Abstract:
An interference fit screw driver has a polygonal driving surface to impart torque to a corresponding surface in a screw. The end of the polygonal driving surface has a frusto-conical shaped gripping member which fits into an aperture in the screw to create a friction fit between the screw and screw driver. This allows the screw to be attached to the screw driver to facilitate installation of the screw. The friction fit is tight enough to hold the screw to the driver, yet allows the driver to be easily disengaged from the screw by pulling the driver away from the screw once the screw is lodged in place. The interference fit screw driver is particularly useful in turning bone screws into the spine of a patient during orthopedic surgery.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. The Field of the Invention  
           [0004]    The present invention relates generally driving tools, and more particularly, but not necessarily entirely, to screw drivers capable being attached to screws by an interference fit for use in orthopedic surgery, to turn bone screws into the spine of a patient.  
           [0005]    2. Description of Related Art  
           [0006]    A typical screw driver known in the art has a driving surface or bit which mates with a complementary recess on a screw to allow the screw driver to impart torque to the screw. An area of concern related to screw drivers has been retaining the screw on the bit of the driver while positioning the screw. This aspect of screw drivers is particularly important in the medical profession, for example, as bone screws are inserted into the spine of a patient during orthopedic surgery.  
           [0007]    Screw drivers known in the prior art have used various techniques to attach the screws to the bits. For example, U.S. Pat. No. 4,970,922 (granted Nov. 20, 1990 to Krivec) discloses a rotatable driving tool having a plurality of substantially circularly helical driving portions with small helix angles. The driving portions mate with lobes of the fastener recess but are slightly inclined with respect to the lobes. This provides a wedge fit to retain the fastener with the tool. However, the inclined driving portions allow the driver to efficiently apply torque only in one direction. If torque is applied to the fastener in the opposite direction, the inclined driving portions force the driver out of engagement with the lobes of the fastener. Thus, a reverse driver may be required to remove the screw.  
           [0008]    Another approach has been to use a tapered bit on the driving tool which is adapted to wedge into the recess of the screw. For example, U.S. Pat. No. 4,269,246 (granted May 26, 1981 to Larson et al.) discloses a driver bit with multiple lobes. The lobes are tapered axially, converging toward the tip end of the driver. The driver bit enters a socket of the fastener to a predetermined depth before wedging the fastener to the driver. A disadvantage of this arrangement is that the bit engages the fastener only at the outer end of the socket. This results in inefficient transfer of the torque from the driving member to the fastener. Also the concentration of force at one contact location tends to wear and deform the socket in the contact region. Furthermore, close tolerances are necessary in order to provide the proper wedge fit in a consistent manner.  
           [0009]    U.S. Pat. No. 5,277,531 (granted Jan. 11, 1994 to Krivec) discloses another technique for attaching a fastener to a tool. This patent discloses a polygonal shaped tool fitting a socket recess formed in a fastener. The recess has planar drive surfaces alternating with sloping retaining surfaces. The polygonal tool is wedged in contact with the sloping retaining surfaces to retain the fastener on the tool. This configuration also reduces the contact area between the tool and the fastener. Furthermore, if significant torque is applied to the fastener, the retaining surfaces may become tightly wedged to the tool making it difficult to release the fastener.  
           [0010]    An additional type of retention technique is the use of a magnetized bit on the driving tool. However, this type of retention is only useful in screws formed of magnetic material.  
           [0011]    The prior art is thus characterized by several disadvantages that are addressed by the present invention. The present invention minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.  
           [0012]    In view of the foregoing state of the art, it would be an advancement in the art to provide an interference fit screw driver which is simple in design and manufacture which is attached to a screw by a friction fit to facilitate positioning the screw. It would be a further advancement in the art to provide such a screw driver which is capable of applying torque in two directions and which engages the screw over a large surface area such that wear and deformation of the screw and screw driver are reduced, and which can be released from the screw even after a large torque has been applied to the screw by the screw driver. It would be an additional advancement in the art to provide a screw driver which is capable of retaining contact with screws made of nonmagnetic materials.  
         BRIEF SUMMARY AND OBJECTS OF THE INVENTION  
         [0013]    It is therefore an object of the present invention to provide an interference fit screw driver which is simple in design and manufacture.  
           [0014]    It is an additional object of the present invention to provide a screw driver which is capable of driving a screw by an interference fit.  
           [0015]    It is a further object of the present invention to provide an interference fit screw driver which is attached to a screw by a friction fit to facilitate positioning the screw.  
           [0016]    It is another object of the present invention to provide such a driver which is capable of applying torque in two directions.  
           [0017]    It is a further object of the present invention, in accordance with one aspect thereof, to provide a screw driver which engages the screw over a large surface area such that wear and deformation of the screw and screw driver are reduced.  
           [0018]    It is an additional object of the invention, in accordance with one aspect thereof, to provide an interference fit screw driver which can be released from the screw even after a large torque has been applied to the screw by the screw driver.  
