Abstract:
An orthopedic implant device and system support the fusion of bones in the medial column of the human foot. An exemplary implant uses an improved lag screw and supplemental components, including a washer plate and transverse screws which penetrate transverse through holes in the shaft of the lag screw. Additionally, a surgical procedure is described which may employ such an implant. The improved lag screw may be inserted at the plantar surface of the first metatarsal and support the fusion of, for example, the transverse tarsal joint, the cuneonavicular joint, and the tarsometatarsal joint.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of U.S. Provisional Patent Application No. 62/027,639, filed Jul. 22, 2014, the complete contents of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention generally relates to the field of orthopedic implant devices and, more particularly, surgical solutions for midfoot arthrodesis. 
       BACKGROUND 
       [0003]    Midfoot arthrodesis, which involves the fusion of bones in the medial column, is a common treatment for a number of foot deformities and conditions. Conventional lag screws are a common surgical implant device employed for midfoot athrodesis. A surgical lag screw generally comprises a head and a shaft which is partially threaded at the end which is opposite the head. A portion of the shaft immediately adjacent the head is generally smooth and without threads. For purposes of bone fusion, a lag screw generally works by engaging the threaded portion of the shaft into a bone and, in combination with the head of the screw, compressing the bone in which the threads are imbedded against a neighboring bone through which the smooth portion of the screw shaft passes. The compressive forces between the bones is referred to as the lag effect. Lag screws may be cannulated with a hollow center. This permits alignment with a guide wire or guide pin. 
         [0004]    IO Fix (trademark of Extremity Medical) is a surgical implant generally comprising a pair of screws, with a first screw imbedded in a single bone and a second screw imbedded in two adjacent bone structures, supplying a lag effect therebetween. The first screw gives support to the second screw and changes the distribution of compression forces. 
         [0005]    The Midfoot Fusion Bolt (by DePuy Synthes) is an intramedullary implant that can be used to fuse the medial metatarsocuneiform, naviculocuneiform, and talonavicular joints. The implant generally comprises a solid bolt which is inserted through the upper center of the first metatarsal head close to the dorsal cortex. 
         [0006]    US Patent App. Pub. No. 2010/0256639 A1 describes an intramedullary fixation assembly having a first member and a second member. After insertion into the bone, the first and second members are held in place by pairs of retaining screws which pass through the first and second members. 
         [0007]    In spite of existing solutions, post-operative problems still exist for some patients after a midfoot arthrodesis procedure. The main problems with diabetic Charcot medial column fusion are non-rigid fixation and wound infection with prominent hardware. Furthermore, many existing solutions are undesirable and cost prohibitive. Despite the surgery&#39;s intent to rigidly fix the bones for which fusion is desired, small and potentially deleterious movement does sometimes still occur between the bones, inhibiting or preventing the body&#39;s induced fusion by ossification. This arises from limitations of the existing surgical implants. Traditional lag screws, for example, may sometimes rotate or shift and loosen the compressive forces between the bones. In addition, many of the existing solutions do not allow for fusion of more than two bones. As a result, it is not uncommon for several implants, e.g. several conventional lag screws, to be needed for the fusion of three or more bones in the midfoot. 
       SUMMARY 
       [0008]    An orthopedic implant device and system are described which support the fusion of bones in the medial column of the human foot. An exemplary implant uses an improved lag screw and supplemental components, including a washer plate and transverse screws which penetrate transverse through holes in the shaft of the lag screw. Additionally, a surgical procedure is described which may employ such an implant. The improved lag screw may be inserted at the plantar surface of the first metatarsal, a novel insertion point over existing solutions for midfoot arthrodesis. In an exemplary embodiment, an implant is provided which supports the fusion of the transverse tarsal (talonavicular) joint, the cuneonavicular joint, and the tarsometatarsal joint of the midfoot. 
