Abstract:
A device and method for targeting objects and specifically for locating intramedullary screw openings is described. The device and method include a target magnet and a sensor comprising an elliptical array of magnetoresistive elements, designed to give information on the three-dimensional orientation of the magnet. The sensor array is designed such that each magnetoresistive element is a member of an opposing pair and relays information on their alignment with the target magnet. The array is connected to a display such that the position of the sensor in relation to the target magnet is easily discerned. The invention is lightweight and portable, capable of operating on batteries and can be used in primitive situations where a stable supply of electricity is not available.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 USC §119(e) to U.S. Provisional Patent Application 60/415,952 filed Oct. 3, 2002, the entirety of which is incorporated herein by reference. 
    
    
     REFERENCE TO CITATIONS 
     Complete bibliographical citations to the references can be found in the list preceding the claims. 
     FIELD OF THE INVENTION 
     The present invention relates to a targeting device in general and specifically to a method and device for positioning locking screws for intramedullary nails. The invention describes sensitive methods for magnetic detection of transverse interlocking screw openings in real time. 
     DESCRIPTION OF THE PRIOR ART 
     Each year, approximately 14,000 femoral and 12,000 tibial internal fracture repairs are performed by orthopedic surgeons in the United States. Internal fixation of long bones has allowed shorter hospitalization times and earlier weight bearing for the patient, compared to plaster cast or external fixation. 
     The medical procedure involves the fixation of long-bone fractures by inserting an intramedullary nail (“IMN”), also known as a locking rod, a long, thin-walled, metallic nail, into the medullary canal of the damaged bone. The surgeon introduces the implant by creating an opening in the proximal end of the bone, exposing the medullary canal. The bone fragments are aligned, and the IMN is passed through the fragments, creating a means for internal support. 
     The IMN is prefabricated with openings in the proximal and distal ends, which are designed to accept transverse interlocking screws. These screws are essential to control the rotation and translation of the bone fragments with respect to each other. To insert the transverse, interlocking screws, it is necessary to align and drill through the bone to meet the proximal and distal interlocking screw openings of the IMN. 
     One of the most difficult parts of intramedullary nailing of long bones is locating and drilling the interlocking screw openings. IMN interlocking screw placement requires the surgeon to locate the openings in the nail, center the drill and advance the bit through the bone to meet them. The interlocking screws are then inserted. Proximal interlocking screw placement is relatively easy because the openings can be located with an external guide attached to the end of the IMN. However, this technique does not work well for distal interlocking screw placement. 
     Complicating the process of identifying and drilling the distal interlocking screw openings is the deformation that routinely occurs to the IMN upon implantation in the medullary canal. Studies have shown that deformation occurs in several planes due to medial-lateral and anterior-posterior flexion of the distal nail after it has been inserted (Krettek et al., 1996; 1997; 1998). On insertion, the distal IMN may exhibit a mean lateral deflection of 4.5±3.0 mm and dorsal deflection of 7.8±5.8 mm. In addition, rotational deformation of the distal IMN has been measured at 0.3±0.7 degrees. The distal nail may deform from its original shape in any of these coordinates to some degree. The nail can deform to the shape of the medullary canal upon insertion. The shape of the canal varies widely from person to person, and it is not possible to predict how the nail will deform accordingly. Therefore, it is difficult to determine what the resultant location of the distal interlocking openings will be relative to their initial position. 
     In addition, there are narrow tolerances between the screw and interlocking opening. To avoid any complications, it is important to place the interlocking screws accurately. The physical tolerance between the screw opening and screw must be taken into account when targeting and drilling to allow room for proper insertion. 
     If the drill bit is not within the tolerance constraints or misses the opening, a second attempt must be made. Unfortunately, once a hole in the bone is started, it is difficult to correct. In some cases, the bone may be too weak to accommodate another hole, and then bone grafting or other means of fracture fixation must be employed. 
     Prior Art X-Ray Technology 
     One prior art method for providing a precise location of the transverse distal openings in IMN&#39;s uses X-rays. Correct alignment is indicated when the desired interlocking opening appears as a perfect circle under X-ray fluoroscopy with the drill bit in the exact center. If alignment is lost, the drilling must be stopped and the drill bit realigned using further X-ray imaging. In its most rudimentary form the opening is drilled with freehand means. The process of drill bit centering requires the soft tissue of the patient&#39;s extremity to be separated all the way to the bone so the surgeon has sufficient room to maneuver. X-ray imaging requires long periods of X-ray exposure first, to identify the location of the transverse distal opening and second, to correctly drill the opening. Thus, X-ray exposure may accumulate to dangerous levels for both the surgeon and patient. In addition, X-ray imaging necessitates moving X-ray equipment into and out of position, allowing numerous opportunities for loss of alignment each time the equipment is repositioned. 
     The need to reduce damage to soft tissue during these procedures has led to the use of less invasive techniques. These techniques include percutaneous methods wherein surgical instruments are inserted through small incisions in the skin, thus reducing soft tissue damage. Methods to optimize percutaneous techniques include aiming devices, which rely on mechanical approaches to locate the distal screw openings. 
     The simplest of these mechanical devices uses an external arm with openings that correspond to the screw opening location in the IMN. Once the IMN is implanted, the external guide arm is attached firmly to its proximal end creating a solid link. The openings corresponding to the screw openings then serve as a drill sleeve for drill alignment. Unfortunately, due to the tight tolerances required for screw location and the degree of distal nail deformation occurring, this approach still requires an inordinate amount of X-ray exposure and still carries the risk of misplacing the opening. Further, while methods to limit exposure of individual patients to X-rays have been explored, the need to perform the surgery using X-rays for detection means that the surgical team is serially subjected to X-ray exposure. 
     Prior Art Magnetic Technology 
     The desire to target accurately without X-ray imaging has led to recent attempts to use magnets for targeting of the distal IMN screw openings. Devices have been developed that use external magnetic sensors to find the position of a flux field induced in the IMN by permanent magnets or electromagnets (U.S. Pat. No. 4,621,628 to Brudermann). Some devices have even tried to magnetize the whole IMN and look for variations in the magnetic flux that occur around the interlocking openings (Zacheja et al., 2000). 