           [0019]    It is another object of the present invention to provide a screw driver which is capable of retaining contact with screws made of nonmagnetic materials.  
           [0020]    The above objects and others not specifically recited are realized in a specific illustrative embodiment of an interference fit screw driver. The device preferably includes a body having a proximal end and a distal end. The distal end of the body preferably has a polygonal driving means for driving the driven member. The device also includes a gripping means for gripping the driven member to attach the driven member to the tool. The gripping means preferably has a substantially frusto-conical shape and may be disposed on the end of the driving means to fit into an aperture in the driven member. The gripping means wedges against an edge of the aperture to attach the driven member to the tool with a friction fit.  
           [0021]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention without undue experimentation. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The above and other objects, features and advantages of the invention will become apparent from a consideration of the subsequent detailed description presented in connection with the accompanying drawings in which:  
         [0023]    [0023]FIG. 1 is a side view of a screw driver and screw made in accordance with the principles of the present invention, the upper portion of the screw being a cross-sectional view;  
         [0024]    [0024]FIG. 2 is an end view of the screw driver of FIG. 1.  
         [0025]    [0025]FIG. 3 is a break-away cross-sectional view of an alternative embodiment of the screw driver and screw of FIG. 1.  
         [0026]    [0026]FIG. 4 is an end view of the alternative embodiment of the screw driver of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    For the purposes of promoting an understanding of the principles in accordance with the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications of the inventive features illustrated herein, and any additional applications of the principles of the invention as illustrated herein, which would normally occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention claimed.  
         [0028]    Referring now to FIG. 1, a side view is shown of an exemplary embodiment of a screw driver  10 , also referred to herein more generally as a tool, and a screw  20 , also referred to herein more generally as a driven member, in accordance with the principles of the present invention. The screw driver  10  is preferably substantially cylindrical shaped having a longitudinal axis  16 , and includes a body  30  having a proximal end  32  and a distal end  34 . The body  30  preferably includes a fitting  26  disposed on the proximal end  32 , to allow a user to attach an implement, such as a wrench, to facilitate applying torque if necessary. The fitting  26  may be a polygonal surface or socket for example. However, as those skilled in the art will appreciate, various different configurations may be used as fittings within the scope of the present invention. The body  30  also preferably includes a handle  28  disposed on the proximal end  32  to facilitate gripping the body  30  so that torque may be applied more easily without slipping. The handle  28  may be formed of a series of grooves or knurls in the body  30  to create a frictional surface. Other varieties of handles known in the art may also be used within the scope of the present invention.  
         [0029]    A driving means  12  is preferably disposed on the distal end  34  of the body  30 . The driving means  12  has a first end  36 , a second end  38 , and preferably has a constant cross-sectional configuration throughout the length of the driving means  12 . The constant cross-section allows efficient transfer of torque from the screw driver  10  to the screw  20  without the tendency to separate as is common with tapered driving surfaces. Also, the constant cross-sectional configuration allows the drive means  12  to impart torque efficiently when rotated in both clockwise and counter-clockwise directions. This feature is an improvement over driving surfaces with curved or angled driving surfaces which work transfer torque efficiently only in one direction.  
         [0030]    The driving means  12  is preferably polygonal in shape. For example, the polygonal shape of the driving means  12  may be a hexagonal shape as shown most clearly in FIG. 2. However, as those skilled in the art will appreciate, driving surfaces of various shapes, such as star shapes, cross shapes, blade shapes, or fluted configurations, may be used within the scope of the present invention.  
         [0031]    A gripping means  14  is preferably disposed on the second end  38  of the driving means  12  for gripping the screw  20  to removably attach the screw  20  to the screw driver  10 . The gripping means  14  is preferably substantially frusto-conical in shape, however other configurations are possible within the scope of the present invention. The gripping means  14  is preferably located along the axis  16  such that a break  46  is disposed radially from the longitudinal axis  16  between the driving means  12  and the gripping means  14 . The separation of the gripping means  14  from the driving means  12  allows the gripping means  14  to function independently from the driving means and vice versa. The function of the gripping means  14  is described more fully below.  
         [0032]    The driving means  12  preferably has a radial dimension  48  from the longitudinal axis  16  of the tool which is larger than a radial dimension  50  of the gripping means  14 . The difference in radial dimension forms the radial break  46  to separate the drive means  12  from the gripping means  14 , and to provide a larger surface area to the driving means  12  for applying torque than for gripping the screw. Preferably the radial dimension  48  of the driving means  12  is sized in a range of between approximately two to four times the radial dimension  50  of the gripping means  14  from the longitudinal axis  16 .  
