Abstract:
A torque-limiting fastener driver for driving a bone fastener into bone includes a handle supporting a driver shaft. The driver shaft includes limited components which are designed to provide feedback to a user regarding the applied torsional force. In addition, the driver shaft is designed to controllably fail before any potential uncontrolled failure of the distal driving tip. This prevents application of too much torque and controlled design of the shape and size of the failed components.

Description:
FIELD OF THE INVENTION 
     This invention relates broadly to surgical instruments. More particularly, this invention relates to instruments for driving fasteners in orthopedic applications. 
     STATE OF THE ART 
     During some bone plating procedures, surgeons may have difficulty aligning the fastener driver with the bone plate fastener due to limited access, visualization, etc. If the driver tip is not properly engaged with the fastener head while attempting to drive the fastener into bone, there is danger of damaging the driver tip or the fastener head. Fastener drivers for driving very small fasteners, such as used for attaching small, low profile bone plates to bone, are especially susceptible to this type of damage. 
     During a plating procedure, the surgeon may use the fastener driver for several fasteners, resulting in accumulated wear of the driver tip. Therefore, it is also desirable to provide a sterile, low cost driver that may be discarded after use in one surgical procedure. 
     Further, fastener drivers are known which limit the torque that can be applied to the fastener that is being driven. Such torque-limiting drivers use some type of clutch or shear-pin mechanism in the driver shaft or handle and the mechanism comprises numerous components. 
     SUMMARY OF THE INVENTION 
     A fastener driver for driving a bone fastener into bone includes a handle and a driver shaft coupled to the handle in rotatably fixed manner. In a preferred embodiment, the driver shaft is removably attached to the handle. The driver shaft is a one-piece metal component having a proximal end coupled to the handle and a distal end configured into a driver tip for driving engagement with a head of a bone fastener. The driver shaft further includes an elongate middle portion stepped down in dimension from the proximal and distal ends. The middle portion has a longitudinal axis and is configured to bend elastically in any plane containing the longitudinal axis within a predetermined angular range. The middle portion also is configured to twist elastically within a predetermined angular range of twisting about the longitudinal axis when a predetermined torsional force is exceeded, thereby providing a tactile feedback to a user gripping the handle and operating the driver. According to one embodiment a visual indicator is also provided to reference the degree of twist to which the shaft is subject, and indicate whether the applied torque is within predetermined acceptable limits. According to a preferred aspect of the invention, a portion of the driver shaft is purposefully weakened to become a frangible segment and function as a fuse that controllably breaks at a predetermined location should the applied torque exceed a yield strength of the frangible segment under the applied torsional force. This ensures that any such breakage results in a dislocated piece of the driver shaft having a predetermined relatively non-injurious shape to the patient and of a sufficient size to be readily recovered from the surgical site. 
     Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation of a fastener driver according to the invention. 
         FIG. 2  is an enlarged isometric view of the distal end of the shaft of the fastener driver of  FIG. 1 . 
         FIG. 3  is an isometric view of the shaft provided with a torque gauge in accord with the invention, shown in a neutral position. 
         FIG. 4  is an isometric view of the fastener driver provided with the torque gauge and shown subject to a maximum acceptable torque. 
         FIG. 5  is an enlarged view of the distal end of the shaft with a torque gauge. 
         FIG. 6  is a view similar to  FIG. 5 , with the torque gauge indicating a maximum acceptable torque applied to the fastener driver. 
         FIG. 7  is an isometric view of the fastener driver provided with the shaft having an alternate gauge. 
         FIG. 8  is an enlarged view of the distal end of the shaft, illustrating the controlled breaking of the driver tip. 
         FIG. 9  is a side elevation view of a driver shaft according to a second embodiment of the fastener driver. 
         FIG. 10  is a longitudinal section view of the driver shaft and tube of the embodiment shown in  FIG. 9 . 
         FIG. 11  is side elevation view of the driver shaft and tube of  FIG. 10  shown in loaded low torque configuration. 
