Abstract:
An inserter for a bone screw assembly includes a hollow outer shaft with an inner shaft concentrically disposed therein. The distal ends of the inner shaft and outer shaft each define a feature for engaging the bone screw assembly. A collar is concentrically disposed about the proximal end of the inner shaft and a rotational interface is provided between the collar and inner shaft. The rotational interface is configured so that rotation of the collar relative to the inner shaft moves the collar into abutting engagement with a hub defined at the proximal end of the outer shaft. This abutting engagement locks the inner and outer shafts together when the distal ends of the shafts are engaged to the bone screw assembly for use in introducing the bone screw assembly into a bone.

Description:
PRIORITY CLAIM 
     This application claims priority to provisional application No. 61/538,892, filed on Sep. 25, 2011, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of surgical instrumentation, and more particularly relates to a surgical instrument for driving a threaded member, such as a bone screw, into a substrate, such as a bone. 
     BACKGROUND 
     Various types of fasteners are used to engage implants and other devices to bone. In the spinal field, bone screws are commonly used to attach plates, rods and other types of implants and devices to one or more vertebrae. In some instances, a relatively high degree of precision is required to engage the bone screws in the proper position and orientation relative to the spinal column. In certain procedures, the surgeon may need to manipulate tissue and/or other anatomical structures while holding the bone screw in position with one hand, and at the same time grasping and rotating a screwdriver with the other hand to drive the screw into engagement with vertebral bone. In some instances, the bone screw may be held in position via the use of a holding instrument that is manipulated in one hand while grasping and manipulating a screwdriver with the other hand. The non-positive engagement between the holding instrument, the screwdriver and the bone screw may lead to instability, thereby making the process of driving the bone screw into bone more difficult, awkward and time consuming. 
     There is a need for a screw inserter that permits one-handed operation throughout the entire process. 
     SUMMARY 
     A bone screw inserter comprises a hollow outer shaft with an inner shaft concentrically disposed therein. The distal ends of the inner shaft and outer shaft each define a feature for engaging the bone screw assembly. A collar is concentrically disposed about the proximal end of the inner shaft and a rotational interface is provided between the collar and inner shaft. The rotational interface is configured so that rotation of the collar relative to the inner shaft moves the collar into abutting engagement with a hub defined at the proximal end of the outer shaft. This abutting engagement locks the inner and outer shafts together when the distal ends of the shafts are engaged to the bone screw assembly for use in introducing the bone screw assembly into a bone. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of a screw inserter according to the present disclosure. 
         FIG. 2  is a side cross-sectional view of the screw inserter shown in  FIG. 1 . 
         FIG. 3  is an exploded view of the components of the screw inserter shown in  FIG. 1 . 
         FIG. 4  is a side cross-sectional view of a bone screw assembly suitable for introduction using the screw inserter disclosed herein. 
         FIG. 5  is a perspective view of a yoke of the bone screw assembly depicted in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     A screw inserter  10 , shown in  FIGS. 1-3 , includes an elongated outer shaft  12  that defines a central passageway  14 . Apertures  13  may be provided to communicate with the passageway  14 . A hub  16  is incorporated at the proximal end  15  of the outer shaft  12 . The distal end  18  includes an externally threaded tip  20 . The threaded tip is configured to engage an internally threaded portion at the proximal head of the bone screw to be inserted, such as the INERTIA® pedicle screw sold by NEXXT Medical LLC. The internally threaded portion may be integral with the bone engaging portion of the bone screw or may be included in a yoke or similar structure used to engage the bone screw to an elongated fixation rod. 
     A suitable bone screw assembly is depicted in  FIGS. 4-5 . The bone screw assembly  50  includes a bone engaging fastener, or screw,  52 , a yoke  50  through which the screw extends and a set screw  56  for clamping a rod R within the yoke and in engagement with the screw. As shown in more detail in  FIG. 5 , the yoke  54  includes a body  60  which defines a bore  61  through which the screw  52  extends, and pair of opposed arms  62  extending from the body. The proximal ends of the arms define internal threads  64  for engaging the set screw  56 . The arms are spaced apart to define a channel  66  for receiving the rod R therethrough. It can be appreciated that the threaded tip  20  of the outer shaft  12  is configured to engage the internal threads  64  of the yoke  54 . 
     The screw inserter  10  further includes a collar  25  that is configured for manual engagement. The collar defines a central passageway  27  with optional perforations  26  communicating with the passageway. As best seen in  FIG. 2 , the passageways  14  and  27  are collinear. The collar passageway  27  includes an internally threaded portion  29 . The internally threaded portion  29  may extend along the entire length of the central passageway  27 , or may have a limited length positioned adjacent the hub  16 , as shown in  FIG. 2 . 
     A third component of the screw inserter  10  is an inner shaft  35  that is sized to extend through the collinear passageways  14 ,  27 . The diameters of the passageways  14 ,  27  are sized for a close running fit with the inner shaft  35  so that the outer shaft  12  may rotate and translate relative to the inner shaft  35 . The distal end  36  of the shaft  35  is larger than the inner diameter of the passageway  14 . The distal end  36  is configured to engage interior surfaces at the proximal head of the bone screw and/or yoke, depending upon the structure of the bone screw. In one embodiment, the distal end  35  includes laterally extending wings  38  that may be configured to seat within a transverse slot or rod channel in the bone screw and/or yoke. The distal end  35  may further include a tip  39  configured to fit within an internal bore of the bone screw. 