     Other devices target on a magnet placed inside the IMN at the same position as the opening. For instance, U.S. Pat. Nos. 5,049,151, 5,514,145 and 5,703,375 to Durham et al. teach the use of a pivotable magnetic targeting device to position a guide wire by which a cannulated drill is directed to align the drill bit with the interlocking screw opening. The targeting device is a second, pivoting magnet, attached to a drill sleeve acting as a compass to direct the drill bit toward the target magnet. The Durham et al. device uses a magnet placed inside the IMN, directly aligned with the axis of the distal screw opening to be targeted. The magnet is inserted on a rod through the proximal opening in the hollow nail, while its insertion depth is fixed by a locking pin. Once the magnet is placed adjacent, generally proximal to the distal openings, a skin incision is marked using a magnetic compass to locate the position of the internal magnet which projects central flux lines parallel to the axis of the opening. After the skin and tissue are separated to allow working room, another magnet on a central pivot inside a tube is inserted down to the bone surface. These two magnets attract each other and align a guide wire, which is then inserted in the bone surface. The magnets are removed and a cannulated drill bit is advanced over the guide wire, which is now aligned directly with the axis of the opening. Finally the interlocking screw is inserted and the procedure is repeated for the more proximal opening. 
     Advantages of Magnetic Targeting 
     Magnetic targeting has some significant advantages. Magnetic fields can penetrate the IMN and human tissue without being distorted or causing physiologic damage, unlike X-rays. Also, magnetic devices can be to require little power, allowing portable, battery operation. 
     Disadvantages of Prior Art Magnetic Targeting 
     A notable drawback is that most targeting devices are manufactured to work only with specific nails and are not adaptable to others. In addition, the magnetic field must be powerful enough to be detectable at distances of 10 cm. This is the average maximum distance encountered between the center of the IMN and the exterior of the patient&#39;s limb at the thickest site of IMN implantation, usually about the femur. While electro-magnets can generate stronger fields, devices that use electric current inside the body to create magnetic fields require stringent FDA approval because of their inherent danger. 
     U.S. Pat. No. 4,621,628 to Brudermann describes a method for the magnetic identification of transverse locking openings wherein the sensor is inserted into the IMN and the magnet is placed percutaneously on the broken limb. In this disclosure the sensors, in the form of intersecting Hall elements, are inserted into the IMN to the area of the transverse screw opening and are connected to an external display. The magnet is placed on the surface of the skin until the axis of the field is aligned, wherein a zero point indication is signaled on the display. While Brudermann teaches a non X-ray means of detecting the transverse screw opening, it suffers from the draw backs of inserting an electrical device inside the medullary cavity, the low sensitivity of the Hall Effect sensors used to detect the magnets, the lack of three-dimensional resolution to the display and the lack of portability to the entire device. 
     In a more recent use of magnetic targeting, U.S. Pat. No. 6,162,228, to Durham describes a method of using a target magnet inserted into the IMN and a target sensor, which is essentially mechanical, having a compass that indicates the position of the target magnet in the IMN. This invention is similar to the other Durham patents identified above with the exception that the target magnet produces an output comprising a light or buzzer when the targeting unit is aligned. While this device solves the problem of excessive exposure to X-rays, it has neither the sensitivity to penetrate the combined tissue layers nor the ability to discriminate the orientation of the screw opening in three-dimensional space. It does allow real time feedback while drilling because the target magnet occupies a space offset from the internal diameter of the screw opening during targeting. 
     Due to the aforementioned problems with locating the distal screw openings of IMNs, including excessive X-ray exposure, excessive soft tissue damage, the need for expensive and bulky equipment and the desire for real time imaging, there is a need for a sensitive and easily visualized sensing device that is both portable and safe for use in locating distal transverse screw openings in IMNs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and device for percutaneously locating transverse screw openings in IMNs using a magnetic target and a sensitive and accurate targeting device. 
     Specifically, the invention is directed to a targeting device for detecting a precise location and position within a hollow object having an opening, comprising a magnet adapted to be positioned within the opening of the tube for providing a directional field, wherein the magnet includes a three-dimensional orientation on an x-axis, a y-axis and a z-axis; and a target device, including sensing means for identifying the magnet location along the three-dimensional orientation of the magnet; and a display means, wherein the display means represents the orientation of the sensing means. 
     The present invention is also directed to a method for detecting a precise location and position within a hollow object having an opening and an external surface, comprising inserting and positioning a magnet having a three-dimensional orientation in the opening of the object at a discrete position in the tube, wherein the magnet includes a three-dimensional orientation on an x-axis, a y-axis and a z-axis; providing a target device external to the discrete position, wherein the targeting device comprises sensing means for identifying the magnet location and position along the three-dimensional orientation of the magnet and a display means indicating the position of the sensing means in relation to the magnet; and moving the target device along the external surface location until the sensing means senses the three-dimensional orientation of the magnet. 
     The present invention is further directed to a targeting device for percutaneously detecting the location and position of screw openings within an intramedullary nail for the internal fixation of long bones, wherein the intramedullary nail has a longitudinal opening and screw openings, comprising a magnet adapted to be positioned within the longitudinal opening of the intramedullary nail for providing a directional field, wherein the magnet includes a three-dimensional orientation on an x-axis, a y-axis and a z-axis; and a target device, including sensing means for identifying the magnet location along the three-dimensional orientation of the magnet; and a display means, wherein the display means represents the orientation of the sensing means. 
