Abstract:
A fastener is provided for fixating two objects in a fixed spatial relationship, such as bone segments, so that resultant stress is maintained at a constant value along a shank portion of the fastener in order to minimize stress concentrations and optimize the strength to size relationship of the fastener, thereby reducing the likelihood of fastener failure. The fastener additionally comprises an improved head which temporarily engages a driving instrument to facilitate fixation. A driving instrument for deploying the fastener is also provided.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This patent application claims priority from U.S. Provisional Patent Application Serial No. 60/131,483, filed Apr. 29, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a fastener for holding two objects in a fixed spatial relationship with respect to one another, and more particularly to a screw-type fastener, and associated driving device, for fixating two bone segments in a fixed relationship. 
     The use of screw fixation to hold bone segments has been established as common practice in the surgical management and treatment of bone fractures. When fastening two segments together, it is often desirable to use a lag screw, having threads engaged in a distal bone segment and a smooth shank engaged in a proximal bone segment. Rotably engaging the fastener develops tension in the fastener assembly by advancing the distal bone segment along the long axis of the fastener until contact is made with the proximal bone segment. As the fastener assembly is further tightened, compressive stress is developed at the mating interface of the two bone segments, which has been shown to assist the fracture healing process. 
     As is well known, any device implanted into the human body, however, causes some type of tissue reaction to the implanted foreign material. For this reason, it is desirable to use the smallest feasible fasteners for fixating bone segments. Also, in some instances, there are definite anatomic limits to the size of fastener which may be employed for a particular condition. 
     One area where the anatomic limit is particularly notable is in anterior odontoid fracture fixation. In this condition, the dens portion of the second cervical vertebra has fractured, resulting in a bone segment being loose within the ring formed by the first cervical vertebra. Normally, the dens acts as a pivot around which the first cervical vertebra rotates. However, this pivot function is lost when the dens is fractured from the body of the second cervical vertebra. In order to restore the ability of the first cervical vertebra to rotate, and to prevent injury to the spinal cord, the dens is surgically accessed by an anterior inferior approach. An angled hole is drilled through the third cervical vertebra starting from the anterior face of the vertebral body, and leading in a path stopping at the inferior endplate of the second cervical vertebra. A smaller screw hole is drilled and then tapped from the caudal endplate of the second cervical vertebra through the center of the fractured dens. A screw, preferably a lag screw, is inserted through the body of the second cervical vertebra and into the fractured dens using some form of driving device. Rotably engaging the screw draws the fractured bone segment to the cephalad surface of the second cervical vertebra. 
     Effectively, the screw acts as a mechanical support for the dens to restore its function of acting as a pivot for the first cervical vertebra. The screw also allows compressive force to be developed at the fracture faces of the dens and the body of the second cervical vertebra, assisting the fracture healing process. 
     Surgical experience shows that standard straight-shank screws of the maximal diameter anatomically allowed often are insufficient for the surgical management of odontoid fractures. Often, the implant itself will fracture during healing due to high bending stresses imparted by the first cervical vertebra on the dens, and thereby in turn on the implant. Failures usually occur in areas of high stress concentrations, such as are developed in straight-shank beams loaded in bending conditions. Surgical experience also shows that, during the treatment of bone fractures and other surgical procedures, implantable fasteners can slip from the driving devices and into the patient. 
     Accordingly, an improved fastener is required to surgically manage odontoid fracture, effectively fixate other structures, and to prevent the accidental entry into the human body. 
     BRIEF SUMMARY OF INVENTION 
     The bone fastener of the present invention addresses and overcomes problems found in the prior art. In one aspect of the invention, a fastener for fixating two bone segments is provided, wherein the fastener has a helical thread portion for engaging a distal bone segment, a shank portion for spanning a proximal bone segment, and a head portion for acting as a stop or brake against the proximal bone segment. Also, a means to drive the fastener into the bone segments is provided. 
     In another aspect of the present invention, the thread portion of the fastener may be any sort of helical screw thread. It may be right-handed, left-handed, a machine thread, a cancellous bone thread, a buttress thread, or any other thread as is known in the art. Though a threaded fastener is described in a preferred embodiment, it is contemplated that the threaded portion may be replaced or augmented by non-threaded fastener means, such as bone hooks or anchors, or expanding barbs, including means which may be developed through the use of shape memory alloys. 