         FIG. 12  is a view similar to  FIG. 11  shown when a maximum recommended torque is applied. 
         FIG. 13  is a side elevation view of the driver shaft of a third embodiment of a fastener driver. 
         FIG. 14  is a view similar to  FIG. 13 , rotated 90° about a longitudinal axis of the driver shaft relative to  FIG. 13 . 
         FIG. 15  is a broken longitudinal sectional view of the coupling of the handle to the shaft according to the third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to  FIGS. 1 and 2 , a torque-limiting fastener driver  10  for driving a bone fastener into bone is shown. The driver in the embodiment shown is particularly designed to drive small bone screws into low profile plates, such as to stabilize fractures in the hand. It is understood that the driver can be modified as necessary to drive bone fasteners for other orthopedic applications. 
     The driver  10  includes a handle  12  and a driver shaft  14  rotatably fixed relative to the handle. The driver shaft  14  is a once-piece metal component having a proximal end  16 , a distal end  18  configured into a driver tip  20  for driving engagement with a head of a bone fastener, and a middle portion  22  between the proximal and distal ends. In a preferred embodiment, the shaft  14  is made from  440  stainless steel having a hardness of at least 48 RC. This material is chosen for its high tensile strength. 
     The proximal end  16  of the driver shaft  14  is preferably removably coupled to the handle  12  so that the shaft can be replaced within the handle as the driver tip  20  is worn from use. Well-known quick coupling mechanisms  24  can be used between the proximal end  16  and the handle  12 , e.g., an AO connector releasable via a slidable collet  26 . Referring to  FIG. 3 , for such engagement, the proximal end  16  of the shaft may include a longitudinally extending flat  27  and a partially circumferential groove  28 . Alternatively, any other releasable coupling mechanism can be used with suitable structure provided to the shaft for the engagement. Moreover, as discussed in more detail below, the fastener driver is specifically designed with a limited number of components and in a manner permitting an inexpensive construction so that the entire driver can be discarded (both  12  handle and shaft  14 ) once the driver tip  20  is no longer capable of stably driving screws; i.e., a single patient/procedure device. 
     Referring back to  FIG. 2 , the driver tip  20  of the shaft  14  includes a round proximal portion  30  which extends into facets  32  defining four sides extending towards a distal tip  34 . The faceted portion includes, in distal to proximal order, a constant diameter first portion  36 , a flared diameter second portion  38  extending from the first portion, a constant diameter third portion  40  extending from the second portion, and a flared diameter fourth portion  42  extending from the third portion. In the preferred embodiment, each of the first, second, third, and fourth portions  36 ,  38 ,  40 ,  42  have a square cross-sectional shape. The first portion  36  provides facets  36   a  that extend parallel in a longitudinal direction to provide high surface area contact against the sides of a recess for high transmission of torque. The first portion  36  also has a beveled end  44  to aid in guiding the tip  20  into the driver recess of a fastener. The flared second portion  38  interferes with the entry of the driver recess in a fastener to provide a temporary engagement therebetween to facilitate inserting or retrieving a fastener. The third portion  40  provides facets  40   a  that extend parallel in a longitudinal direction. The third portion  40  is sized larger than the first portion (in cross-section) and shaped to be readily received within and engage a tubular drill guide of the type adapted to be pre-assembled in plurality to a plate, such as the tubular drill guides disclosed in US Pub. No. 20060149250, which is hereby incorporated by reference herein in its entirety. The facets  40   a  provide regions of high surface area contact against the sides of the recess within such a tubular guide for high transmission of torque thereagainst for insertion or removal of the guide relative to a plate. The flared fourth portion  42  interferes with the entry of the tubular guide to provide a temporary engagement therebetween to facilitate removing a guide from the plate. It is appreciated that the driver tip  20  alternatively can be configured as a hex driver with six facets and the respective portions having a hexagonal shape. Other suitable non-circular cross-sectional shapes for driving orthopedic fasteners can also be provided in accord with the invention. ‘Diameter’ herein refers to the maximum cross-sectional dimension regardless of the cross-sectional shape. 
