Abstract:
A distal targeting device for locating at least one distal screw hole in an intramedullary nail that is positioned within a medullary canal of a bone, the device including an elongated and reconfigurable probe that is positionable inside a lumen of the nail.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/467,614, filed Mar. 25, 2011, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to locating the distal screw holes in intramedullary nails without using X-rays. In particular, the invention is used for locating distal screw holes by determining the anterior-posterior and/or the medial-lateral displacement of the distal screw holes and using this information to align an external targeting jig. 
       BACKGROUND 
       [0003]    Fractures of long bones are often treated with an intramedullary (IM) nail. An IM nail is a tubular metal implant bent a certain way to conform to the anatomy of the long bone being treated, such as the femur. An IM nail is inserted into the medullary canal of the bone and locked to the proximal and distal bone fragments with interlocking screws through holes located in the proximal and distal portions of the nail. Although locking both portions of the IM nail in place can be difficult because the nail is inside the bone and the screw holes cannot be visualized, locking the IM nail to the proximal portion of the bone can be somewhat easier to perform with the aid of targeting jigs attached to the proximal end of the nail, since the proximal portion is closer to the entry point of the nail into the bone than the distal portion. On the other hand, distal locking is more difficult because the nail is further from the entry point of the nail into the bone and targeting jigs attached to the proximal end of the nail can become misaligned with the distal holes when the nail deforms as it is inserted into the bone. 
         [0004]    One common method of locating the distal screw holes is by using a fluoroscope or C-arm image intensifier to visualize the screw hole and target it from outside the bone with a hand-held drill. However, X-ray exposure from repeated use of a C-arm poses risks to surgeons who accumulate significant amounts of scattered X-ray radiation throughout their professional careers. In addition, C-arms are expensive and not commonly available in all hospitals. 
         [0005]    Several targeting jigs have been proposed and created to locate the distal screw holes without the use of X-rays. However, many of these devices do not work consistently because of the misalignment that can be caused when the nail deforms. Thus, there is a need for improved devices and methods for locating the distal screw holes in an IM nail without the use of X-rays. 
       SUMMARY 
       [0006]    The devices and tools of the invention provide for accurate locating of distal screw holes in intramedullary nails. In a first configuration of the invention, a distal targeting device is provided for locating at least one distal screw hole in an intramedullary nail that is positioned within a medullary canal of a bone, the device comprising an elongated and reconfigurable probe that is positionable inside a lumen of the nail. The reconfigurable probe can include a plurality of segments extending longitudinally along a probe axis, wherein each of the plurality of segments is flexibly connected to at least one adjacent segment by a hinge. The plurality of segments of a particular probe can be rigid, flexible, or can include a combination of rigid and flexible segments, and the hinges can include a pivoting hinge or a flexible hinge. The plurality of segments can include one or more segments having ends with an associated extending member that is larger in at least one dimension than the segment end, but smaller than the inner lumen of an intramedullary nail in which it will be positioned. 
         [0007]    The distal targeting device can further include an adjustable external jig that is adjustable in at least one of an anterior-posterior direction and a medial-lateral direction to correspond to a location of the at least one distal screw hole in response to information received from at least one data-gathering member of the probe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention will be further explained with reference to the appended Figures, wherein like structure is referred to by like numerals throughout the several views, and wherein: 
           [0009]      FIG. 1  is a perspective view of a prior art intramedullary nail; 
           [0010]      FIG. 2  is a perspective view of a measuring probe of the invention; 
           [0011]      FIG. 3  is an enlarged perspective view of one of the flexible hinges of a measuring probe of the type illustrated in  FIG. 2 ; 
           [0012]      FIG. 4   a  is a detailed perspective view of a flexible hinge of a measuring probe, with a strain gage attached to the hinge; 
           [0013]      FIG. 4   b  is a detailed perspective view of a flexible hinge of a measuring probe, with two strain gages attached to the hinge; 
           [0014]      FIG. 5  is a perspective view of an embodiment of the invention including a measuring probe and an external jig; 
           [0015]      FIG. 6  is a perspective view of an embodiment of the invention being used with an intramedullary nail; 
           [0016]      FIG. 7  is a perspective view of another embodiment of a measuring probe of the invention; 
           [0017]      FIG. 8  is an enlarged perspective view of a hinge of the measuring probe of  FIG. 7 ; 
           [0018]      FIG. 9  is a perspective view of another embodiment of a measuring probe of the invention; 
           [0019]      FIG. 10  is a perspective view of another embodiment of an external jig, in accordance with the invention; and 
           [0020]      FIG. 11  is a perspective view of the external jig of  FIG. 10  and the measuring probe of  FIG. 9  in use with an intramedullary nail. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    External targeting jigs that are currently being used to locate distal screw holes for intramedullary (IM) nails are often inadequate due to bending deformations of the nail as it is inserted into the bone. For most nailing applications, the most significant deformation of the nail that affects the accuracy of external jigs is the bending that occurs in the anterior-posterior direction. If the displacement of the screw holes in the anterior-posterior direction can be determined, the screw holes can be targeted accurately using an external jig. The devices and methods of the invention are used to measure the displacement of distal screw holes in the anterior-posterior direction by using a probe placed into the lumen of the IM nail. An adjustable external jig is then aligned with the distal screw holes using the probe measurements. 