     In yet another aspect of the present invention, the shank portion of the present invention has a circular cross-section with a diameter that varies along the length of the shaft in a manner such that the resultant stress developed due to load applied in the distal bone segment is always a constant value. Maintaining the resultant stress at a constant value minimizes the development of stress concentrations and optimizes the strength vs. size relationship of the implanted component. 
     In a further aspect of the present invention, the head portion of the bone fastener has a diameter larger than the largest diameter of the shank portion of the fastener, so that the head portion acts as a stop against the proximal bone segment. In one present embodiment, the head portion also has a broached hex cavity and a smaller diameter internally threaded hole to facilitate driving and removing the fastener with a unique driving instrument. 
     In a final aspect, a driving instrument has a ball-type hex driver at its tip for engaging the broached hex cavity located in the head portion of the screw fastener. The driving instrument is further cannulated and accommodates a draw rod for engaging the threaded hole of the head portion of the bone fastener. Rotably engaging the draw rod while the ball-type hex is seated into the broached hex develops tension in the draw rod and bone fastener assembly, thereby firmly drawing the bone fastener to the driving instrument. The bone fastener may be driven without fear of it dropping off of the instrument and into the patient. Once the threaded portion of the fastener is engaged with the first bone segment, the draw rod may be removed, thereby allowing the ball-hex driver to drive the fastener at an angle from its centerline. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal view of the bone fastener according to an embodiment of the present invention; 
     FIG. 2 is an axial view of the bone fastener according to an embodiment of the present invention; 
     FIG. 3 is a representative view of the bone fastener according to an embodiment of the present invention engaging a proximal and distal bone segment; 
     FIG. 4A is a perspective view of a bone fastener and driving mechanism according to an embodiment of the present invention prior to engagement of the bone fastener by the driving mechanism; 
     FIG. 4B is a detailed perspective view of FIG. 4A showing, in partial cross section, the head portion of a bone fastener according to an embodiment of the present invention, and the tip of the driving instrument just prior to engagement; 
     FIG. 5 is a view of a bone fastener and driving mechanism according to an embodiment of the present invention, showing the draw rod and ball-type hex driver and the bone fastener engaged for driving or removing; and 
     FIG. 6 is a view of a bone fastener and driving mechanism according to an embodiment of the present invention, showing the draw rod removed for angular driving. 
     FIG.  7 ( a ) depicts an embodiment of the invention wherein the internal cavity is slotted; 
     FIG.  7 ( b ) depicts an embodiment of the invention wherein the internal cavity comprises a generalized open irregular polygonal cylinder having at least three sides; 
     FIG.  7 ( c ) depicts a further embodiment of the invention wherein the internal cavity comprises a generalized open irregular polygonal cylinder having at least three sides; 
     FIG. 8 depicts an embodiment of the invention wherein the shank portion further comprises a non-circular cross-section; 
     FIG.  9 ( a ) depicts an embodiment of the invention wherein the fastener comprises a distal end having a non-threaded fastening mechanism in the form of an expanding barb attached thereto, wherein the expanding barb is depicted in the retracted position; 
     FIG.  9 ( b ) depicts the fastener of FIG.  9 ( a ) wherein the expanding barb is of a shape memory alloy and is depicted in the expanded position; 
     FIG.  10 ( a ) depicts an embodiment of the invention wherein the fastener comprises a non-threaded fastening mechanism further comprising bone hooks; and 
     FIG.  10 ( b ) depicts an embodiment of the invention wherein the fastener comprises a non-threaded fastening mechanism further comprising bone anchors. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of understanding the principles of the invention, references will now be made to the embodiments illustrated in the drawings. For ease of understanding, and for uniformity in structural terminology, the description will be directed to a fastener used in a surgical context. It should be appreciated, however, that the present invention is also contemplated for use in connection with fixation with body structures other than odontoid fixation. Similarly, the present invention also finds use for fixation of structures not found in the human body. 
     Referring now to the drawings, FIG. 1 shows a longitudinal view of a bone fastener  10  according to a preferred embodiment of the present invention. 