     Thus, with respect to the bone screw assembly  50 , the wings  38  are configured to seat within the channel  66  between the two arms  62  of the yoke. The tip  39  may project into a recess  53  ( FIG. 4 ) of the bone engaging fastener. The recess  53  may be configured to receive a driving tool in a conventional manner. In that case, the tip  39  is provided in a hex configuration to make with the recess to serve as the driving tool for the screw  52 . 
     It can be appreciated that the threaded portion  20  at the distal end  18  of the outer shaft  12  and the distal end  36  of the inner shaft  35  may be configured to cooperate to rigidly engage a bone screw in a manner that permits rotating the bone screw and manually driving the screw into the bone, such as by threading the screw into the bone. The wings  38  and/or tip  39  can further facilitate threading the screw into the bone, particularly for a bone screw assembly such as the assembly  50  that utilizes a separate yoke  54 . It can further be appreciated that the structures of the distal ends  18  and  36  of the two shafts are configured to permit easy release of the screw inserter  10  from the bone screw in situ, once the bone screw has been driven into the bone. Moreover, the distal ends are configured so that the ends do not project outside the profile of the bone screw or yoke, which allows the inserter to be used with a working channel cannula. 
     The proximal end  41  of the inner shaft  35  is coupled to a handle  45  that is configured for manual engagement. The proximal end  41  may be couple to the handle in a conventional manner, such as by a locking thread engagement or other suitable means for fixing the two components together for the transmission of torque and rotation from the handle through the inner shaft. The handle  45  may be configured as a “palm” driver that allows the surgeon to apply downward force while rotating the inner shaft. The handle  45  includes a lower portion  46  that extends into the collar  25 . The collar defines a recess  28  collinear with the central bore  27  and sized to receive the lower portion  46 . The recess is defined at a depth into the collar sufficient to allow the shaft  35  to slide distally before the lower portion  46  of the handle  45  abuts the base of the recess  28 . 
     The inner shaft  35  is further provided with a threaded sleeve  43  with threads configured to mate with the internal threads of portion  29  of the collar  25 . The sleeve  43  is affixed to the inner shaft in a conventional manner or may be integrally formed on the shaft. The threaded sleeve  43  and threaded portion  29  are used to lock the inserter components together once the bone screw has been engaged, as explained below. The sleeve  43  and threaded portion  29  define a rotational interface that permits relative rotation between the collar  25  and inner shaft  35 . Although the rotational interface in the illustrated embodiment is a threaded interface, other arrangements are contemplated that permits controlled relative rotation between the components, such as a bayonet-type interface or a spiral groove and follower arrangement. 
     In a first step for using the inserter  10 , the distal end  36  of the inner shaft  35  is engaged within the bone screw to be inserted. In the illustrated embodiment, the wings  38  and tip  39  are seated within the rod channel and central bore of the bone screw assembly, such as the channel  66  of the yoke  54  ( FIG. 5 ). In the next step, the outer shaft  12  is advanced into contact with the bone screw, and particularly, the threaded tip  20  is positioned for engagement with the internal threads of the bone screw, such as the internal threads  64  of the yoke. The hub  16  can be used to rotate the outer shaft  12  to fully engage the threaded tip  20  with the bone screw. It is appreciated that this engagement step is performed prior to introduction of the bone screw into the surgical site. 
     Once the bone screw is fully engaged, the collar  25  is rotated so that the collar advances along the threaded sleeve  43  of the inner shaft. The collar is advanced until it engages the hub  16  of the outer shaft  12 , as depicted in  FIG. 2 . It is understood that the distal end  36  of the inner shaft  35  is simultaneously drawn proximally toward the distal end  18  of the outer shaft. The inner shaft thus becomes anchored to the outer shaft by abutment of the inner shaft distal end  36  to the outer shaft distal end  18  and abutment of the collar  25  to the hub  16 . The hub and collar thus act as locking nuts locked down onto a bolt to prevent unthreading of the nut. In this instance, the engagement between the hub and collar ensure that the inner and outer shafts remain firmly engaged to the bone screw. 
     With the bone screw thus engaged, the inserter  10  may be manipulated to position the bone engaging threads of the screw at the surgical site for introduction of the screw into the bone. The handle  45  may be used to manually drive the bone screw into the bone in a conventional manner. It can be appreciated that the engagement of the threads  20  with the bone screw as well as engagement of the wings  38  and/or tip  39  to the bone screw allow the transmission of torque to the bone screw to thread it into the bone. Once the bone screw is fully seated within the bone, the handle  45  is grasped while the collar  25  is rotated in the reverse direction to loosen the collar from the hub  16 . Once the hub and collar are unlocked, the hub  16  may be rotated to unthread the threaded tip  20  of the outer shaft  12  from the bone screw. When the threaded tip is fully disengaged the distal end  36  of the inner shaft  35  may be removed from the bone screw. 
     It can be appreciated that the screw inserter  10  disclosed herein provides a simple, easy-to-use device for holding and driving a bone screw into bone. The interface between the collar  25  and hub  16  locks the components of the inserter together to provide an essentially unitary structure for driving the bone screw. This same interface can be readily released once the bone screw is properly seated. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.