     The present invention is still further directed to a method for detecting the location and position of interlocking transverse screw openings within an intramedullary nail for the internal fixation of long bones, wherein the intramedullary nail includes a longitudinal opening and interlocking screw openings, comprising inserting and positioning a magnet having a three-dimensional orientation in the opening of the intramedullary nail to a discrete position proximal to the transverse interlocking screw opening; providing a targeting device external to the discrete position, wherein the targeting device comprises sensing means for identifying the magnet location along the three-dimensional orientation of the magnet and a display means indicating the position of the sensing means in relation to the magnet; and moving the target device along the external surface location until the sensing means senses the three-dimensional orientation of the magnet. 
     The present invention also provides a target magnet that can be affixed to the end of an insertion rod and inserted in the cavity of an IMN to a discrete position proximal to the transverse interlocking screw opening. The magnet has an axisymmetric flux field thereby relating information on its orientation in relation to the screw opening. In addition, the method is designed such that the flux field is detectable at a distance of 10 cm or more. Ten cm is the average maximum distance encountered between the intramedullary canal and the outside of the patient&#39;s limb. 
     The invention further describes a method for sensing the flux field whereby commercially available magneto-resistive (MR) elements are arranged in an elliptical pattern thereby being aligned perpendicular to the lines of flux. The targeting device comprises a sensor comprised of eight MR elements arrayed in an elliptical pattern with the elements comprising four pairs opposing member elements. Because direct centering of the target between members of a pair of MR sensors elicits the same magnitude response from each member of the pair, the difference in output between pair members is relative to the spatial difference of each pair member from the target magnet. By comparing voltage offset between opposing sensors in the array, it can be determined which direction in the field the sensors must be moved to elicit an equivalent output, thus indicating exact centering over the target magnet. 
     The targeting device locates a permanent magnet locked in place offset from the openings to be drilled. The north pole of the magnet must face medially (along the z-axis) so that it projects a magnetic field having a central line of flux parallel to the axis of the interlocking opening. From outside the extremity in which the IMN is inserted, the targeting is performed by an array of magnetic sensors held parallel to the medial plane. These sensors are embedded in a targeting device handle which has at least one and preferably two drill sleeves attached at its distal end. The surgeon can advance the drill bit through the bone without hitting the magnet, while maintaining alignment feedback in real time. A display on the handle of the targeting device includes a position indicator, preferably in the form of a “bull&#39;s-eye” of light emitting diodes (LED&#39;s). The outside of the display will consist of a ring of lights, with one offsetting light in the center. The ring of lights to indicate the position the drill sleeve must move to have correct alignment. When properly aligned, the ring of lights will be off and the central light will be lit. The surgeon is then ready to advance the drill bit through the drill sleeve and drill a hole in the bone in order to insert the interlocking screws without aid of fluoroscopy or extraneous targeting systems. 
     In a further embodiment of the invention, the sensor display comprises a handle with the display in the middle and a drill sleeve in the distal end. By this means, the sensor can be aligned with the target magnet and the interlocking drill opening drilled while simultaneously, in real time, monitoring the position of the drill bit in relation to the interlocking screw opening. 
     It is another aspect of the invention that the target magnet is designed to relay information about its position and orientation in relation to the axis of the transverse interlocking screw opening. Thus, the magnet is designed to have a non-circular, axisymmetric flux field allowing the sensor to distinguish rotation about the z-axis, while the peak flux lines perpendicular from the magnet indicate its exact center. 
     Advantages: 
     Advantageously, the system of the present invention can use some of the existing magnet insertion techniques, but applies an electronic approach to the targeting issue. The targeting device of the present invention locates a permanent magnet that will be locked in place offset from the opening to be drilled. 
     The present invention makes a significant contribution to orthopedic surgeries involving IMN interlocking. Approximately 30,000 of these surgeries are performed each year, providing a larger market for the device. The present invention has advantages that alleviate many problems that arise during IMN distal interlocking screw opening identification. 
     The advantages of the present invention include: portability, low power requirement; X-ray independent identification, targeting independent of IMN deformation, providing a non-invasive/non-radioactive imaging technique, accurate and repeatable identification of the distal IMN, adaptability for use with existing IMN&#39;s, ease of learning and ease of use, and simple design and concomitant inexpensive means of manufacture. In addition, there are no in vivo active or passive electronics; no x-ray imaging is needed for targeting; there is real time feedback of alignment; and the system is battery operable. 
     The magnetic targeting device can improve orthopedic surgeons&#39; ability to target and drill distal IMN interlocking screw openings. The device has significant advantages that will appeal to orthopedic surgeons that perform IMN insertions and interlocking. 
     This device is able to resolve all degrees of freedom needed to accurately align the drill bit with the central axis of the interlocking opening, within the given tolerances. This device gives feedback of position in real time, so that alignment can be maintained during drilling. The prototype device achieves targeting without x-ray exposure. Although fluoroscopy may be employed to check proper screw interlocking, this device has the potential to eliminate x-ray use during targeting. 
     The application of this prototype allows for a percutaneous approach to interlocking screw opening targeting and drilling. Also, it can be used to locate the exact location of skin incisions needed above the interlocking openings for insertion of the drill bit. A visual positioning display was created to provide feedback of drill alignment during targeting. It is also possible to provide configure the prototype to provide audible and tactile feedback as well. The prototype includes a calibration circuit used to zero the sensors prior to targeting. This calibration can negate the effects of extraneous magnetic field present in the operating room. 
     This device has additional benefits. The prototype&#39;s target magnet could be adaptable to any nail, providing the nail is hollow and non-ferrous. The cylindrical magnet shape, with a diameter preferably less than 3 millimeters (“mm”), allows the magnet to be placed lengthwise in the smallest, hollow IMN&#39;s used for bones such as the humorous or tibia. The device has low power requirements and can be powered by battery. The prototype can be incorporated with existing drill sleeves, IMN&#39;s, and magnet insertion rods, while only needing a handle to be fabricated to connect all the pieces. 