     As used herein, the term longitudinal axis shall take on its ordinary meaning in referring to an imaginary axis that runs through the length of the bone fastener  10 . Likewise, as used herein, the term radial dimension shall take on its ordinary meaning in referring to a dimension that is measured orthogonally to the longitudinal axis in a given cross section of the bone fastener. In FIG. 1, the bone fastener  10  is a nonlinear taper lag screw. The bone fastener  10  comprises, generally, a proximal end  30 , a distal end  60 , and a variable diameter shank portion  70  therebetween. 
     The proximal end  30  typically further comprises a head  40 . The head  40  further comprises an internal cavity  42  and a smaller internal threaded portion  44 . 
     The distal end  60  typically further comprises an externally threaded portion  62  further comprising external threads  64 . The external threads  64 , as is known in the art, have associated therewith a minor root and a major root. 
     The shank portion  70  of the preferred embodiment has a circular cross-section that varies in diameter along the length of the shank portion  70  so that the resultant stress due to load applied in distal bone segment  66  is constant. Since the resultant stress remains constant, local stress concentrations decrease, thus greatly reducing the potential for failure of the bone fastener  10 . 
     For the particular case of odontoid fixation described above, the shank portion  70  diameter is determined by the equation: 
     
       
           D ( z )=[ D (0)][(1- z/L ) (⅓)  ] 0 &lt;z&lt;L, D ( z )&gt;0.  (Equation 1) 
       
     
     where, 
     z=the axial coordinate along the long axis of the screw; 
     L=the length of the bone fastener  10  in the current embodiment; and 
     D(z) is the diameter of the shank portion  70  in the current embodiment at a given coordinate, z. 
     It is contemplated that in some variations of the present invention, the variable L represents the length of only the variable diameter shank portion  70  of the bone fastener  10 . In the preferred embodiment, z=0 at the head  40  of the bone fastener  10 , and z=L at the most distal portion of the bone fastener  10 . In the alternative variations of the present invention, z= 0  at the head  40  of the bone fastener  10 , and z=L at the most distal end of the shank portion  70 . 
     From Equation 1: 
     
       
         1 im D ( z ) z→L =0   (Equation 2) 
       
     
     Equation 2 indicates that the actual varying diameter is carried out functionally only to the diameter of the minor root of the external threads  64 . 
     It is further contemplated that in some variations of the present invention, a fastener with a non-circular cross section will replace the diameter variable with another appropriate dimension, such as the distance across flats in a hexagonal cross section, or the base or height in a rectangular cross section, the major or minor axes of an elliptical section, and so forth. One skilled in the art would readily appreciate that the form of Equation 1 would be appropriately modified for the non-circular cross section employed. 
     FIG. 2 shows an axial view of the bone fastener  10  of the preferred embodiment. As illustrated, the head  40  contains an internal cavity  42  and an internal threaded portion  44 . The internal cavity  42  defines, generally, a cavity or pocket taking the shape of an open polygonal cylinder. By this is meant that the open cylinder that is the cavity appears, in axial view, as a polygon. Referring to FIGS. 1 and 2, it can be seen that the internal cavity  42  further comprises at least two sidewalls  68 . If the internal cavity  42  takes the shape of a slot, the internal cavity  42  will have two sidewalls  68 . If the internal cavity  42  takes the shape of a cube or rectangular box, it will have four sidewalls  68 . In the preferred embodiment depicted in FIGS. 1 and 2, the internal cavity  42  takes the shape of a hexagonal polygonal open cylinder, and therefore has six sidewalls  68 . It is to be appreciated that the cavity further could take the shape of any regular open polygonal cylinder having at least three sides. Furthermore, the cavity could take the shape of any non-regular open polygonal cylinder, such as a star, or further any other irregular shape, and could also contain sidewalls  68  having differing dimensions. 
     FIGS. 1 and 2 also show that the head  40  of the preferred embodiment further has an internal threaded portion  44 . The internal threaded portion  44  is used primarily to threadedly engage the bone fastener  10  with a driving instrument  100  in preparation of implantation, as will be described below. Once the bone fastener  10  and the driving instrument  100  are threadedly engaged, the bone fastener  10  will not accidentally fall into the patient. 