         [0009]    No other system currently on the market has an insertion point on the plantar surface of the first metatarsal, contains through-holes as described herein, or includes a washer plate on the first metatarsal. The proposed device, system, and methods simplify the process of installing compression along Meary&#39;s Line. They may be considered an improvement to the standard lag screw. Generally, exemplary embodiments provide a more rigid configuration with interlocked fusion sites and are not prominent as in a plate/screw construct which can be compromised with wound breakdown. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIGS. 1A-1C  show different views of an exemplary lag screw; 
           [0011]      FIG. 2  shows an exemplary washer plate; 
           [0012]      FIGS. 3A and 3B  shows an exemplary orthopedic implant installed in a midfoot for arthrodesis of the transverse tarsal joint, the cuneonavicular joint, and the tarsometatarsal joint; 
           [0013]      FIGS. 4A and 4B  show elements of an exemplary guide for drilling and installing transverse screws in a lag screw; 
           [0014]      FIGS. 5A and 5B  show a guide in use with an implant; and 
           [0015]      FIG. 6  shows a surgical method of midfoot arthrodesis. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring now to the figures, and more particularly  FIGS. 1A-1C , a lag screw  101  is provided which, after implantation, penetrates two or more bones of one or more joints and induces compressive forces therebetween which are required for fusion. In some exemplary embodiments, the lag screw  101  is of sufficient length to penetrate the bones of one or more of the transverse tarsal (talonavicular) joint, the cuneonavicular joint, and the tarsometatarsal joint. As used herein, “penetrate” describes insertion of one object into another. Penetration may be only partial, in that the penetrating element is inserted into the penetrated element without reemerging at another location. Alternatively, penetration may be such that the penetrating element passes through the penetrated element such that it enters at one location and emerges at a second location of the penetrated element. In the case of a lag screw  101  which is linear, penetration through a bone means the lag screw enters one side of the bone and exits at an opposite side of the bone. On the other hand, if a lag screw  101  simply penetrates a bone, it may pass entirely through the bone or only penetrate up to a certain depth, the inserted end remaining imbedded at some depth within the bone. 
         [0017]    As illustrated in  FIGS. 1A-1C , a lag screw  101  includes a head  104  at end  102   a  and a shaft  106  extending from the head  104  to end  102   b.  The end  102   a  may sometimes be referred to as the distal end of the lag screw  101 , and the end  102   b  may sometimes be referred to as the proximal end of the lag screw  101 . The use of “proximal” and “distal” may correspond to the placement of the device in a patient&#39;s body, where the proximal end is the portion closest to the body proper and the distal end is the portion further away. Generally, a shaft  106  will have an unthreaded portion  106   a  adjacent to the head  104  and a threaded portion  106   b  at or near the end  102   b.  The lag screw  101  may be made with a material which includes tantalum and/or other elements or chemicals which encourage in-growth. The dimensions indicated in the drawings are by way of example and are not intended to be limiting. The core diameter  108  and the length  110  of the shaft may vary, especially in order to accommodate the different sized anatomies of different patients. For example, the core diameter  108  and length  110  of a shaft for a child&#39;s implant may be smaller than those for an adult&#39;s implant. As one specific example, a lag screw  101  may have a total length of 100 mm with 25 mm of threading and may furthermore be cannulated for a 1.6 mm diameter k-wire. Transverse through holes (e.g., 3 mm in diameter) may be positioned in locations where transverse screws will be inserted into the middle of each bone. These example measurements were taken from x-ray scans. Variations of screw length and transverse through hole positions/placement may be used to create a large variety of lag screws for use in a broad range of patients. In addition, a length  110  may also vary according to the number of joints which are to be fused (e.g. a shorter length  110  may be used if fusing fewer joints). For example, in different embodiments, one joint, two joints, or three joints may be fused. 