         [0033]    The screw  20  preferably includes a first aperture  24  and a second aperture  22 . The first aperture  24  is preferably aligned coaxially with the second aperture  22  and smaller than the second aperture  22 , such that the second aperture  22  circumscribes the first aperture. The first aperture  22  is defined by a circumferential edge  40  which is engaged by the gripping means  14  to attach the screw  20  to the screw driver  10 . The first aperture  22  and edge  40  are preferably circular in shape, however, various shapes may be used within the scope of the present invention. The second aperture  22  is preferably defined by a socket  42  which is engaged by the driving means  12  of the screw driver  10  to form an interference fit. The socket  42  preferably has a shape which corresponds to the shape of the driving means  12 , such as hexagonal for example. However, it will be appreciated that the socket  42  may have various corresponding shapes of polygons, stars, crosses, blades, or fluted configurations for example, within the scope of the invention. The screw  20  also preferably has threads  44  of any variety known in the art. However, the principles of the present invention may be applied to any such driven member which may employ other engaging means such as flanges or pins for example, besides threads.  
         [0034]    In use, the screw  20  is attached to the screw driver  10  by inserting the gripping means  14  into the first aperture  24  to the point where the edge  40  engages the gripping means  14  to wedge the gripping means  14  against the edge  40  with a friction fit. Preferably, the area of contact between the gripping means  14  and the edge  40  is large enough to supply sufficient force to attach the screw  20  to the screw driver  10 , yet small enough such that the screw  20  may be released when desired without undue effort. As the gripping means  14  enters the first aperture  24 , the driving means  12  is aligned against the socket  42 . The surface area of the contact between the driving means  12  and the socket  42  is large as compared to the contact between the gripping means  14  and the edge  40 . This relationship allows efficient transfer of torque from the screw driver  10  to the screw  20  without imposing concentrated loads on a single point. Furthermore, since the contact between the driving means  12  and the socket  42  is separate from the contact between the gripping means  14  and the edge  40 , the screw  10  can be easily released from the screw driver  10 , regardless of how much torque is applied. In other words, a high torque placed on the socket  42  by the driving means  12 , has no effect on the frictional connection between the gripping means  14  and the edge  40 . Once it is desired to release the screw  20  from the screw driver  10 , the screw driver  10  is simply pulled from the screw  20  with a force sufficient to overcome the frictional fit between the gripping means  14  and the edge  40 .  
         [0035]    Manufacturing of the present invention is facilitated since the gripping means  14  and edge  40  need not be constructed to exact dimensions to allow proper attachment of the screw  20  to the screw driver  10 . The tapered surface of the gripping means  14  allows a friction fit against the edge  40  at various longitudinal locations along the gripping means  14  depending upon the size of the first aperture  24 . Furthermore, since the screw  20  is attached to the screw driver  10  by a friction fit, the use of magnetic materials is not necessary. The screw driver  10  and screw  20  may be constructed of various materials know to those skilled in the art.  
         [0036]    Reference will now to made to FIG. 3 to describe a second embodiment of the present invention. As previously discussed, the presently preferred embodiments of the invention illustrated herein are merely exemplary of the possible embodiments of the invention, including that illustrated in FIG. 3.  
         [0037]    It will be appreciated that the second embodiment of the invention illustrated in FIG. 3 contains many of the same structures represented in FIGS.  1 - 2  and only the new or different structures will be explained to most succinctly explain the additional advantages which come with the embodiments of the invention illustrated in FIG. 3. The second embodiment of the invention includes a tapered member  14   a  disposed on the screw  20 , and a gripping means including a first aperture  24   a  and edge  40   a  disposed on the screw driver  10 . The function of the second embodiment of the invention is similar to that of the first embodiment. An advantage of the second embodiment is that the tapered member  14   a  is protected within the socket  42 . Therefore, damage to the tapered member  14   a  is less likely so that a proper fit between the screw  20  and the screw driver  10  is allowed.  
         [0038]    In accordance with the features and combinations described above, a preferred method of driving a driven member  20  with a tool  10  includes the steps of:  
         [0039]    A) inserting a gripping means  14  into a first aperture  24  to attach the driven member  20  to the tool  10 ; and  
         [0040]    B) inserting a driving means  12  into a second aperture  22  to transfer a driving force from the tool  10  to the driven member  20 .  
         [0041]    In view of the foregoing, it will be appreciated that the present invention provides an interference fit screw driver which is simple in design and manufacture which is attached to a screw by a friction fit to facilitate positioning the screw. The present invention also provides such a screw driver which is capable of applying torque in two directions and which engages the screw over a large surface area such that wear and deformation of the screw and screw driver are reduced. The present invention also provides a screw driver which can be released from the screw even after a large torque has been applied to the screw by the screw driver. The present invention also provides a screw driver which is capable of retaining contact with screws made of nonmagnetic materials.  
         [0042]    It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements. Thus, while the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications, including, but not limited to, variations in size, materials, shape, form, function and manner of operation, assembly and use may be made without departing from the principles and concepts set forth herein.