         [0022]    Referring now to the Figures, wherein the components are labeled with like numerals throughout the several Figures, and initially to  FIG. 1 , an exemplary prior art IM nail  1  with a lumen  2  going through its entire length and distal screw holes  3   a  and  3   b  is shown, which is one embodiment of an intramedullary nail. However, it is understood that other intramedullary nails having different configurations can also be used relative to the locating devices of the invention. 
         [0023]      FIGS. 2 and 3  illustrate a measuring probe  4  of the invention, which includes a probe base  9 , probe segments  6   a  and  6   b , and two flexible hinges  5  between probe base  9  and segment  6   a , and between segments  6   a  and  6   b . While two hinges are shown and discussed herein, it is understood that a particular measuring probe can include more or less than two hinges. The flexible hinges  5  have a relatively thin cross-section, which may be rectangular, for example, and which allow bending in one plane, as shown in  FIGS. 3 ,  4   a , and  4   b.    
         [0024]    In operation, measuring probe  4  is inserted from the proximal end into lumen  2  of an IM nail, such as nail  1 , such that the bending planes of flexible hinges  5  correspond to the bending plane of the nail (for example, parallel to the sagittal plane for femoral nails). As measuring probe  4  is inserted, the flexible hinges  5  can bend to allow the measuring probe  4  to conform to the bent or curved shape of lumen  2 . Probe base  9  and probe segments  6   a  and  6   b  are relatively rigid compared to the flexible hinges  5 . Due to the relative flexibility of flexible hinges  5  as compared to the adjacent areas of the probe structure, any deformation of measuring probe  4  as it is inserted into lumen  2  will be isolated and concentrated mainly or exclusively to the areas of the flexible hinges  5 . The base end  7   a  of probe base  9  and segment ends  7   b ,  7   c ,  7   d , and  7   e  of probe segments  6   a  and  6   b  are shown as spherical end portions having a diameter that closely matches the diameter of lumen  2  and which is larger than the diameter of the middle portions of probe base  9  and probe segments  6   a  and  6   b . This configuration will allow probe  4  to deform in a relatively consistent manner since the contact points are limited to predetermined locations at base end  7   a  and segment ends  7   b ,  7   c ,  7   d  and  7   e . The middle portions of probe base  9  and probe segments  6   a  and  6   b  can also be designed to have a bent or curved shape in order to better approximate the contour of lumen  2  and avoid contact with lumen  2  other than at the spherical ends. 
         [0025]    It is noted that the use of the term “spherical” relative to “spherical end portions” or “spherical members” throughout the description is not intended to solely encompass an end portion shaped as an actual sphere. Rather, the use of the term spherical herein with regard to the end portions of the invention can instead have a different shape, such as elliptical, cubic, triangular and the like. In order to provide the advantages described herein relative to contact between these members and the inside of a lumen of an intramedullary nail, however, at least one of the dimensions of the “spherical” members at the ends of probe segments should be larger than at least one dimension of the outer surface of the corresponding probe segment. Thus, these spherical members are alternatively referred to herein as “extending members.” 