         [0018]    To create a higher level of fusion stability, the lag screw  101  may have one or more transverse through holes  103  in key locations which permit installation of one or more transverse screws  105 . The transverse through holes  103  are spaced away from the end  102   a  towards the end  102   b  of the lag screw  101 . In some exemplary embodiments, at least one transverse screw  105  is installed in each bone of the two or more bones which are to be fused. The transverse through holes  103  may be arranged at any non-zero angle with respect to a center longitudinal axis  107  of the lag screw  101  or, more particularly, of the shaft  106  thereof. In an exemplary embodiment, the transverse through holes  103  are perpendicular to the center longitudinal axis  107  of the shaft  106 . This arrangement provides advantages such as easier alignment of the transverse screws  105  with the transverse through holes  103  when the transverse screws  105  are being installed in the transverse through holes  103 . 
         [0019]    The transverse screws  105  help ensure the lag screw  101  does not loosen over time by locking. In some exemplary embodiments, a lag screw  101  has a plurality of transverse through holes  103 . Generally, an individual transverse screw  105  may be arranged in each of the transverse through holes  103 . However, it is not required in all embodiments that each transverse through hole  103  have a transverse screw  105  installed. As an example, a lag screw  101  may have three transverse through holes  103 , such as is shown in  FIGS. 1A and 1B , and yet any combination of one, two, or three such transverse through holes  103  may have a transverse screw  105  installed. A transverse screw  105  is preferably solid to avoid breaking. 
         [0020]    In some exemplary embodiments, a lag screw  101  is cannulated, as shown by the cannula passage  112  shown in  FIG. 1C . This permits the use of a guide wire (e.g. a k-wire) or a guide pin for drilling and inserting the lag screw  101 . 
         [0021]    In order to achieve greater compression across the joints as well as an even level of compression on the proximal head of the first metatarsal, a washer plate  201  such as is shown in  FIG. 2  may be arranged between the head  104  of the lag screw and the surface of the first metatarsal. Generally, a washer plate  201  may include an inferior hole  203  and a superior hole  204 . The inferior hole  203  is sized such that the shaft  106  of the lag screw  101  may pass through but the head  104  cannot. That is to say, the diameter of the inferior hole  203  must be at least as large as the core diameter  108  of the shaft of the lag screw and should be smaller than a diameter of the head  104 . A countersink, counterbore, or other depression configured to receive an underside of the head  104  may be provided with the inferior hole  203 . A superior hole  204  may be included with a position displaced and at an angle with respect to the inferior hole  203 , such that when the washer plate  201  is fitted to a plantar surface of the first metatarsal, the center axis of the inferior hole  203  is directed toward the adjacent bone (i.e. the medial cuneiform) while the center axis of the superior hole  204  is directed toward the diaphysis of the first metatarsal. This configuration permits installation of a screw in the diaphysis of the first metatarsal at the superior hole  204 . The diameter of the superior hole  204  should be smaller than the head of the screw installed in the diaphysis to prevent the head from passing through the washer plate  204 . The size of a washer plate  201  may be optimized to ensure sufficient distribution of compressive forces along the washer plate to bone interface to prevent bone from the first metatarsal breaking off from excessive loading. In an alternative exemplary embodiment, a bored out coned washer may be used instead of a washer plate  204 . 