     Other Uses of Invention: 
     While the preferred embodiment of this invention will be described with respect to the use of an IMN for repairing long bones, such as femurs, it is within the scope of the present invention to have other uses. These include: tracking and positioning of medical instruments, including endoscopes, catheters and implants within the body; use of location and targeting devices used in industry, particularly with materials that are X-ray sensitive; replacement of jigs and other measurement systems used in industry and manufacturing; providing positioning feedback for robotic devices; and, any process requiring blind hole targeting in non ferrous materials including, precise positioning of opposing elements such as in cabinetry making, fiberglass fabrication and construction and processes involving ceramic and tile fabrication and installation. As previously discussed, the use of electromagnets is not recommended for in-vivo uses; however, electromagnets may be well suited to these other uses of the invention. 
     The objects and advantages of the invention will appear more fully from the following detailed description of the preferred embodiment of the invention made in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the invention illustrating the target magnet inserted in the IMN and attached to an insertion rod with the target sensing device connected to a drill sleeve. 
         FIG. 2  is a representation of the flux lines produced by the magnet. The elliptical array is shown in which each sensor lies perpendicular to the flux field. This arrangement allows for equivalent voltage outputs from all the sensors when the array is centered in the flux field over the magnet. 
         FIGS. 3   a  and  3   b  are graphs illustrating plots of flux density in the y-z and x-z planes above the chosen cylindrical magnet. The peak is centered exactly over the magnet, parallel with the z-axis and is detectable at the required targeting distance of 10 cm. 
         FIG. 4  is a schematic diagram of the sensing device with an insert representing the target magnet and flux lines and the elliptical sensing array in three-dimensional space. 
         FIG. 5  is a diagrammatic illustration of the electronic system of the present invention. 
         FIG. 6  is partially exploded perspective view of the targeting device illustrating the placement of the electronic system in the device. 
         FIG. 7  is a plan view showing the invention in operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As embodied and broadly described herein, the present invention is directed to a method and device for detecting the position of interlocking transverse screw openings within an IMN for the internal fixation of long bones. The IMN device consists of electronics which interface to magnetic sensors and a display to indicate target proximity. The housing supports the electronics and a drill sleeve. The unit is typically powered by a battery. 
     Intramedullary Nail 
     Referring now to  FIG. 1 , there is illustrated a hollow intramedullary nail (IMN)  10 , known to the art. Examples of IMN&#39;s are prevalent in the prior art. For example, reference is made to U.S. Pat. No. 6,503,249 to Krause and the patents to Durham (cited herein), the contents of which are incorporated herein for a description of IMN&#39;s and manners of use. The IMN  10  is an elongated metal rod having a hollow body portion or shaft  15 . The IMN  10  includes a first locking screw opening  12  and a second more distal locking screw opening  14 . While the screw openings  12 ,  14  of typical IMNs  10  are transverse, i.e., positioned at a ninety degree angle in relation to the nail as illustrated in  FIG. 1 , it is within the scope of the present invention to have non-transverse screw openings, i.e., openings at other than ninety degrees in relation to the length of the IMN  10 . For purposes of this disclosure such openings are termed “oblique.” Prior to placement of the IMN  10 , a reaming rod known to the art is worked through the medullary cavity of a long bone  20 , such as a broken femur, tibia or humerus bone. The IMN  10  is then placed within the medullary cavity for securing within the bone  20  by means of cross-locking screws or bolts positioned through the screw openings  12 ,  14  (not illustrated in  FIG. 1 ). 
     Magnet  30   
     In order to align and advance the drill bit ( 60  in  FIG. 7 ) through the bone  20  accurately, the surgeon must have accurate knowledge of the position of the drill sleeves  47 ,  48  in relation to the axes  35 ,  39  of the transverse locking screw openings  12 ,  14 . This requires a target magnet that provides a suitable magnetic field to resolve all degrees of freedom. Therefore, the magnetic field within the IMN  10  must have a shape and polarity that affords unique targeting information in all possible planes. For targeting with this approach, the flux lines  50 , illustrated in  FIG. 2 , have a peak and a non-circular field shape about the axis of each plane so that the targeting sensors  44  may be aligned. A non-circular, axisymmetric field was selected; allowing the sensors  44  to distinguish rotation about the z-axis  36 , while the peak flux lines  50  perpendicular from the magnet  30  indicate its exact center. 
     In verification of this design, Ansoft&#39;s Maxwell 3 D magnetic modeling program (http://www.ansoft.com/products.com/max3d) was used to compare various magnet shapes and orientations. The magnetic field that was found to afford the required properties for targeting would have a peak and a non-circular field shape about the axis of each plane so that the targeting sensors may distinguish position from any direction. The shape and polarity that was found to afford the optimal field was a cylindrical Neodymium Iron Boron (NdFeB) magnet that is polarized across its axis. A preferred size for the magnet has a diameter of about 3 mm and a length of about 7 mm. The field from this magnet must be detectable at a maximum distance typically encountered between the center of the IMN  10  and the outside of the patient&#39;s limb, which is approximately 10 centimeters (“cm”). For the small percentage of large patients who have an IMN place in an extremity of exceptional diameter, the surrounding tissue can be compressed to bring the distance below 10 cm. 
     It is within the scope of the present invention to use different magnet shapes and materials can be used as long as the sensor array used to target them is adjusted to match the flux field of the magnet. It must also provide the desired flux field for feedback of discriminate targeting in all required planes. Additionally, an electro-magnet may be used to achieve a similar field if desired. 
     Referring back to  FIG. 1 , a target magnet  30 , attached to a magnet insertion rod  32  or other like device, is inserted into the IMN  10  in a specified orientation to a locking point  34  at the most distal transverse locking screw opening  14 . A reaming rod, known to the art for conducting such a procedure, can be adapted for use as a magnet insertion rod  32 . The adaptation would require a means for attaching the target magnet  30  to the distal end of the rod  32 , with provisions for maintaining correct depth, rotation, and centering of the magnet  30  within the IMN  10 . It is also within the scope of the present invention to eliminate the insertion rod  32  and provide an IMN  10  with a permanent magnet  30  mounted within the longitudinal open shaft  15  of the IMN  10 . In this manner the IMN  10  would be formed with a previously mounted magnet  30  situated within the shaft  15  of the IMN  10  at the locking point  34 . 
