Abstract:
A medical screwdriver configured to avoid disengaging from a screw during implantation of the screw at a surgical site, the screwdriver including an outer shaft having a housing in which is located a piston and a spring, an inner shaft, and a collar. When the collar is rotated in a first direction the collar is positioned at a first position and the inner shaft is rotationally coupled to the outer shaft. When the collar is rotated in a second direction opposite the first direction, the collar moves to a second position and the inner shaft is unlocked from the outer shaft.

Description:
FIELD 
     This disclosure relates to the field of medical screwdrivers. More particularly, the disclosure relates to a medical screwdriver configured to avoid disengaging prematurely before a screw is properly installed at the surgical site. 
     BACKGROUND 
     Medical screwdrivers of the type used to install pedicle screws typically have an inner portion that can be driven to install a screw and an outer portion to engage the screw during installation. 
     One problem associated with conventional screwdrivers is that the screwdriver can often at least partially disengage from a screw, such as a pedicle screw, during installation of a screw. 
     Accordingly, what is desired is an improved screwdriver structure that avoids disengagement from the screw during installation of the screw at the surgical site. 
     The disclosure relates to an improved screwdriver structure that enables selective engagement and disengagement of an inner shaft and an outer shaft of the screwdriver. The shafts are engaged for installation of a screw, then disengaged to allow the inner shaft to rotate freely from the outer shaft and disengage from the screw so that the screw does not disengage prematurely before it is properly installed at the surgical site. 
     SUMMARY 
     The disclosure relates to a medical screwdriver configured to avoid disengaging prematurely before the screw is properly installed at the surgical site. 
     In one aspect, the screwdriver includes an outer shaft having a housing in which is located a piston and a spring, an inner shaft, and a collar. 
     The outer shaft includes a first end, a housing defined at an opposite second end of the outer shaft, and a central bore extending through the length of the outer shaft. The housing includes a cavity located to intersect the central bore; a spring positioned within the cavity; and a piston positioned within the cavity of the housing of the outer shaft. The piston has an end extending outwardly of the cavity, an opposite base in contact with the spring, and a bore extending through the piston, the bore including an interior projection. 
     The inner shaft has a first end, an opposite second end, and a toothed circumferential surface located between the ends of the shaft. The inner shaft is positioned within the central bore of the outer shaft and passes through the bore of the piston and is located so that the toothed surface of the inner shaft is proximate the bore of the piston for permitting selective engagement of the interior projection of the piston with the toothed surface of the inner shaft. 
     The collar is rotatably positioned on the housing of the outer shaft and is rotatable between a first position and a second position. The collar includes a sidewall and an interior profiled cam located along an interior portion of the sidewall. The cam includes first and second cam surfaces. 
     When the collar is rotated in a first direction, such as clockwise, the collar moves to the first position with the first cam surface adjacent the piston. The first cam surface allows sufficient clearance for the spring and piston to translate. The spring forces the interior projection of the piston to engage with the toothed circumferential surface of the inner shaft. As the collar is further rotated in the first direction, the outer shaft is engaged and rotates in the first direction along with the collar and the outer shaft engages with the surgical screw. As the collar and outer shaft rotate in the first direction the piston indexes to a subsequent tooth of the shaft as the spring ensures it remains in contact with the circumferential teeth of the inner shaft. Once the outer shaft is sufficiently tightened to the surgical screw, the piston remains locked to the inner shaft until the surgical screw has been installed at the surgical site. 
     When the collar is rotated in the opposite direction, such as counterclockwise, the second cam surface contacts the piston and compresses the spring. The piston disengages from the inner shaft and unlocks the outer shaft from the inner shaft allowing removal of the screwdriver from the installed surgical screw. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the disclosure are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1  is a perspective view of a medical screwdriver according to the disclosure. 
         FIG. 2  shows the screwdriver of  FIG. 1  engaged with a pedicle screw. 
         FIG. 3  is an exploded view of the screwdriver of  FIG. 1 . 
         FIG. 4  shows a collar component of the screwdriver of  FIG. 1 . 
         FIG. 5  shows a piston component of the screwdriver of  FIG. 1 . 
         FIG. 6  shows the piston component of  FIG. 5  located on an inner shaft component of the screwdriver of  FIG. 1 . 
         FIGS. 7 and 8  show an outer shaft component of the screwdriver of  FIG. 1 . 
         FIG. 9  shows an inner shaft component of the screwdriver of  FIG. 1 . 
         FIG. 10  and  FIG. 11  show the piston component positioned within the collar component of the screwdriver of  FIG. 1 , with  FIG. 10  showing the collar transparent. 
         FIG. 12  is an assembled interior view with the collar transparent. 