         [0022]      FIGS. 3A and 3B  show an implant which has been arranged in a midfoot. In this exemplary example, a lag screw  101  has been inserted into a hole which penetrates each of the first metatarsal  301 , the medial cuneiform  303 , the navicular  305 , and the talus  307 . As a result, this exemplary implant gives midfoot arthrodesis of the transverse tarsal joint, the cuneonavicular joint, and the tarsometatarsal joint. A washer plate  201  is fitted on a plantar surface of the first metatarsal  301 . The lag screw  101  penetrates through each of the first metatarsal  301 , the medial cuneiform  303 , and the navicular  305 . In this example, only unthreaded portion  106   a  of the shaft  106  is located within the through hole drilled through these three bones  301 ,  303 , and  305 . In contrast, the entirety of the threaded portion  106   b  is embedded in and engaged with the talus  307 . The lag screw  101  is inserted and screwed into place in such a manner that the washer plate  204  and all four bones  301 ,  303 ,  305 , and  307  are pulled toward one another, creating compressive forces between abutting surfaces. The compressive forces between two bones stimulate ossification and thus fusion at the sites of compression. As shown in  FIGS. 3A and 3B , transverse through holes  103  are spaced along the shaft  106  such that at least one transverse through hole  103  is positioned inside of each of the bones  303 ,  305 , and  307 . As a result, at least one transverse screw  105  may be inserted into each of these bones. Alternatively, just one or any combination of two transverse screws  105  may be inserted into any of the three transverse through holes  103  and the bone corresponding thereto. The transverse screws  105  should have a length sufficient to at least partially be present in a transverse through hole  103  and at least partially be present in one of the bones  303 ,  305 , and  307 . In some exemplary embodiments, one or more transverse screws  105  may be passed through transverse through holes  103  such that they are at least partially imbedded in bone tissue to each side of a transverse through hole  103 . 
         [0023]    Because the lag screw  101  must be fully inserted into the two or more bones of the joints being immobilized prior to installing the transverse screws  105 , alignment of the transverse screws  105  at the external surfaces of the bones with the transverse through holes  103  hidden inside of the bones may be challenging. To assist in alignment of the screws  105  with the holes  103 , a guide  401  may be provided which identifies positions of the transverse through holes  103  with respect to the two or more bones. “Position” as used here may include longitudinal location along longitudinal axis  107  as well as rotational orientation about longitudinal axis  107 . In an exemplary embodiment, a guide  401  such as is shown in  FIGS. 4A and 4B  includes a plate  403  with one or more guide holes  405  the centers of which are spaced apart identically to the spacing of the transverse through-holes  103  in the lag screw  101 . An arm  407  is extendable from the plate  403  to the head  104  of the lag screw  101 . In one embodiment, the arm  407  has a hexagonal cross section which fits into a hexagonal inset in the head  104 . The head  104  and the transverse through holes  103  of the lag screw  101  have a fixed relationship to one another, typically because the lag screw  101  may be formed as one integral piece. More specifically, each transverse through hole  103  has a fixed longitudinal distance from head  104  and a fixed rotational orientation such that rotation of the head  104  an arbitrary number of degrees about the longitudinal axis  107  likewise gives a rotation of the transverse through holes  103  about the longitudinal axis  107  by exactly the same number of degrees. As a result, the position (e.g. both longitudinal location and rotational orientation) of the head  104  in three dimensional space and with respect to the bones may be used to ascertain the position of the transverse through holes  103  in three dimensional space and with respect to the bones. The guide  401  is configured to use this relationship between the head  104  and the transverse through holes  103 . Once in place, the guide  401 , using the position of the head  104 , aligns the guide holes  405  with the transverse through holes  103  such that aligned pairs are substantially coaxial. Guide sleeves  410  may be inserted into the guide holes  405  to facilitate guiding drilling for and installation of transverse screws  105 . 
         [0024]      FIGS. 5A and 5B  show a guide  401  in use with a lag screw  101 ′ which is not presently inserted in a midfoot ( FIG. 5A ) and a lag screw which is inserted in a midfoot ( FIG. 5B ). As shown, a drill bit  502  is aligned by the guide  401  for drilling through bone tissue along a center axis of a transverse through hole  103  of the lag screw  101 ′. Note that lag screw  101 ′ differs from a lag screw  101  as shown in  FIGS. 1A-1C  according the length of threaded portion  106   b  as compared to unthreaded portion  106   a.    