     The magnet  30  is designed to be axisymmetric with non-circular flux lines. The north pole of the magnet  30  must face medially (along the z-axis  36 ) so that it projects a magnetic field having a central line  37  of flux parallel to the axes  35 ,  39  of the interlocking screw openings  12 ,  14 . Designed in this manner, the flux field  50  (shown in  FIG. 2 ) of the magnet  30  relates information about its three-dimensional orientation along the x-axis  41 , the y-axis  43 , and the z-axis  36 . 
     Magneto-resistive Sensors  44   
     As illustrated in  FIG. 1 , a targeting device  40  is then applied percutaneously to the approximate region of the interlocking screw openings  12 ,  14 . In the embodiment illustrated, the targeting device  40  includes at least one pair of sensors and preferably a sensor array  42 , described more fully below, and a handle  46 . Located at the distal end of the targeting device  40  are drill sleeves  47 ,  48  situated within channels  47   a  and  48   a  (illustrated in  FIG. 7 ). It is known to the art that drill sleeves  47 ,  48  are slibably positioned within channels  47   a ,  48   a . In this manner, the sleeves  47 ,  48  can be slidably positioned directed on the bone  20  after an incision is made in the skin for accurate drilling. 
     Referring to  FIGS. 1 ,  2  and  4 , the sensor array  42  relates to sensors  44 , which are designed to detect the magnetic flux lines  50  of the target magnet  30  and can be aligned to precisely identify the magnet  30  location. The target sensors  44  are designed to have a sufficient sensitivity and resolution to operate at a range of approximately 10 cm from the magnet  30 . The field strength of the target magnet  30  at this distance will be below 1 Gauss, which is close to the value of the Earth&#39;s magnetic field. Therefore, a sensor  44  is needed that can sense very small changes in magnetic field. Sufficient resolution is gained by using a sensor  44  that has a sensitivity range of −2 to +2 Gauss. Such components are commercially available. For example, Phillips Semiconductors (Sunnyvale, Calif.) currently makes a MR field sensor that requires only 120 milliwatts of power, which is appropriate for battery powered operation (Phillips Semiconductors KMZ10B). The KMZ10B is comprised of a Wheatstone bridge arrangement of MR elements. The resistance of the MR elements changes in proportion to the orientation and strength of an external magnetic field in opposition to its own internal magnetization. Magnetic field measurements are obtained by supplying a voltage to the KMZ10B and then reading the differential voltages across the bridge. This output voltage is proportional to the angle and magnitude of the magnetic field and is sensitive over a field strength range of +/−2 kA/m. The field strength of the target magnet, at the maximum 10 cm, distance falls into this range. These sensors produce a maximum output when flux lines are perpendicular to its sensitive axis, +Hy. 
     The sensitivity of the KMZ10B is 20 millivolts per kA/m when supplied with 5 volts. The targeting circuitry allows the sensor outputs to be zeroed so that they only represent the magnitude of flux lines emitted from the target magnet. This allows compensation for extraneous fields from other sources in the operating room. Such extraneous sources could be the surgical drill, video monitors, lighting, and even the Earth&#39;s magnetic field. In addition, these sensors can provide sensing feedback for small variations of magnetic fields such as those present at a distance of 10 cm from the target magnet. The KMZ10B sensors have a sensitivity of 10 millivolts change per Gauss. They can also be nulled so that their outputs only represent changes in the flux lines emitted from the target magnet, thus allowing extraneous fields from other sources in the operating room to be ignored. Further, the KMZ10B sensors are extremely versatile; being very robust, able to withstand extremes in temperature, chemical challenges as well as having a low energy requirement. Further, these sensors can be configured in an array so that their collective outputs may be used for targeting. 
     It is within the scope of the present invention to utilize one sensor  44  in the invention. However, greater accuracy can be achieved by utilizing two or more sensors  44  preferably in pairs. In its preferred embodiment the array  42  of sensors includes eight MR sensors  44   a - h  in an elliptical array forming four pairs ( 44   a - e ,  44   b - f ,  44   c - g , and  44   d - h ). Each sensor  44  in a pair opposes the other member of the pair. Each sensor  44  produces a maximum output when flux lines  50  are perpendicular to its sensitive side. This allows the angle and magnitude of the detected field to be known. As illustrated in  FIG. 2 , the elliptical arrangement of the sensors  44  allows them to be geometrically aligned with flux lines  50  of the target magnet  30 . Each individual sensor  44  is oriented perpendicularly to the flux lines  50  that project radially outward from the target magnet  30 , while being centered about the peak flux lines emitted along the z-axis  36 , as illustrated in  FIG. 2 . This arrangement guarantees that each sensor  44  in the array  42  will be excited by the same magnitude and angle of flux when perfectly centered about the z-axis  36  of the magnet  30 , and will produce the same output voltage. 
     Feedback for alignment is obtained by comparing the output voltages of opposing sensors pairs  44   a - e ,  44   b - f ,  44   c - g ,  44   d - h  within the array  42 . It can be seen that if one sensor  44  in the pair is further from the target magnet  30 , it will be exposed to a smaller field, showing a voltage imbalance, indicating misalignment. A visual display is used to indicate direction for correct alignment based on these voltage outputs. The same principle applies if a sensor pair  44   a - e ,  44   b - f ,  44   c - g ,  44   d - h  is rotated from correct alignment about an axis, where outputs will not be equivalent unless the angle of the flux seen by the sensor pairs is equal and opposite. 
     A plot of the flux lines in the x-y plane is illustrated in  FIGS. 3   a  and  3   b , which shows flux densities above the magnet  30 , as well as orientation of the magnet polarity. It can be seen that there is a definitive peak that remains parallel to the z-axis  36  regardless of distance. This is important because the targeting device  30  and corresponding drill sleeves  47 ,  48  must remain parallel to the openings  12 ,  14  at all depths. 