         [0026]      FIGS. 4   a  and  4   b  illustrate exemplary embodiments of flexible hinge  5  in more detail. In the embodiment of  FIG. 4   a , a strain gage  8  is shown attached to one side surface of flexible hinge  5 . Optionally, two strain gages can be attached, one on each side of flexible hinge  5 , as shown in  FIG. 4   b . As measuring probe  4  deforms about one or more of the hinges  5  while being inserted into lumen  2 , the amount of bending deformation at each flexible hinge  5  is measured by strain gage  8 . As long as the bending stresses on flexible hinge  5  are kept within the elastic range, this bending deformation will have a linear correlation with the angular displacement of each segment of probe  4  relative to the adjacent segment, such as angular displacement of probe segment  6   a  relative to probe base  9 . Multiplying the angular displacement by the length of the segment allows calculation of the displacement of each segment end relative to the axis of the adjacent segment, such as the displacement of segment end  7   c  relative to axis  9 ′, where axis  9 ′ is shown in  FIG. 3 . The displacement of segment end  7   b  relative to segment end  7   a  can be neglected due to their close proximity to each other, or a certain correction factor can be factored into the computations. Similarly, the displacement of segment end  7   e  relative to axis  6   a ′ (see  FIGS. 2 and 3 ) can be computed from the angular displacement between segments  6   a  and  6   b . Adding these displacements together will allow the calculation of the total displacement of the segment end  7   e  relative to base  9  of measuring probe  4 . In operation, segment end  7   e  can be positioned near either of distal holes  3   a  or  3   b  so that the position of segment end  7   e  will closely correspond to the location of the distal hole relative to probe base  9 . 
         [0027]      FIG. 5  illustrates base  9  attached to an external jig  10 . External jig  10  has a nail attachment portion  11 , which can be rigidly attached to the proximal end of nail  1 , a pivoting joint  12 , and an extension arm  13  with guide holes  14   a  and  14   b . Extension arm  13  is calibrated in such a way that the distance of the guide holes  14   a  and  14   b  from the proximal end of nail  1 , when measured along the axis of the nail, corresponds to the distance of the distal holes  3   a  and  3   b  from the proximal end of nail  1 , respectively, when measured along the nail axis. Using the known position of segment end  7   e , as can be computed from the strain gage readings as discussed above, the user will be able to position extension arm  12  in the anterior-posterior direction such that guide holes  14   a  and  14   b  will be aligned with the distal holes  3   a  and  3   b  of intramedullary nail  1  respectively, as is shown in  FIG. 6 . 
         [0028]    Any or all of the strain gages  8  can be connected to appropriate electronic circuitry and devices to measure the strains at flexible hinges  5 . The strain values in turn can be converted to displacement data by calibration or by using appropriate equations and conversion factors known to those skilled in the art. Although not illustrated in the figures, spaces for electrical wiring to the strain gages can be made, for example, by hollowing out or cutting grooves along the lengths of segments  6   a  and  6   b , and/or of base  9 . 
         [0029]      FIG. 7  illustrates another embodiment of a measuring probe  15  of the invention. In this embodiment, rather than having rigid segments connected by flexible hinges discussed relative to the above embodiment,  FIG. 7  provides for a probe  15  composed of a rigid base member  16 , a flexible hinge  17 , and a flexible segment  18 . Base member  16  is composed of a roughly cylindrical portion  16   a , along with two spherical portions  16   b  and  16   c  having diameters that closely match the diameter of lumen  2  of the intramedullary nail  1  in which the probe  15  will be positioned. The diameters of the spherical portions  16   b  and  16   c  can also be larger than the diameter of cylindrical portion  16   a . Flexible hinge  17  is attached to base  16  and also to flexible segment  18 , as shown in  FIG. 8 . Flexible hinge  17  has a thin rectangular cross section and can also be equipped with one or more strain gages, such as strain gage  19 . Flexible segment  18  is composed of a flexible portion  18   a  and a spherical portion  18   b  at its distal end, where the spherical portion  18   b  can have a diameter that closely matches the diameter of lumen  2  of nail  1 , and which is larger than the diameter of flexible portion  18   a . Flexible portion  18   a  is constructed to be much more flexible than rigid base member  16 , such that when subjected to the same bending moment, the deformation of rigid base member  16  will be significantly less than the deformation of flexible portion  18   a.    