         [0025]      FIG. 6  shows a method  600  of midfoot arthrodesis of one or more joints comprising two or more bones which include a first metatarsal in a foot. To install a lag screw  101  in a foot, incisions are generally first made on the foot in order to access the bones, e.g. the talus, navicular, medial cuneiform, first metatarsal, and the respective joints connecting these bones (step  601 ). These incisions will generally be made on the medial and plantar aspects of the foot and are used to arrange the bones (step  602 ). Arrangement of the bones includes arranging their positions relative to one another and removing soft tissue that would interfere with fusion. Once the bones are arranged, the insertion point for the lag screw may be prepared (step  603 ). To accomplish this, an incision may be made on the plantar surface of the foot slightly distal to the proximal head of the first metatarsal, and any material between the opening and the proximal head should be cleared out of the way. The plantar surface of the first metatarsal will serve as the insertion point of the lag screw  101 . 
         [0026]    At this point a hole may be drilled through the plantar surface of the first metatarsal penetrating each of the two or more bones (step  604 ). This may include first drilling a proposed line of action, originating at the plantar surface of the proximal head of the first metatarsal and crossing, for example, the transverse tarsal joint, the cuneonavicular joint, the tarsometatarsal joint, and ending in the talus. Then a k-wire may be installed, followed by a subsequent drilling or reaming to enlarge the hole to match the core diameter  108  of the lag screw  101 . Once the hole has been cleared, a washer plate  201  may be fitted to the plantar surface of the first metatarsal (step  605 ). When this is done, some of the plantar surface of the first metatarsal near the proximal head may be shaped (e.g. bone tissue removed therefrom) in order to provide a flat surface for the washer plate  201  to be stable and evenly engaged with the metatarsal when compression has been induced. Once fitted, the superior hole of the washer plate  201  may have a standard screw drilled through it, engaging the diaphysis of the metatarsal due to the larger amount of cortical bone present (step  606 ). This bicortical screw helps ensure that the washer plate  201  does not loosen over time and may improve fusion and compression as the entire first metatarsal will be pulled during compression to fuse with the surface of the medial cuneiform. 
         [0027]    Next, the lag screw  101  will be placed through the inferior hole of the washer plate  201  and inserted into the hole penetrating the bones, the insertion including rotation (e.g. clockwise) to engage the threading in the bone (step  607 ). Rotation is generally performed until two-finger tightness has been obtained from the compressive forces caused by the lag effect being transferred from the talus to the washer plate  201 . When inserting the lag screw it is preferred that rotation be completed such that at its completion, the transverse through holes  103  line-up perpendicular to the median sagittal plane of the foot. 
         [0028]    Next, transverse screws  105  will be installed in the transverse through holes  103  (step  608 ). This may be done using the incisions made earlier to arrange the bones for compression (steps  601 / 602 ). In some exemplary embodiments, a correctly placed guide  401  will lay parallel with the median sagittal plane of the foot, in line with the transverse through holes  103  of the inserted lag screw  101 . The fixed positions of the transverse through-holes  103  with respect to the head  104  helps ensure that the transverse through holes  103  and the guide holes  405  of the guide  401  will be aligned even if the transverse through holes  103  are not directly perpendicular with the median sagittal plane. With the guide  401  in place, transverse screws  105  may be installed through any one or more of the transverse through holes  103  of the lag screw  101 . X-ray in surgery may be used to verify the placement and penetration depth of the transverse screws  105  (e.g. to verify a bicortical nature, meaning that a transverse screw  105  goes through one bone cortex (i.e. the medial cortex of the bone) and then engages the cortex on the other side of the bone (i.e. lateral cortex, in a sagittal plane reference frame)). Generally, the heads of the transverse screws placed in the transverse through holes will sit on the surface of whatever bone they are installed in. It is desirable that the transverse screws penetrate into but not totally through the bone (e.g. the talus, navicular, or medial cuneiform). After the lag screw  101  and the transverse screws  105  are fully installed, the implant is complete and the incisions may be closed (step  609 ). 
         [0029]    While exemplary embodiments of the present invention have been disclosed herein, one skilled in the art will recognize that various changes and modifications may be made without departing from the scope of the invention as defined by the following claims.