female threads are provided in a section of the cannulation. At the proximal end of the female threads of the subtalar implant, the cannulation expands through a neck into a fitting recess. The fitting recess expands through a shoulder into a cylindrical recess. 
     The positioning element comprises a cylindrical rod with a male thread at its distal end. The male thread is adapted to engage the female threads in the cannulation of the subtalar implant. 
     During surgical implantation, the male threads of the positioning element engage the female threads of the cannulation of the subtalar implant, and the positioning element is used for positioning control and manipulation of the subtalar implant into the sinus tarsi of a foot during surgery. Once the subtalar implant is properly positioned in the sinus tarsi, the positioning element is twisted to unscrew the male threads from the female threads in the cannulation of the subtalar implant, and the positioning element is removed from the subtalar implant positioned in the sinus tarsi. A head of a driver is positioned in the fitting recess of the subtalar implant, and a guide rod is positioned in a cannulation of the driver and corresponding cannulation of the subtalar implant. The guide rod is used as a point of reference to ensure proper positioning of the subtalar implant as it is being screwed into final position in the sinus tarsi by the driver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exploded view of the subtalar implant and positioning element in accordance with the present invention; 
         FIG. 2  illustrates a sectional view of the side of the subtalar implant engaging the positioning element in accordance with the present invention; 
         FIG. 3  illustrates a perspective view of the distal end of the subtalar implant; 
         FIG. 4  illustrates a perspective view of the side of the subtalar implant; 
         FIG. 5  illustrates a perspective view of the proximal end of the subtalar implant; 
         FIG. 6  illustrates a sectional view of the side of the subtalar implant; 
         FIG. 7  illustrates an exploded view of the subtalar implant, a guide rod and a driver in accordance with the present invention; and 
         FIG. 8  illustrates a partial sectional side view of the subtalar implant engaged with the head of the driver all in line with a guide rod. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 through 8  in which similar reference characters refer to similar parts throughout the several views thereof, the present invention comprises an implant  20  and a positioning element  60  and methods for using the positioning element  60  to position the implant  20  in the sinus tarsi adjacent the subtalar joint (STJ). The correct positioning of the implant  20  with the positioning element  60  in the sinus tarsi is essential for correcting manifestations of problems associated with various types of flat foot disorders. 
     Referring generally to  FIGS. 1-8 , the implant  20  is generally frustoconically shaped. The implant  20  is configured with the external features of a low domed distal end  22  with preferred radii, a threaded surface  32  with v-shaped threads  34 , and a flat proximal end  24  with preferred diameters as shown particularly in  FIGS. 4 and 6 . The v-shaped threads  34  consist of one continuous thread that begins at the low domed distal end  22 , wraps around the implant  20  to provide the threaded surface  32 , and terminates adjacent the flat proximal end  24  of the implant  20 . The v-shaped threads  34  have a leading edge  36  and a trailing edge  38  as shown in  FIG. 6 . Generally, the depth V of the v-shaped thread  34  has a 1.0 mm vertical distance between the trough and the peak of the v-shaped thread  34  as shown in  FIG. 6 . The implant  20  is configured with a less aggressive thread so as not to provide a leading edge with a greater height than a trailing edge, which are presented by threads of a self-tapping screw. All preferred sizes of the implant  20  have a thread pitch P of 2.15 mm. The thread pitch P is the measured distance between two sequential peaks of the v-shaped threads  34  as shown in  FIG. 6 . 
     Referring to  FIG. 6 , the frustoconical shape of the implant  20  is configured such that the angle α between the longitudinal midline and the threaded surface  32  provides a preferred shape for anatomical fit. The angle α provides the implant  20  with a preferred anatomical fit when the implant  20  is positioned in the sinus tarsi adjacent the STJ of a foot. The frustoconical shape of the implant  20  is provided by the preferred embodiments of the proximal end diameter D, angle α, length L, and distal end radius R as shown in  FIG. 6 . 
