Abstract:
A method is provided and may include rotating a biocompatible, non-metallic threaded fastener at a rotational speed greater than 15,000 revolutions per minute and driving said fastener into a bone at said rotational speed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/040,310, filed on Feb. 29, 2008, which claims the benefit of U.S. Provisional Application No. 60/905,157, filed on Mar. 6, 2007, and U.S. Provisional Application No. 60/904,678, filed on Mar. 2, 2007. The disclosures of each of the above applications are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to threaded fasteners, and more specifically to insertion methods for threaded fasteners. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    Typical fastener insertion techniques may include applying a torque to a threaded fastener to rotationally drive the fastener into a structure. The torsional strength of the fastener is typically greater than the torque applied to drive the fastener to prevent breaking of the fastener before insertion is completed. The torque applied to the fastener increases when a self-tapping fastener is used due to the additional force required to cut threads into the structure. 
       SUMMARY 
       [0005]    A method is provided and may include rotating a biocompatible, non-metallic threaded fastener at a rotational speed greater than 15,000 revolutions per minute and driving said fastener into a bone at said rotational speed. 
         [0006]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0007]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0008]      FIG. 1  is a perspective view of a powered driver according to the present disclosure; 
           [0009]      FIG. 2  is a translucent plan view of an adapter assembly and fastener according to the present disclosure; and 
           [0010]      FIG. 3  is an additional translucent plan view of the adapter assembly and fastener of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0012]    With reference to  FIGS. 1-3 , a driving assembly for a fastener may include a powered driver  12  and an adapter assembly  14 . Powered driver  12  may be any driver capable of driving adapter assembly  14  in the manner discussed below. Powered driver  12  shown in  FIG. 1  is a powered driver from MicroAire Surgical Instruments L.L.C. of Charlottesville, Va. Adapter assembly  14  may be engaged with and driven by powered driver  12 . 
         [0013]    More specifically, adapter assembly  14  may include a plunger  16  and a driving member  18 . Plunger  16  may include a central body portion  20 , a connection shank  22 , and an ejector shaft  24 . Central body portion  20  may include a generally cylindrical body  26  having first and second ends  28 ,  30 . An elongate passage  32  may extend radially through cylindrical body  26  and may have an axial extent along cylindrical body  26 . Connection shank  22  may extend axially from first end  28  of cylindrical body  26  and may engage powered driver  12  for driving adapter assembly  14 . Ejector shaft  24  may be generally cylindrical or may be in the form of a drill bit for drilling a pilot hole, as discussed below, and may extend axially from second end  30  of cylindrical body  26  and may be slidably disposed within driving member  18 , as discussed below. 
         [0014]    Driving member  18  may include an elongate body  34  having first and second ends  36 ,  38 . A central opening  40  may extend axially through elongate body  34  and first and second ends  36 ,  38 . A first portion  42  of elongate body  34  may include a first portion  44  of central opening  40  passing therethrough. First portion  44  of central opening  40  may extend through first end  36  and may have a first diameter greater than the diameter of a central portion of cylindrical body  26  and second end  30  thereof and less than the diameter of first end  28  of cylindrical body  26 . A set of apertures  46 ,  48  may extend radially through first portion  42  of driving member  18  near first end  36 . 
         [0015]    A second portion  49  of elongate body  34  may include a second portion  50  of central opening  40  passing therethrough. Second portion  50  of central opening  40  may have a second diameter that is less than the first diameter of first portion  44 , forming a step  52  therebetween. The second diameter of second portion  50  may be greater than the diameter of ejector shaft  24  and less than the diameter of cylindrical body  26  of plunger  16 . 