     Arrangement of Sensors  44   
     The arrangement of the preferred embodiment of sensor pairs  44   a - e ,  44   b - f ,  44   c - g ,  44   d - h  so described allows each sensor  44  in the array to be excited by the same magnitude and angle of flux  50  when centered about the z-axis  36  of the magnet  30 , and will produce the exact output voltage. The sensor array  42  can move in a plane perpendicular to the z-axis  36  and retain the same feedback of position because the field shape in that plane remains constant. The reading and accuracy of the target device  40  becomes stronger as the sensors  44  move closer to the target magnet  30 , as best illustrated in  FIG. 2  which shows the results of computer modeling of these flux lines in the x-y plane along with the optimal placement of the sensors  44 . 
     Because the sensors  44  are aligned in opposing pair members, centering each pair over the target magnet  30  elicits the same magnitude output from each member of the pair. Upon exact centering, one member will “cancel out” the other member. Any deviation from exact center, in either rotation or translation, will cause an offset in opposing members of the sensor pair. 
     By comparing voltage offset between opposing sensors in the array  42 , the direction in the field can be determined. Reference is made to  FIG. 4  for a schematic of a preferred embodiment, in which the inset represents the sensor array  42  relative to the flux lines  50  generated by the magnet  30  (illustrated in  FIG. 1 ). As described, the targeting device  40  allows centering with feedback of offset in the x-y plane, while providing feedback of rotation about x, y, and z axes  41 ,  43  and  36 . Further, modifications to the present display can allow distinct, absolute and differential measurements of distance and rotation in separate axis (x, y and z), output to the user. Because the magnetic flux lines  50  lie in three dimensions, the target device  40  comprises a multi-axis targeting device. Further, the disclosed configuration of the target device  40  can compensate for constant, uniform (DC) magnetic fields that exist in the operating room, e.g., earth&#39;s magnetic field, lights, and instruments, but it is recommended that the operating table and other fixtures within an effective radius of the targeting magnet be non-ferrous. This compensation is possible because the targeting circuitry uses a differential measurement between sensors pairs, so as to affectively cancel any extraneous fields which would provide equal but opposite outputs. 
     Using the sensor array output, continuous feedback is provided for the surgeon to center the drill  60  (illustrated in  FIG. 7 ) in each of the drill sleeves  47 ,  48  above the interlocking openings  12 ,  14  in the medial plane. The targeting electronics, known to the art, are used to compare opposing sensor outputs and determine their location within the magnetic field, drive a display  52  that indicates this position and performs calibration of the sensors  44 . An example of circuitry which can be adapted to the present invention can be found in  Semiconductor Sensors Data Handbook SC 17, Philips Electronics, September 2000. It will be appreciated that the sensitivity of the targeting device  40  to movement is almost infinitely adjustable via the electronics. 
     The sensors  44  in the sensor array  42  should be sensitive to small changes in magnetic field, thereby making it possible to determine the position of the magnet  30  in the field with a resolution of less than a millimeter in translation and less than one degree of rotation. Calibration is necessary because each sensor  44  has an inherent offset at zero field due to manufacturing tolerances. Additionally, it is necessary to null any extraneous fields present in the operating room. Circuitry, known to the art, is provided that zeros the output of each sensor  44  so that the array  42  is ready for targeting. Calibration must be done with the sensors  44  away from any strong magnetic field, including that of the target magnet  30 , so that the reading is not biased. 
     Once calibrated, it is possible to use the sensor array  42  to provide positioning data. When the array  42  is centered exactly over the target magnet  30  in the medial plane, all sensors  44   a - h  will have equal voltage outputs. Any deviation from exact center, in either rotation or translation, will cause an offset in opposing members of the sensor pair. By comparing voltage offset between opposing sensors in the array, the direction of the sensor in the field can be determined until the opposing sensors have equivalent outputs. Those having skill in the art will appreciate that the sensor array  42  must correspond to the magnetic field shape of the magnet  30  to allow feedback of position about the desired axis for a specific application. It will also be appreciated that for any specific application, the magnet size and material may be changed, as long as the correct magnetic field shape is maintained. 
     Readable Display  52   
     In a more preferred embodiment, illustrated in  FIGS. 1 and 4 , the outputs from the sensor array  42  are converted into a readable display  52  allowing the surgeon to precisely determine the location of the screw opening  14 . In this example, the display  52  resembles a “bulls-eye” of light emitting diodes (LEDs) comprising a ring of colored lights  54   a - h  around a central “bulls eye” light  56  of another color. For example, the ring of lights  54   a - h  could be a yellow color and the central light  56  could be an offsetting color such as red. Each LED is tied to the corresponding sensor  44   a - h  in the elliptic array. If a sensor pair has a voltage difference between them, it will be indicated on the LED display  54   a - h  (illustrated in  FIG. 5 ). An illuminated light means that the targeting device  40  must be moved in the direction of the light. The position indicating LEDs have a variable brightness, which decreases as the targeting device  40  moves toward correct alignment. When all target sensors  44   a - h  are properly aligned, each sensor  44  cancels the output of its opposite pair member, all lights  54   a - h  shut off and the central light  56  is illuminated. The lit central light  56  indicates correct placement of the drill sleeves  47 ,  48  for drilling the hole through the bone  20  and for correct placement of the transverse interlocking screws in the IMN interlocking screw openings  12 ,  14 . In a further embodiment, it is well within the scope of the present invention to substitute the visual display described above with audible, tactile, or other feedback mechanisms to indicate alignment. Such mechanisms are well-known to the art. 
     Electronics 
     The electronics perform the functions of acquiring and conditioning signals from the sensors  44 , processing these signals, and driving the display  52 . Reference is made to  FIG. 5 , which illustrates the system hardware block diagram generally referenced at  70  and  FIG. 6  which illustrates the targeting device  40  without half of its casing  45 . Within the system  70 , there is a microcontroller  72  which is the heart of this system. The microcontroller  72  includes an on-board analog-to-digital converter (not shown) which is used to digitize signals from the instrumentation amplifiers  74 . The instrumentation amplifiers  74  interface directly to the sensors  44 . Computations are performed by the microcontroller  72  to determine which, if any of the display lights  54  should be illuminated in the sensor array  42 . 