     The implant of the present invention can have several sizes, such as a proximal end diameter D of between about 4.0 mm to 20.0 mm, an angle α of between about 1.0 degree to 15.0 degrees, a length L of between about 10.0 mm to 20.0 mm, and a distal end radius R of between about 2.0 mm to 5.0 mm 
     Several preferred sizes of the implant  20  are provided by the present invention. In one embodiment, the implant  20  has a proximal end diameter D of 8.0 mm, angle α of 10.0 degrees, length L of 14.6 mm, and a distal end radius R of 3.6 mm. In a second embodiment, the implant  20  has a proximal end diameter D of 9.0 mm, angle α of 8.1 degrees, length L of 14.7 mm, and a distal end radius R of 3.7 mm. In a third embodiment, the implant  20  has a proximal end diameter D of 10.0 mm, angle α of 6.1 degrees, length L of 14.8 mm, and a distal end radius R of 3.8 mm. In a fourth embodiment, the implant  20  has a proximal end diameter D of 11.0 mm, angle α of 4.3 degrees, length L of 14.9 mm, and a distal end radius R of 3.9 mm. In a fifth embodiment, the implant  20  has a proximal end diameter D of 12.0 mm, angle α of 2.1 degrees, length L of 15.0 mm, and a distal end radius R of 4.0 mm. The aforementioned preferred embodiments are provided for a preferred anatomical fit to correct flat foot conditions in various sized patients. 
     Referring generally to  FIGS. 1-8 , the implant  20  is configured with several internal features. One internal feature is a cylindrical recess  26  at the proximal end  24  of the implant  20 . The cylindrical recess  26  is centered along the longitudinal axis of the implant  20 . The cylindrical recess  26  has a diameter that is smaller than the diameter D of the proximal end  24  of the implant  20 . The cylindrical recess  26  has a depth of approximately 2.0 mm into the proximal end  24  of the implant  20 . The cylindrical recess  26  is continuous with a fitting recess  30 . A shoulder  29  is provided with the implant  20 , which delineates the end of the cylindrical recess  26  and the beginning of the fitting recess  30 . 
     Referring generally to  FIGS. 1-8 , the fitting recess  30  of the implant  20  is configured to receive a corresponding fitting head  88  of a driver  80  used in the control and positioning of the implant  20  into and in the sinus tarsi of a foot. The fitting recess  30  is preferably hexagonal with a width of 4.0 mm from flat surface to flat surface of the recess. Other preferred embodiments of the fitting recess  30  are apparent to those skilled in the art and include a star shape, a D shape, a square shape and any other suitable shape. The fitting recess  30  of the implant  20  has a depth of 2.0 mm. The fitting recess  30  is in continuous communication with a female thread  42  of the implant  20 . 
     Referring specifically to  FIGS. 2 and 6 , the female thread  42  of the implant  20  is positioned between the fitting recess  30  and a cannulation  40 . The fitting recess  30  is connected through a neck  28  to the female thread  42  of the implant  20 . The female thread  42  is configured in the circular wall adjacent the cannulation  40  of the implant  20  along a length of 4.0 mm. The female thread  42  can have either a left-hand or right-hand configuration. The female thread  42  is adapted for fit with a corresponding left hand or right-hand configured male thread  68  of the positioning element  60 , which is rod shaped with a proximal end  66  and a distal end  64 . The positioning element  60  may have a handle  62  or other gripping means to provide friction with the hand of an individual positioning the implant  20  during surgery so long as male threads  68  are provided at the distal end  64  of the positioning element  60 . Other embodiments of the positioning element  60  would be apparent to a skilled artisan. 
     Referring specifically to  FIGS. 2 and 6 , a cannulation  40  is provided along the longitudinal axis of the implant  20 . The cannulation  40  provides communication through the low domed distal end  22  along the longitudinal axis to the female thread  42  of the implant  20 . Further communication is provided between the cannulation  40  through the proximal end  24  of the implant  20  by way of the female threads  42 , the fitting recess  30 , and the cylindrical recess  26 . Regardless of the size configuration of a preferred embodiment of the implant  20 , the cannulation  40  has a uniform diameter of 2.0 mm in all embodiments. The cannulation  40  provides a receiving space for a guide rod  82  used during surgery. 