     The middle portion  22  of the shaft  14  has a length of 60-100 mm and more preferably approximately 80 mm. The middle portion  22  is reduced in cross-section dimension relative to the proximal end  16  and a proximal portion  30  of the driver tip  20 . The middle portion is round in cross-section, and preferably 1.8 mm in diameter along its entire length. The middle portion has a longitudinal axis A extending throughout its length, and is configured to bend elastically in any plane containing the longitudinal axis within a predetermined angular range, e.g., ±20° relative to the axis A. The middle portion  22  also has inherent torsional flexibility such that it can twist elastically (in the direction of arrow  44 ) within a predetermined angular range of twisting about the longitudinal axis A when a predetermined torsional force is exceeded. The amount of twist is directly proportional to the torque applied to the driver. The twist is easily felt by the surgeon, thereby providing a tactile feedback to the surgeon gripping the handle and operating the driver. Further, once a user feels a twist, e.g., approaching 30°-40°, corresponding to greater than 7 in-lbs, such is sufficient feedback to indicate that torque should be reduced to prevent unintended driver tip breakage. 
     Referring to  FIGS. 3 and 4 , optionally, a longitudinally stiff outer tubular member  50  is provided to the fastener driver over the shaft  14 . The tubular member  50  includes a proximal end  52  and a distal end  54 . The proximal end  52  of the tubular member  50  is rotational fixed to the proximal end  16  of the shaft  14 . Alternatively, the tubular member  50  can be rotationally fixed directly to the handle  12 . One or more venting holes  51  are provided along the length of the tubular member  50  to aid in sterilizing the driver during autoclaving. While the inclusion of the tubular member  50  limits the longitudinal flexibility of the driver, it permits further gauging the applied torque, as now described. The distal ends of the shaft and the tubular member  18 ,  54  include indicia, respectively  56 ,  58  that can be referenced against each other to provide a visual indication of the degree of twist to which the driver tip  20  of the shaft is torqued relative to the proximal end  16  of the shaft  14 . The indicia  56  on the driver tip  20  is a longitudinally extending stripe. As seen best in  FIG. 5 , the indicia  58  on the tubular member  50  is a gauge including a central line  60  identifying 0 in-lbs torque (at rest), and flared indicators  62   a ,  62   b  leading to markings  64   a ,  64   b  of maximum acceptable limits of torque for both insertion and removal of a fastener. The maximum acceptable limit indicators  64   a ,  64   b  are preferably located 30°-40° on either side of the central line  60 . By way of example, for a 1.3 mm fastener with a square driver, a preferred maximum acceptable torque causing alignment of the stripe  60  with the maximum torque line  64   a  is 10 in-lbs ( FIG. 6 ). 
     Turning now to  FIG. 7 , another embodiment of the fastener driver  110  is shown with a driver shaft  114  provided with a tubular member  150 . The distal end  118  of the shaft  114  and distal end  154  of the tubular member  150  are coupled together at  155  in a rotationally fixed manner. For example, ends  118 ,  154  can be crimped, welded or bonded together at  155 . The proximal end  152  of the tubular member  150  is free to rotate relative to the proximal end  116  of the shaft  114 . The tubular member  150  limits the bending of the shaft  114 , thus reducing the stress on the shaft. The proximal ends of the shaft and the tubular member  116 ,  152  include indicia, respectively  156 ,  158  that can be referenced against each other to provide a visual indication of the degree of twist to which the driver tip  120  of the shaft is torqued relative to the proximal end  116  of the shaft  114 . One of the indicia  156  is a longitudinally extending stripe and the other indicia  158  defines a gauge within which the stripe  156  is safely when the maximum acceptable limits of torque for both insertion and removal of a fastener are not exceeded. When the driver tip  120  is engaged with a fastener and the driver is operated to drive the fastener, the torsionally flexible middle portion of the shaft will twist causing the proximal end of the tubular member to rotate relative to the stationary portion of the shaft (as indicated by the indicia  156 ,  158  defining the gauge) in proportion to the amount of torque applied to the fastener. 