         [0016]    A third portion  54  of elongate body  34  may include a driving geometry. More specifically, third portion  54  may include a third portion  56  of central opening  40  extending therethrough. Third portion  56  may extend through second end  38  of driving member  18  and may define a series of flats  58  or some other form of driving geometry on an inner wall of third portion  56 . Third portion  56  of central opening  40  may have a third diameter that is greater than the second diameter of second portion  50 , forming a step  59  therebetween. 
         [0017]    Cylindrical body  26  may be disposed in first portion  44  of central opening  40  and a pin  61  may pass through apertures  46 ,  48  in driving member  18  and elongate passage  32  in plunger  16 , slidably coupling plunger  16  to driving member  18 . Ejector shaft  24  may extend into first and second portions  44 ,  50  of central opening  40  when plunger  16  is in a retracted position (seen in  FIGS. 2 and 3 ) and a may be extended into third portion  56  of central opening  40  to abut and/or eject a fastener  60 , as discussed below. A spring  63  may be engaged with step  52  and second end  30  of cylindrical body  26 , urging plunger  16  into the retracted position. 
         [0018]    Fastener  60  may be disposed within third portion  56  of central opening  40 . Fastener  60  may include a torque limiting feature, such as first and second portions that are removable from one another when a predetermined torque limit is exceeded. For example, the first portion may include a breakaway portion  62  and the second portion may include a fastening portion  64 . Breakaway portion  62  may be coupled to fastening portion  64  and may include a series of flats engaged with flats  58  within third portion  56  of central opening  40 , providing for driving of fastener  60  by the driving assembly. Breakaway portion  62  may be retained within third portion  56  of central opening  40  through an interference fit engagement therewith. Fastening portion  64  may include a head  66  having upper and lower portions  68 ,  70  and a threaded shank  72 . Breakaway portion  62  may be integrally formed with and coupled to upper portion  68  of head  66  at a reduced diameter breakaway region  74  and threaded shank  72  may extend from lower portion  70 . A series of recesses (not shown) may be formed in a perimeter of head  66 . 
         [0019]    Fastener  60  may be formed from a variety of materials including, but not limited to metals (including titanium, titanium alloys, stainless steel, zirconium, and CoCr), biocompatible non-resorbable materials (including polyetheretherketone (PEEK) and polyetherketoneketone (PEKK)), biocompatible resorbable materials, ceramics, composite materials, allograft or xenograft (including demineralized bone matrix and coral), or combinations thereof. 
         [0020]    The fasteners and fastener insertion method discussed below may include a variety of applications such as to craniofacial procedures, neurosurgical procedures, spinal procedures, orthopedic procedures, suture anchors (Glencord anchors), small bone fixation/anchors, anterior cruciate ligament (ACL) fixation devices (tendon repair devices), and soft tissue anchors. Further, while discussed with respect to powered driver  12 , adapter assembly  14 , and fastener  60 , it is understood that a variety of alternate driving assemblies and fasteners may be used as well. 
         [0021]    As seen in  FIGS. 2 and 3 , fastener  60  may be fixed to another structure  76  such as bone, wood, or some other media. A pilot hole  78  may be drilled into structure  76  and fastener  60  may be inserted into pilot hole  78 . More specifically, fastener  60  may cut threads into pilot hole  78 . Fastener  60  may therefore act as a self tapping screw. In order to insert a self-tapping screw, work is performed on the screw. Work is generally defined as a force imparted over a distance: 
         [0000]      Work= F   i   ×D;    
         [0000]    where F i  is force and D is distance. 
         [0022]    In the context of the present disclosure, the force (F i ) noted above in the work definition may generally include a sum of the force (F c ) needed to cut (or tap) threads and the force of friction (F f1 ) from the threads on structure  76 . The driving assembly may apply the force (F i ) to fastener  60  in order to insert fastener  60  into pilot hole  78 . The force (F s ) applied by fastener  60  may generally include the sum of the force (F t ) from the torque imparted on fastener  60  and the force (F k ) from the kinetic energy of fastener  60 . Therefore, in order to insert fastener  60  into structure  76 , force (F e ) should be greater than force (F i ). When fastener  60  is inserted at low rotational speeds, the kinetic energy force component (F k ) may be small relative to the torque force component (F t ). 
         [0023]    Kinetic energy is generally defined as: 
         [0000]        K= ½ Iω   2 ;
 