         FIGS. 13 and 14  are cross-sectional views showing the positioning of the piston relative to the collar when the piston is disengaged from the inner shaft. 
         FIG. 15  is a cross-sectional view showing the positioning of the piston relative to the collar when the piston is engaged with the inner shaft. 
         FIGS. 16 and 17  are partially assembled views of the screwdriver of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, the disclosure relates to a medical screwdriver  10  having a drive  12 , a collar  14 , a pin  16 , a piston  18 , a spring  20 , an outer shaft  22 , an inner shaft  24 , an exterior sleeve  26 , and a retaining ring  28 . The screwdriver  10  is particularly configured for installing a pedicle screw  30  of the type having a polyaxial head  31  below a tulip-shaped head  32  having internal threads. In regards to the following description, it will be understood that the screw  30  is configured to have bone engaging threads  34  configured to tighten the screw  30  into the bone as the screw  30  is rotated clockwise. Accordingly, the screwdriver  10  is described herein with respect to installation of such screws. 
     As previously noted, one problem associated with conventional screwdrivers is that the screwdriver can often at least partially disengage from a screw during installation of the screw. The screwdriver  10  is configured to avoid disengaging prematurely before the screw is fully installed at the surgical site. 
     The drive  12  is configured to connect a handle or driver operated by a surgeon to the screwdriver  10  so that force can be transmitted to install a screw. In one embodiment, the drive  12  includes a square drive surface  36  at one end for connecting to a handle. The opposite end of the drive  12  defines a blind bore  38  having ridges defined around the inner periphery thereof. The bore  38  slips over an end of the inner shaft  24 . 
     The collar  14  includes a cylindrical sidewall  40  having an open end  42  and an opposite substantially closed end  44  having a central aperture  46 . An interior profiled cam  48  is located along an interior portion of the sidewall  40  adjacent the closed end  44 . The cam  48  includes cam surfaces  48   a  and  48   b  at opposite ends of the cam  48 . As explained more fully below, when the collar  14  is rotated in a clockwise direction the cam surface  48   a  provides clearance room to permit the piston  18  to be positioned to lock the inner shaft  24  to the outer shaft  22 . When the collar  14  is thereafter rotated in the opposite direction, the cam surface  48   b  positions the piston  18  to unlock the inner shaft  24  from the outer shaft  22 . 
     The pin  16  is installed within an aperture  50  located on the collar  14  adjacent the open end  42 . The pin  16  is oriented to extend into the open space of the interior of the collar  14 . The pin  16  retains the collar  14  from dislocating or otherwise moving axially on the outer shaft  22 . 
     The piston  18  is shaped to include a substantially planar circular base  52 , a cylindrical sidewall  54  extending from the base, and a pointed upper end  56 . A bore  58  extends through the mid-section of the piston  18  to define opposite open ends through which the inner shaft  24  extends. The bore  58  includes a tooth or other interior projection  60  defined on a lower central portion of the bore  58 . 
     The spring  20  is a compression type spring and may be provided as by a coil or wave spring. The use of a wave spring is preferred for reduced spring height. The spring  20  is provided to bear against the base  52  of the piston  18 , as explained more fully below. 
     The outer shaft  22  includes an open-ended elongate slotted cylindrical member  70  having a threaded distal end  72  and a housing  74  defined at the opposite proximal end of the shaft  22  having stop surfaces  74   a  and  74   b . A slot  75  is defined along a portion of the exterior of the housing  74  for receiving the pin  16 . A central bore  76  extends through the length of the outer shaft  22  for receiving the inner shaft  24 . The housing  74  includes a cavity  78  located perpendicular to the bore  76  and configured to receive the piston  18  and the spring  20 . Slots  79  extend through the sidewall of the cylindrical member  70 . The slots  79  are provided for facilitating sterilization and cleaning. 
     The inner shaft  24  has a proximal end  80  configured to receive the bore  32  of the drive  12 . A peripheral toothed surface  82  defining a plurality of teeth  822  is located adjacent the proximal end  80 . The teeth  82   a  are configured for engaging the interior projection  60  of the piston  18 , as described more fully below. A distal end  84  of the inner shaft  24  is configured for engaging the pedicle screw  30 . For example, the distal end  84  may include a drive tip  84   a  configured to drivingly engage a corresponding drive surface (such as a hex or slot) of the polyaxial head  31  of the screw  30  and a cross-piece  84   b  configured for being received by side slots of the tulip head  32   
     The exterior sleeve  26  includes an elongate slotted open-ended cylindrical member  90  having an exterior rim  92  defined on the proximal open end. The exterior sleeve  36  rotates independently from the outer shaft  22  and the inner shaft  24  and is configured so that a user may grasp for guidance of the screw during installation. The exterior rim  92  provides a hand stop used to avoid contact with the collar  14  during screw installation. A central bore extends through the length of the sleeve  26  for receiving the outer shaft  22 . The retaining ring  28  fits onto the distal open end of the sleeve  26 . For example, the ring  28  may have a slightly reduced diameter portion that fits within the end of the sleeve  26  to seat the retaining ring  28  onto the end of the sleeve  26  while still permitting the outer shaft  22  to pass through the end of the sleeve  26 . 