     Microcontrollers are known to the art. A representative example of a microcontroller is the Microchip PIC16F877 microcontroller (Microchip Technology Inc., Chandler, Ariz.). The Microchip PIC16F877 has the required 8 analog-to-digital converter inputs and enough outputs to drive an LED display, while still having left over ports for additional tasks in future revisions to the device. There are many benefits to using digital control. It uses less power, less area, and is lower cost than the analog components that would be required to perform the same functions. The microcontroller-based approach also allows easy adjustment of feedback sensitivity and other parameters during prototyping. These adjustments can be made by simply changing software, rather than having to change circuit components and hardware. The printed circuit board  73  will have a small connector that will allow a wired interface to the microcontroller to facilitate in-circuit programming. 
     There are several steps involved in using the microcontroller  72  for converting the sensor outputs into a visual display of alignment. The first task is to read the analog voltage inputs from the sensors  44  and convert them to digital format. A calibration mode can be entered by activating the calibration switch. This will put the software in a loop which cycles thru each analog-to-digital converter channel and records the offset present in each sensor  44  when held in a null field. These offset values are saved and then later subtracted from their respective channel values when in normal targeting mode, effectively canceling any extraneous fields or tolerance differences between sensors  44 . 
     When in normal targeting mode, after the input is calibrated, the microcontroller  72  performs a comparison of sensor pair outputs  44 A-E,  44 B-F,  44 C-G, and  44 D-H. The input of each sensor  44  in a pair is subtracted to determine which one falls in a higher flux field. If one of the sensors  44  in a pair indicates a higher flux field, another software loop will light the appropriate LED  52  on the microcontroller&#39;s  72  output port, which indicates the desired direction for correct alignment. When all sensor pairs read voltages that are close in value, below a predetermined threshold, only the central LED  56  on the output port will be lit. At any time, the targeting device  40  can be removed from the field of the target magnet  30  and recalibrated if needed. 
     Each of the eight sensors  44 A-H is supplied with an excitation of energy preferably from a battery  76  and generally about 5 volts. The differential output of each sensor  44  is fed into a signal conditioning instrumentation amplifier  74  in the microcontroller  72 . Signal conditioning instrumentation amplifiers  74  are known to the art. A representative example of such an amplifier is identified as AD623 (Analog Devices, Norwood, Mass.). The amplifier  74  amplifies and shifts the sensor output signal to a level usable by the analog-to-digital converter of the microcontroller  72 . 
     Power for the targeting device  40  is typically derived from a 9 volt battery  76  which runs through a voltage regulator  78  that provides a constant 5 volt supply for the system. Each component is set to operate at this voltage. The main power switch  80  (on/off) disconnects the battery, minimizing battery drain during storage. The switch  80  or a calibration switch  81  is used to put the device into calibration mode. The circuitry includes a crystal oscillator  82  used as a clock reference for the microcontroller  72 . Connected to the output ports  84  of the microcontroller  72  is the sensor array  42 , which includes low-current light emitting diodes  52  for visual positioning feedback of the sensors  44 . Computations are performed by the microcontroller  72  to determine which, if any of the LED&#39;s  52  should be illuminated. 
     Method of Use 
     While the operation of this invention should be self-explanatory from the foregoing description, a brief description of the procedure will now be presented with specific reference to  FIG. 7 . 
     The magnet  30  provides no useful information unless it is fixed at a desired location to be targeted. In this case, the magnet  30  must be fixed at an exact known distance from the locking screw openings  12 ,  14 . This distance must match exactly the distance between the center of the sensor array  42  and drill sleeves  47 ,  48  so that when the sensor array  42  is aligned with the magnet  30 , the drill sleeves  47 ,  48  are aligned with the interlocking openings  12 ,  14 . The small size and shape of the preferred magnet  30  allows it to be inserted in the cannula of the IMN  10  and locked at the correct position proximal to the openings  12 ,  14  for targeting. The preferred magnet  30  has a diameter of 3 mm, which corresponds to the inside diameter of many IMN&#39;s, and thus can be attached to the end of a 3 mm reaming rod  32  for insertion. 
     For proper targeting, the reaming rod  32  and IMN  10  must be adapted to accommodate a mechanism that locks the magnet  30  in place during the drilling procedure. The magnet  30  only remains locked within the IMN  10  during targeting, and can be removed after interlocking is complete. It is possible of course, in special cases, to incorporate the magnet  30  permanently within an IMN  10 . For IMNs  10  with an inside diameter larger than 3 mm, the target magnet  30  may need a carriage built around it (not shown) to maintain centering. The rod  32  adapted to position the target magnet  30  must also have provisions to maintain rotational alignment, so that the north pole remains parallel to the axes  35 ,  39  of the openings  12 ,  14  and points in the lateral direction. 
     In order to align a drill bit  60  with the axes  35 ,  39  of the desired interlocking openings  12 ,  14  in  FIG. 7 , the surgeon must have feedback of positioning for rotation and translation in three dimensions. With specific reference to interlocking opening  12 , a coordinate axis is used where it is assumed that the axis  35  of the interlocking screw opening  12  is z  36 , and the x-y plane  41 ,  43 , normal to the face of the interlocking screw opening  12 , is coplanar with the medial plane. This is the direction from which the surgeon will be locating and drilling the hole in the bone  20 . The magnet  30  and targeting device  40  containing the sensing array  42  provide feedback for the surgeon to align the drill sleeve  47  within channel  47   a  so that it is parallel to the axis  35  of the interlocking screw opening  12  for drilling. The same procedure is used for interlocking opening  14 . 