         [0000]    where K is kinetic energy, I is moment of inertia, and w is angular rotational velocity. Angular rotational velocity is directly proportional to rotational speed. Therefore, the kinetic energy force component (F k ) may be directly proportional to the square of the rotational speed that fastener  60  is being driven at. As rotational speed of fastener  60  increases, the torque force component (F t ) needed to drive fastener  60  into structure  76  may be reduced. More specifically, fastener  60  may be driven at a rotational speed that reduces the torque force component (F t ) required to drive fastener  60  into structure  76  below the torsional strength of fastener  60 . As such, fastener  60  may cut threads into structure  76 , even where structure  76  has a torsional resistance that is greater than the torsional strength of fastener  60 . 
         [0024]    However, once fastener  60  is fully inserted into structure  76  and head  66  abuts structure  76 , the force (F i ) required to further insert fastener  60  into structure  76  may be increased by the force of friction (F f2 ) from head  66  on structure  76 . In order to maintain rotation of fastener  60 , force (F s ) must be increased. If the rotational speed that fastener  60  is being driven at remains constant, the torque force component (F t ) increases. Once the torque force component (F t ) exceeds the torsional strength of breakaway region  74 , breakaway portion  62  may be separated from fastening portion  64 . Alternatively, some other torque limiting feature of fastener  60  or the driving assembly may prevent further transmission of driving torque to fastening portion  64 . 
         [0025]    However, the force (F s ) applied to drive fastener  60  may be limited such that it is greater than the sum of the force (F c ) needed to cut (or tap) threads and the force of friction (F f1 ) from the threads on structure  76 , but less than a strip-out force (F so ), (F so &gt;F s &gt;F c +F f1 ). Strip-out may occur when fastener  60  is located within structure  76  and rotationally driven without further insertion into structure  76 . In order to avoid a strip-out condition once fastener  60  is inserted into structure  76  and fastener head  66  is seated against structure  76 , the force applied to fastener  60  (F s ) may be limited such that it is less than the sum of the force (F c ) needed to cut (or tap) threads, the force of friction (F f1 ) from the threads on structure  76 , and the force of friction (F f2 ) from head  66  on structure  76 , which is less than the strip-out force (F so ), (F so &gt;F c +F f1 +F f2 &gt;F s ). 
         [0026]    While discussed with regard to pilot hole  78 , the arrangement discussed above may also be used where fastener  60  is a self-drilling fastener and there is no pilot hole. In the self-drilling configuration, the discussion above applies equally, except the force (F i ) needed to drive fastener  60  may be increased by a drilling force (F d ). Accordingly, the kinetic energy force component (F k ) of fastener  60  may also be increased in order to keep the torque force component (F t ) below the torsional strength of fastener  60 . 
         [0027]    Pilot hole  78  may have a diameter that is greater than the minor diameter and less than the major diameter of fastener  60 . The rotational speed needed to drive fastener  60  may vary based on the relation between the size of pilot hole  78 , the major diameter of fastener  60 , the length of threaded shank  72 , and the material density of structure  76  relative to the material density of fastener  60 . 
         [0028]    More specifically, as the diameter of pilot hole  78  is increased, the rotational speed needed to drive fastener  60  is reduced and as the diameter of pilot hole  78  is decreased, the rotational speed needed to drive fastener  60  is increased. Similarly, as the length of threaded shank  72  is increased, the rotational speed needed to drive fastener  60  is increased and as the length of threaded shank  72  is decreased, the rotational speed needed to drive fastener  60  is decreased. As the material density of structure  76  is increased relative to the material density of fastener  60 , the rotational speed needed to drive fastener  60  is increased and as the material density of structure  76  is decreased relative to the material density of fastener  60 , the rotational speed needed to drive fastener  60  is decreased. 
         [0029]    In operation, adapter assembly  14  may be coupled to powered driver  12  and a desired driving speed may be selected for powered driver  12 . Fastener  60  may be loaded into adapter assembly  14  and an end of fastener  60  may be placed against an opening of pilot hole  78  and fastener  60  may be axially aligned with pilot hole  78 . Fastener  60  may then be driven into pilot hole  78 , as discussed above. 