     With additional reference to  FIGS. 10-17 , in the assembled state of the screwdriver  10 , the collar  14  fits over the housing  74  of the outer shaft  22 , with the piston  18  and the spring  20  located within the cavity  78  of the housing  74 . The inner shaft  24  extends through the aperture  46  of the collar  14  and the bore  76  of the outer shaft  22 . The inner shaft  24  passes through the bore  58  of the piston  18  such that the piston  18 , which is located in the cavity  78  of the housing  74  and biased by the spring  20 , straddles the toothed surface  82  of the inner shaft  24  and the pointed upper end  56  of the piston  18  bears against the cam  48 . The outer shaft  22  (having the inner shaft  24  therein) is received by the sleeve  26  such that the distal end  84  of the inner shaft  24  and the threaded distal end  72  of the outer shaft  22  extend out of the sleeve  26 , as best seen in  FIG. 1 . The drive  12  is positioned on the proximal end of the inner shaft  24  adjacent the collar  14 . The drive  12  may be connected to a handle or driver operated by a surgeon so that force can be transmitted to the screwdriver  10  to install the screw  30 . 
     In use of the screwdriver  10 , as the drive  12  is rotated clockwise to tighten and install the pedicle screw  30  into a bone at a surgical site, the surgeon will rotate the collar  14  clockwise to bear the cam  48  against the stop surface  74   a  of the housing  74  and to also engage the piston  18  with the inner shaft  24  and lock the inner shaft  24  to the outer shaft  22 . 
     For example, as shown in  FIG. 15 , the cam surface  48   a  of the cam  48  provides the piston  18  sufficient clearance so that the piston  18  is located so that the interior projection  60  of the piston  18  engages one of the teeth  82   a  of the inner shaft  24 . Thus, as the collar  14  is rotated clockwise, the cam  48  rotates to the lock position where the piston  18  bears against the surface  48   a  to position the piston  18  so that the interior projection  60  engages the teeth  82   a  on the inner shaft  24  of the screwdriver  10 . As the collar  14  is rotated further in the clockwise direction, the interior projection  60  of the piston  18  will index to the next tooth  82   a  on the inner shaft  24  until the threaded end  72  of outer shaft  22  of the screwdriver  10  is fully tightened into the threads of the tulip shaped head  32  of the pedicle screw  30 . Once the outer shaft  22  is sufficiently tightened to the surgical screw, the piston  18  remains locked to the inner shaft  18  until the surgical screw  30  has been installed at the surgical site. Rotation of the screwdriver  10  in the clockwise direction is continued until the screw  30  is desirably installed at the surgical site. 
     When the collar  14  is rotated in the opposite direction (counter-clockwise), the cam  48  bears against the stop surface  74   b  and the piston  18  disengages from the inner shaft  24  and unlocks the outer shaft  22  from the inner shaft  24  allowing removal of the screwdriver  10  from the installed surgical screw  30 . 
     For example, as shown in  FIGS. 13 and 14 , the cam surface  48   b  urges the piston  18  away from the inner shaft  24  so that the interior projection  60  of the piston  18  does not engage any of the teeth  82   a  of the inner shaft  24 . Thus, when the collar  14  is rotated counter-clockwise, the cam  48  rotates to an unlocked position, where the piston  18  bears against the surface  48   b  and is depressed and overcomes the force of the spring  12  so that the interior projection  60  on the piston  18  disengages from the teeth  82   a  on the inner shaft  24 . 
     Accordingly, it will be appreciated that the configuration of the screwdriver  10  enables selective engagement and disengagement of the inner shaft  24  and the outer shaft  22  of the screwdriver  10 . This enables threaded securement of the outer shaft  22  to the screw and, once the outer shaft is sufficiently tightened to the surgical screw, the piston remains locked to the inner shaft until the surgical screw has been installed at the surgical site. This avoids premature disengagement of the screwdriver from the screw during installation. After the screw is installed, the collar  14  is rotated counterclockwise to bear the cam  48  against the stop surface  74   b  of the housing  74  and the inner shaft  24  is disengaged from the outer shaft  22  to allow removal of the screwdriver  10  from the installed surgical screw  30 . 
     The foregoing description of preferred embodiments for this disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the disclosure and its practical application, and to thereby enable one of ordinary skill in the art to utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the disclosure as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.