     FIG. 3 is a schematic representation of a longitudinal section of an embodiment of the bone fastener  10  of the present invention as implanted. The externally threaded portion  62  engages distal bone segment  66 , while the shank portion  70  spans a proximal bone segment  67 . Because the largest diameter of the head  40  of the bone fastener  10  is greater than the largest diameter of the shank portion  70 , the head  40  seats against a surface  65  of the proximal bone segment  67 . The bone fastener  10  can be tightened until the proximal bone segment  67  makes contact with the distal bone segment  66  at mating interface  69 . After contact is made, the bone fastener  10  can be tightened further to create increased compressive stresses at mating interface  69  to facilitate the fracture healing process. The shape of the head  40  can be of many configurations and curvatures, including conical, tapered, non-linearly tapered, spherical, and any other configurations known in the art. 
     FIG. 4A is a perspective view of a driving instrument  100  for deploying the bone fastener  10 . The driving instrument  100  has a cannulated shaft portion  110 , a driving end  120  and a draw rod  130 . The driving end  120  has a broached ball-type geometric portion  122  for engaging the internal geometric receiving portion of the head  40  of the bone fastener  10 . The draw rod  130  typically has an external threaded portion  132 , a shaft portion  134 , and an activation portion  136 . The draw rod  130  of the driving device  100  can additionally be fitted with a retention mechanism (not shown) to prevent the draw rod  130  from sliding out of the cannulated shaft portion  110  when the driving mechanism  100  is turned upside down. Such retention mechanisms can include, but are not limited to, ball detentes (comprising a ball and spring combination attached to either the cannulated shaft portion  110  or the draw rod  130 , and a corresponding recess in the draw rod  130  or the cannulated shaft portion  110 ), or annular elastomeric rings attached to the draw rod  130  or the cannulated shaft portion  110 . The external threaded portion  132  of the draw rod  130  is primarily used for engaging the internal threaded portion  44  of the bone fastener  10 . The shaft portion  134  of the draw rod  130  has an external diameter that is smaller than the internal diameter of the cannulated shaft portion  110  of the driving instrument  100 . This facilitates the easy removal of the draw rod  130  from the driving instrument  100 . The activation portion  136  of the draw rod  130  allows the user to rotably engage and disengage the bone fastener  10  to and from the driving instrument  100 . The broached ball-type geometric portion  122  at the driving end of the driving instrument  100  engages the internal cavity  42  of the bone fastener  10  for driving the bone fastener  10 . 
     FIG. 4B is a detailed view of the driving end  120  of the driving instrument  100  and the head  40  of the bone fastener  10 . As can be seen, the broached ball-type geometric portion  122  of the driving instrument  100  engages the internal cavity  42  of the head  40  of the bone fastener  10 . In this manner it is readily apparent how other various geometries can be implemented for the broached ball-type geometric portion  122  of the driving instrument  100  and the internal cavity  42  of the head  40  of the bone fastener  10 . 
     FIG. 5 shows a cut-away view of the driving instrument  100  of FIGS. 4A and 4B engaged with the bone fastener  10 . The external threaded portion  132  of the draw rod  130  is engaged with the internal threaded portion  44  of the bone fastener  10 . The broached ball-type geometric portion  122  of the driving end  120  of the driving instrument  100  is engaged with the internal cavity  42  of the head  40  of the bone fastener  10 . 
     FIG. 6 shows a cut-away view of the driving instrument  100  engaged with the bone fastener  10 . The external threaded portion  132  of the draw rod  130  has been disengaged from the internal threaded portion  44  of the head  40  of the bone fastener  10 , and the draw rod  130  itself has been removed from the driving instrument  100 . In FIG. 6, the driving instrument  100  is shown disposed at an angle of inclination (angle of approach) with respect to the longitudinal axis of the bone fastener  10 . In other words, the angle of inclination between the longitudinal axes of the driving instrument  100  and the bone fastener  10  is greater than zero. Despite the angle of inclination, however, it can be seen that the broached ball-type geometric portion  122  of the driving instrument  100  remains engaged with the internal cavity  42  of the head  40  of the bone fastener  10 . This arrangement allows the bone fastener  10  to be driven into the bone structures at varying angles of inclination. 