     Referring now to  FIGS. 2 and 8 , the distal end  18  of the shaft  14 , preferably at the driver tip  20 , is provided with a frangible construct that causes separation of at least a portion  70  of the driver tip  20  from the shaft  14  should the applied torque during fastener insertion exceed a predetermined torsional limit, i.e., when the proximal end of the shaft is twisted by, e.g., up to a limit of 90°-120° relative to the distal end of the shaft (when corresponding, e.g., to 10 in-lbs or more), which is generally greater than a comfortable amount of angular rotation to be exerted by the surgeon. The frangible construct is formed by using wire electro-discharge machining (EDM) to define a circumferential groove  68  above the contact area between the driver tip  20  and the driver recess in the fastener. The EDM groove  68  is shaped and dimensioned to cause sudden failure and clean separation (shown at  72 ) along a predetermined fracture line when the yield strength for the driver tip is exceeded. This ensures that the applied torque cannot exceed too high a force, and any such breakage results in a dislocated piece of the driver tip having a predetermined relatively non-injurious shape to the patient, e.g., planar break, and of a sufficiently large size to be readily recovered from the surgical site. 
     Turning now to  FIGS. 9 to 11 , another embodiment of the torque limiting driver is provided. The driver shaft  214  includes a proximal end  216 , a distal end  218  with a driver tip  220 , and a middle portion  222  between the proximal and distal ends. A frangible segment  268  is formed as a necked down portion of the middle portion  222  of the shaft, preferably adjacent the driver tip  220 . The frangible segment  268  is intended to function as a mechanical fuse and catastrophically fail if subject to an applied torque greater than a recommend torque for the intended use of the driver in a procedure, e.g., 10 in-lbs. A tube mount is defined on the shaft by cylindrical bearing portions  270 ,  272 , each preferably of the same diameter, located relative to opposites ends of the middle portion  222  of the shaft  214 . A larger cylindrical portion  274  is formed adjacent proximal portion  270 , and a flat  276  is defined through both cylindrical portions  270 ,  274 . The tubular member  250  extends over the shaft, closely contacting the shaft at the cylindrical bearing portions  270 ,  272  of the tube mount. The distal end  254  of the tubular member  250  is fixed to the distal cylindrical portion  272 . The proximal end  252  of the tubular member includes a finger  278  extending therefrom. The middle portion  222  of the shaft is pre-twisted so that the finger  278  rotationally engages the flat  276  with a pre-load. The pre-load is preferably 7 in-lbs. A gauge, represented by indicia  256 ,  258  is provided to reference movement of the proximal shaft  216  relative to the finger  278  of the tubular member  250 , and thus to gauge applied torque. 
     The tubular member, coupled about the cylindrical bearing portions  270 ,  272 , carries the bending loads. This protects the shaft  214  (and frangible segment  268 ) from bending forces. The frangible segment  268  would otherwise be subject to premature failure due to the combination of applied torque and bending forces. The frangible segment  268  preferably has a round cross-section to facilitate control of its size and therefore its strength accurately. 
     In operation, the surgeon uses the driver as a standard driver. Provided the applied torque is less than the pre-load (7 in-lbs), the finger  278  remains in contact with the flat  276  during operation and the driver functions as if the shaft and tubular element are a unitary component. Referring to  FIG. 12 , should the applied torque be greater than the pre-load (i.e., greater than 7 in-lbs), the applied torque will overcome the pre-load and the shaft will twist defining a gap  280  between the finger  278  and the flat  276 . The surgeon will feel the resulting springiness in the shaft, and the gauge  256 ,  258  will indicate the amount of applied torque within a recommended range. Should the applied torque be outside the recommended range and exceed a maximum permitted torque (e.g., 10 in-lbs), the frangible segment  268  will catastrophically fail causing portions of the shaft about the segment to separate from each other. The frangible segment  268  is structured so that the torque required to cause failure of the segment  268  is less than the torque that would cause failure of the driver tip  220  so as to ensure that the frangible segment  268  fails prior to the tip  220 . In addition, after failure the separated pieces are long and easy to recover. A first piece includes the proximal end and middle portion of the shaft. A second piece includes the entire tubular member and distal end of the shaft fixed thereto. Therefore, recovery is not required to be performed from inside the surgical wound, as the proximal end of the both pieces will be readily obtainable from outside the surgical wound. 