     The magnet  30  is placed inside the IMN  10  at a position generally proximal to the interlocking screw opening  12  to be targeted. Unless the magnet  30  is permanently positioned within the shaft  15  of the IMN  10 , the magnet  30  is inserted by the insertion rod  32  through the proximal opening  11  in the IMN  10 , while its insertion depth is fixed by a locking pin  13 . Once the magnet  30  is placed at a fixed position  34  adjacent the interlocking screw opening  12 , a skin incision is marked using the array of magnetic sensors  42  to locate the position of the now-internal magnet  30  which projects central flux lines  50  illustrated in  FIG. 2 , parallel to the axis of the interlocking screw opening  12 . 
     From outside the extremity in which the IMN  10  is inserted, the targeting will be performed by an array of magnetic sensors  44  held parallel to the medial plane, illustrated in  FIG. 1 . These sensors  44  are embedded in the handle  46  of the targeting device  40 , which also includes the drill sleeves  47  and  48 . Acceptable drill sleeves have been developed previously that could be retrofitted to this design. The distance between the center axis  35  of the drill sleeve  47  and the center axis  37  of the magnetic sensor array  42  will be equivalent to the distance between the magnet  30  and the interlocking screw opening  12  inside the IMN  10 . When the sensor array  42  is aligned correctly over the magnet  30 , the drill sleeve  48  is aligned with the interlocking screw opening  12 . The drill sleeves  47 ,  48  are removable from the handle  46 , so that the empty space can be used as a window to mark the skin to indicate the position of the internal target magnet for incision. The drill sleeves  47 ,  48  can then be replaced and the drill returned to position for the percutaneous procedure. The surgeon can advance the drill bit  60  through the bone  20  without hitting the magnet  30  while maintaining alignment feedback in real time. Advantageously, the magnet  30  is off-axis, meaning that the magnet is not located in the axes  35 ,  39  of the drill sleeves  47 ,  48 . 
     The actual locking mechanism  13  is well known and can be retrofitted to the device of the present invention. An example of an acceptable locking mechanism can be found in  Durham and Crickenberger  (1998). 
     The exterior display  52  of the sensor array  42  on the upper face of the handle  46  of the targeting device  40  will read a “bull&#39;s-eye” of LEDs, which indicate the correct position of the sensors  42  directly underneath the display on the underside of the handle  46 . The exterior display  52  indicates the position of the sensors  44  in relation to the target magnet  30 . The display  52  will consist of a ring of lights  54 , with one offsetting light  56  in the center. The lights  54  light to indicate which position the sensor array  42  must move to correctly align with the magnet  30 . When aligned correctly, all lights  54  will be off and the central light  56  will be lit. The drill sleeves  47 ,  48  will then be aligned with the interlocking transverse screw openings  12  and  14 . The surgeon is then ready to drill the holes and insert the interlocking screws without aid of fluoroscopy or extraneous targeting systems. 
     Other embodiments and uses of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclose herein. For example, rather than use the above-described electronics, which presently consists of eight separate instrumentation amplifiers, a microcontroller and a handful of resistors and capacitors, the function of the microcontroller and instrumentation amplifiers could be integrated into a single mixed-signal microchip. 
     The present invention is adaptable to other medical uses, such as tracking and positioning of medical instruments, including endoscopes, catheters and implants within the body. In addition, the present invention can be adapted for use outside the medical industry for locating and targeting areas in materials that are X-ray sensitive and other measurement systems used in industry and manufacturing; providing positioning feedback for robotic devices; and, any process requiring blind hole targeting in non ferrous materials including, precise positioning of opposing elements such as in cabinetry making, fiberglass fabrication and construction and processes involving ceramic and tile fabrication and installation. In embodiments not involving a living body, electromagnets may be used. 
     All references cited herein for any reason, including all U.S. and foreign patents and patent applications, are specifically and entirely incorporated by reference. It is understood that the invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the claims following the Bibliography. 
     BIBLIOGRAPHY OF CITATIONS 
     
         
         1) Durham, A. A. and Crickenberger, D. P. (1998) Magnetic Distal Targeting for Modular Intramedullary Nails.  Techniques in Orthopaedics  13, 71-78. 
         2) Krettek, C., Konemann, B., Mannss, J., Schandelmaier, P., Schmidt, U., and Tscherne, H. (1996) [Analysis of implantation-induced nail deformation and roentgen morphometric studies as the principle for an aiming device for distal interlocking nailing without roentgen image intensification].  Unfallchirurg  99, 671-678. 
         3) Krettek, C., Konemann, B., Miclau, T., Schlandermaier, P., and Blauth, M. (1997) In vitro and in vivo radiomorphic analyses of distal screw hole position of the solid tibial nail following insertion.  Clinical Biomechanics  12, 198-200. 
         4) Krettek, C., Konemann, B., Miclau, T., Kolbli, R., Machreich, T., Kromm, A., and Tscherne, H. (1998) A new mechanical aiming device for the placement of distal interlocking screws in femoral nails.  Arch Orthop. Trauma Surg  117, 147-152. 
         5) Krettek, C., Mannss, J., Miclau, T., Schandelmaier, P., Linnemann, I., and Tscherne, H. (1998) Deformation of femoral nails with intramedullary insertion.  J. Orthop. Res.  16, 572-575. 
         6)  Semiconductor Sensors Data Handbook SC 17, Philips Electronics, September 2000 
         7) Zacheja, J., Bach, T., and Clasbrummel, B. (2000) Application of Microsensors for Minimally Invasive Vascular Flow Measurements and Fracture Repair Systems., Hanover, Germany. 
         8) U.S. Pat. No. 4,621,628 to Brudermann 
         9) U.S. Pat. No. 5,049,151 to Durham et al. 
         10) U.S. Pat. No. 5,514,145 to Durham et al. 
         11) U.S. Pat. No. 5,703,375 to Durham et al. 
         12) U.S. Pat. No. 6,162,228, to Durham 
         13) U.S. Pat. No. 6,503,249 to Krause