     Referring to FIGS. 2,  4 ,  5 , and  6 , it is readily seen that the internal cavity  42  receives the driving end  120  of the driving instrument  100  in such a way that the driving instrument  100  can rotate the bone fastener  10  despite being inclined with respect to the longitudinal axis of the bone fastener  10 . In the preferred embodiment, the internal cavity  42  is hexagonal. However, any geometric shape is possible for the internal cavity  42 , including slotted, triangular, quadrangular, pentagonal, hexagonal, septagonal, octagonal, nonagonal, pentagonal, and n-tagonal, where n represents the number of sides of a regular polygon greater than  10 . In addition, it is readily apparent to one skilled in the art that all other polygons, as well as any irregular shapes for the internal cavity  42  also come within the ambit of the present invention. The figures also show that the head  40  of the preferred embodiment further has an internal threaded portion  44 . The internal threaded portion  44  is used primarily to threadedly engage the bone fastener  10  with the driving instrument  100  in preparation of implantation. Once the bone fastener  10  and the driving instrument  100  are threadedly engaged, the bone fastener  10  will not accidentally fall into the patient. 
     The bone fastener  10  can be made from virtually any material used in making devices and articles to be implanted in the human body. Often, the fastener will be made from a radiopaque material so that during imaging, the fastener can be readily observed. It is contemplated, however, that the fastener can also be made from radiolucent materials that are not readily observed during imaging. Likewise, some combination of radiopaque and radiolucent materials may be employed in order to create a marker that can be readily observed during imaging. 
     In the preferred embodiment of the present invention described above, the bone fastener  10  is described having a circular cross section. However, it would be readily apparent to one skilled in the art that the present invention is readily adaptable to fasteners having other polygonal cross sections, as well as irregular cross sections. Equation 1 need only be modified to apply to the particular non-circular cross section chosen. 
     Furthermore, in the preferred embodiment described above, the broached ball-type geometric portion  122  is hexagonal. However, any geometric shape may be possible for the broached geometric ball-type portion  122 , including flat, triangular, quadrangular, pentagonal, hexagonal, septagonal, octagonal, nonagonal, pentagonal, and n-tagonal, where n represents the number of sides of a regular polygon greater than  10 . In addition, it is readily apparent to one skilled in the art that irregular or other geometries for the broached ball-type geometric portion  122  also come within the ambit of the present invention. 
     Having described the structure of the bone fastener  10  of the preferred embodiment, the use of the bone fastener  10  can now described. Again, a surgical example is chosen for illustrative purposes, it being understood that non-surgical applications are equally applicable. Once the proper surgical preparations have been made, the user selects a bone fastener  10  of the appropriate size, material, and physical specifications. The draw rod  130  is inserted into the cannulated shaft portion  110  of the driving instrument  100  so as to engage the retaining mechanism (not shown), if any. Holding the driving instrument  100  and draw rod  130  in one hand, the user then inserts the broached ball-type geometric portion  122  of the driving end  120  of the driving instrument  100  into the internal cavity  42  of the head  40  of the bone fastener  10 . Then, by rotating the activating portion  136  of the draw rod  130 , the user engages the external threaded portion  132  of the draw rod  130  with the internal threaded portion  44  of the head  40 . The user can continue rotably engaging the bone fastener  10  to the driving end  120  of the driving instrument  100  until the bone fastener  10  is securely engaged thereto. 
     Now, the bone fastener  10  is effectively an extension of the driving instrument  100 . The user is able to drive the bone fastener  10  into the material (for example, into the proximal bone segment) without fear of the bone fastener  10  falling off of the driving instrument  100 . In addition, once the bone fastener  10  has been started in the material, or at any point thereafter, the draw rod  130  can be removed from the driving instrument  100  by rotably disengaging the external threaded portion  132  from the internal threaded portion  44  of the head  40 . The bone fastener  10  may be further driven into the material by rotably engaging the broached ball-type geometric portion  122  with the internal cavity  42  of the head  40 . In addition, because of the broached ball-type geometric portion  122 , the bone fastener  10  may be driven into the material at an angle of inclination from the longitudinal axis of the bone fastener  10 . This facilitates driving the bone fastener  10  in locations or conditions requiring, for whatever reason, an angle of approach greater than zero.