     Turning now to  FIGS. 13 through 15 , another embodiment of a torque limiting driver is provided. The driver shaft  314  includes a proximal end  316 , a distal end  318  with a driver tip  320 , and a middle portion  322  between the proximal and distal ends. The middle portion  322  is not necessarily reduced in diameter relative to the proximal and distal ends  316 ,  318 . The proximal end  316  is provided with a first connector  384 , a second connector  386 , and a reduced diameter frangible segment  368  between the first and second connectors  384 ,  386 . The frangible segment  368  may be further reduced in diameter adjacent second connector  386  to facilitate a clean break thereat. Each of the first and second connectors  384 ,  386  includes structure permitting a quick connect driver handle  24  to releasably engage relative to the connector. For example, such structure on the connectors  384 ,  386  may include an at least partially circumferential recess  390  with a flat  392 . In addition, bearing surfaces  394 ,  396  are provided on either side of the frangible segment  368 . The walls  24   a  of the driver socket of the handle  24  define a tubular member that functions as the tubular member of prior embodiments, bearing against bearing surfaces  394 ,  396 , and carrying the bending loads to thereby limit the frangible segment  368  to be subject only to torque. 
     In operation, the surgeon uses the driver to rotatably insert fasteners. Provided the applied torque is less than a maximum permitted torque (i.e., a torque that would cause failure of the frangible segment  368  (e.g., 10 in-lbs)), the driver functions as a standard driver. However, should the applied torque be greater than the maximum permitted torque, the frangible segment  368  will consistently/predictably fail. The frangible segment  368  is designed to fail prior to any failure of the driving tip  320 . If the frangible segment  368  breaks during a surgical procedure, the surgeon is able to remove the driver shaft  314  and separated first connector  384  from the handle  24 . The driver shaft  314  is re-inserted into the handle  24  with the handle then engaging the second connector  386  of the shaft. While the frangible segment  368  is designed to fail close to the second connector  386 , the quick connect couplings of the handle are capable of functioning with a bit of ‘slop’ even should a portion of the segment  368  remain attached at the distal end of the second connector. Further, if necessary any remaining portion of the frangible segment  368  can be removed with a wire cutter prior to re-inserting the shaft. Re-insertion of the shaft  314  permits the surgeon to complete the procedure, albeit without the safety of the ‘fuse’ of the frangible segment  368 . 
     The torque-limiting fastener driver of the invention has few components and is relatively low cost to manufacture. The shaft can be integrated with the handle for single-patient use or the shaft can be separable from the handle so that the handle can be used with replacement driver shafts. No complex mechanisms are required to monitor and/or limit the applied torque. In addition, the driver shaft may be provided in numerous sizes and with numerous styles of driver tips to accommodate different fasteners. However, the driver is particularly well-adapted to small drivers having corresponding small handles that cannot readily or cost-effectively accommodate torque limiting mechanisms. 
     There have been described and illustrated herein embodiments of a torque limiting fastener driver. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while an outer tubular member attached at either the proximal or distal ends of the shaft has been disclosed for use as part of a torque gauge, it is appreciated that the gauge may be defined by an element other than a tubular member in conjunction with the shaft, e.g., a stiff wire coupled at one end of the shaft and referenced relative to a marking fixed relative to another end of the shaft. In addition, while particular materials, dimensions, and shapes have been disclosed, it will be understood that other suitable materials can be used and that the dimensions and shapes used should be appropriate for the orthopedic application and the fasteners to be driven. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its scope as claimed.