     The implant  20  and positioning element  60  are preferably made of a metal, for example, titanium or stainless steel. 
     During surgery, the implant  20  is positioned in the sinus tarsi adjacent the STJ as an STJ arthroereisis to correct flat foot conditions. The surgical procedure of STJ arthroereisis implantation is known by a skilled artisan. However, for the purposes of disclosing the method of the present invention, the relevant steps of STJ arthroereisis implantation are explained. 
     Prior to implantation of the implant  20 , a single incision is made in the lateral side of the foot of a patient over the sinus tarsi. While incising, it is important to avoid cutting the intermediate dorsal cutaneous nerves and the sural nerve. The deep fascia of the foot should also be incised, and if encountered, the cervical ligament of the foot should be retracted for access to the sinus tarsi. The tarsal canal is dissected rendering palpable access to the calcaneous and talus. The sinus tarsi is minimally dissected. 
     Adjacent the lateral side of the foot, a probing instrument is positioned perpendicular to the lateral wall of the calcaneous and is inserted toward the medial aspect of the foot through the sinus tarsi into the sinus canalis. The tip of the instrument is pushed against the interior surface of the skin along the medial aspect of the foot. The tip of the instrument will present through tenting of the skin along the medial aspect of the foot. An incision is provided at the location of the tenting to provide through and through communication from the lateral side to the medial side of the foot. 
     A plastic implant is screwed into the sinus tarsi to open access to the sinus tarsi and open the sinus canalis. The plastic implant is also used partially as a positioning guide, and as an initial step, the implant is removed before final positioning of the implant  20 . Once access to the sinus tarsi is opened, the male threads  68  of the positioning element  60  are screwed into the female threads  42  of the implant  20 , and the implant  20  is positioned in the sinus tarsi for sizing. An appropriately sized implant  20  is selected for final positioning of the implant  20 , which is controlled by the positioning element  60 . Once the proper implant  20  is selected and positioned, the positioning element  60  is unscrewed from the implant  20 . The guide rod  82  is then inserted through the cannulation  40  of the implant  20  so that the guide rod  82  fully communicates through the incision on the lateral aspect of the foot and through the incision on the medial aspect of the foot. The guide rod  82  is used to position the longitudinal axis of the implant  20  at a suitable angle from the longitudinal bisection of the talus. The distal end  22  of the implant  20  should not cross over the longitudinal bisection of the talus. The distance of the proximal end  24  of the implant  20  from the lateral surface of the calcaneous will vary depending upon the size of the implant  20  and the size of the patient. 
     In the proper position and at a suitable angle from the longitudinal bisection of the talus, the head  88  of the driver  80  (having handle  84  and shaft  86 ) is positioned in the fitting recess  30  of the implant  20  with the guide rod  82  still in position through cannulations  40 ,  81  of the implant  20  and the driver  80  as shown in  FIGS. 7 and 8 . With the head  88  of the driver  80  in the fitting recess  30 , the driver  80  is used to turn the implant  20  either clockwise or counterclockwise into final position. The final position is the position at which the implant  20  acts to significantly reduce excess STJ pronation and other foot movement associated with the various types of flat foot conditions. Once the implant  20  is finally positioned, the driver  80  and the guide rod  82  are removed, and the deep tissue, fascia, subcutaneous and skin layers are closed in a manner that is acceptable in the art. 
     While the present invention is satisfied by embodiments in many different forms, there is shown in the drawings and described in detail herein the preferred embodiments of the invention, with the understanding that the present disclosure is to be considered as exemplary of the principles of the invention and is not intended to limit the invention to the embodiments illustrated. Various other embodiments will be apparent to and readily made by those skilled in the art without departing from the scope and spirit of the invention. The scope of the invention will be measured by the appended claims and their equivalents.