         [0030]    More specifically, powered driver  12  may be rapidly actuated, rather than gradually actuated, to quickly generate a desired rotational speed. Downward force may be applied to fastener  60  by displacement of plunger  16  into engagement with fastener  60  once powered driver  12  has been actuated to achieve the desired rotational speed to drive fastener  60  into structure  76 . Fastener  60  may be formed from a material that has a greater or lesser torsional strength than the torsional resistance of structure  76 . However, even when fastener  60  is formed from a material having a lesser torsional strength than the torsional resistance of structure  76 , fastener  60  may still be driven into pilot hole  78  and may tap pilot hole  78  due to the kinetic energy force component (F k ) and the torque force component (F t ) of fastener  60 , as discussed above. 
         [0031]    More specifically, when fastener  60  is driven into pilot hole  78  at a high rotational speed, the force (F s ) applied by fastener  60  may be great enough to tap structure  76 , such as bone. As indicated above, when the rotational speed of fastener  60  is great enough, the kinetic energy force component (F k ) may reduce the torque force component (F t ) needed to drive fastener  60  to a level below the torsional strength of fastener  60 . Since the entire fastener  60  (breakaway portion  62  and fastening portion  64 ) is rotating at the high rotational speed, a minimal amount of torque is transmitted through fastener  60 , allowing fastener  60  to cut threads into structure  76  even when structure  76  has a torsional resistance that is greater than the torsional strength of fastener  60 . 
         [0032]    As discussed above, once head  66  of fastener  60  bottoms out on an outer surface of structure  76 , an amount of torque required to drive fastener  60  further into structure  76  becomes too great for the kinetic energy force component (F k ) of fastener  60  and the torque force component (F t ) is increased. Torque may then be transmitted to breakaway region  74  causing breakaway portion  62  to separate from fastening portion  64 . Powered driver  12  may then be turned off and breakaway portion  62  may then be ejected from adapter assembly  14  through use of ejector shaft  24 . 
         [0033]    Several parameters may be varied for driving fastener  60  in the method discussed above. For example, parameters that may be used for the appropriate fastener and driving arrangement according to the method described above may include the major and minor diameters of the threaded shank  72 , the body length of threaded shank  72 , the pilot hole diameter, the downward force applied to fastener  60 , the speed ramp up of powered driver  12 , the set speed of powered driver  12 , rate of trigger actuation of powered driver  12 , and the material properties of structure  76  and fastener  60 , such as material densities. More specifically, Table 1 below includes several configurations and associated parameter values for driving fastener  60  in the manner discussed above. The configurations listed below are examples and may generally apply to structure  76  being bone and fastener  60  being formed from a polymer. As indicated above, fastener  60  may be formed from a material that has a greater or lesser torsional strength than the torsional resistance of structure  76 . In the examples listed below, the polymer formed fastener  60  may have a torsional strength less than the torsional resistance of bone. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 major diameter 
                 body length 
                 pilot hole diameter 
                   
               
               
                 (mm) 
                 (mm) 
                 (mm) 
                 driver speed (rpm) 
               
               
                   
               
             
             
               
                 1.5 
                 5 
                 1.3 
                 12,000 
               
               
                 1.5 
                 3 
                 1.1 
                 20,000 
               
               
                 1.5 
                 4 
                 1.1 
                 25,000-30,000 
               
               
                 1.5 
                 4 
                 1.2 
                 25,000-30,000 
               
               
                 1.5 
                 5 
                 1.1 
                 25,000-30,000 
               
               
                 1.5 
                 5 
                 1.2 
                 25,000-30,000 
               
               
                 1.5 
                 8 
                 1.3 
                 25,000-30,000