         [0030]    As probe  15  is inserted into the intramedullary nail  1 , the portion of probe  15  comprising flexible hinge  17  and flexible segment  18  behaves like a cantilever beam and bends to approximate the contour of the distal part of nail  1 , while rigid base  16  remains substantially straight, thereby approximating the straight contour of nail attachment portion  11  and the proximal part of nail  1 . Thus, contact between probe  15  and the inner walls of nail attachment portion  11  and lumen  2  is limited mainly or exclusively to spherical portions  16   b ,  16   c , and  18   b . Measurements from strain gage  19  will be directly proportional to the displacement of spherical portion  18   b  relative to base  16  and can be used to locate distal holes  3   a  and  3   b  of intramedullary nail  1 . Flexible portion  18   a  can also be designed to relatively closely approximate the nail contour when it bends, for example, by having a tapering cross section instead of a constant cylindrical cross section along its length, or by having it pre-bent in a certain way. 
         [0031]      FIG. 9  illustrates another exemplary embodiment of a measuring probe of the invention. As is illustrated in this Figure, a probe assembly  20  is composed of two probes  21  and  24  that can have substantially similar constructions to each other, and which are rigidly connected to each other via a connector member  28 . Probes  21  and  24  can be generally constructed as described above relative to the construction of probe  15 , except that base  25  of probe  24  has only one spherical portion  25   a  as compared to the two spherical portions  22   a  and  22   b  of base  22  of probe  21 . This is intended to avoid redundant supports and provide for a stable and repeatable orientation of base  22  and base  25  relative to an exemplary external jig  29  that is shown in  FIG. 10 . 
         [0032]    Since probes  21  and  24  are substantially similar in construction, and contact to the probes when the probe assembly  20  is in use is limited mainly or exclusively to spherical portions  22   a ,  22   b ,  23   a ,  25   a , and  26   a , the positions of spherical portions  23   a  and  26   a  in the anterior-posterior direction relative to base  22  and base  25  will be the same if the strain gage readings for probes  21  and  24  are the same. A calibration factor may be used to accommodate any variations between probes  21  and  24 . 
         [0033]      FIG. 10  illustrates an external jig  29  and  FIG. 11  illustrates nail  1  attached to the external jig  29 , with probe assembly  20  inserted into nail  1  and external jig  29 . External jig  29  is equipped with guide walls  30  and  31  to simulate the inner wall of nail attachment portion  32  and lumen  2  of nail  1 , and to provide contact points with spherical portions  25   a  and  26   a . In operation, probe  21  is inserted into nail  1  such that spherical portion  23   a  is at or near the distal hole to be targeted, while probe  24  is inserted through the guide walls  30  and  31 . Extension arm  33  is then adjusted in the anterior-posterior direction, while bending flexible segment  26  and flexible hinge  27  of probe  24  in the process, until the strain gage readings in probes  21  and  24  are identical. At this point, the positions of spherical portions  23   a  and  26   a  relative to base  22  and base  25 , respectively, will also be identical. Thus, guide hole  34   a  (and/or guide hole  34   b ) will be aligned with the corresponding nail distal hole being targeted. 
         [0034]    In accordance with the invention described herein, any of the deformable probes can be designed to provide for contact with the inner lumen of an intramedullary nail and/or nail holding instrument, and can be limited to a certain number of predetermined points. The purpose of this is to ensure repeatability and accuracy of measurements. If contact points are not accurately known, the readings received from strain gages will not be repeatable. In other words, if the contact points differ, the readings for the same position of the distal end of the probe can be different. 
         [0035]    The concepts described above can also be modified by using more or less flexible hinges (and corresponding number of segments) than are illustrated in the figures, rigid segments, and flexible segments, if desired, such for the purpose of accommodating sharper or shallower bending of the intramedullary nail, for example. The hinges can also be designed to allow bending in more than one plane to accommodate bending deformations in more than a single plane. 
         [0036]    The present invention has now been described with reference to several embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures described herein, but only by the structures described by the language of the claims and the equivalents of those structures.