Patent Publication Number: US-2012035468-A1

Title: System and method for identifying a landmark

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     This application claims priority to U.S. Provisional Application Ser. No. 61/173,069, filed on Apr. 27, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     This disclosure relates to identification of blind landmarks on orthopaedic implants. 
     2. Description of the Related Art 
     The interlocking nail has significantly widened the scope for intramedullary (IM) fixation of long bone fractures. Anchoring an IM nail to a bone makes the construct more stable longitudinally and stops rotation of the nail within the bone. A typical IM nail fixation surgery involves a combination of jigs, x-ray imaging, and manual “eye-balling” to locate and drill the distal screw holes and to install the screws in the screw holes. 
     In IM nail fixation surgery, an IM nail is inserted into the canal of a fractured long bone in order to fixate the fractured ends together. Typically, the proximal locking is performed first and is usually carried out with a jig. Nail deformation during intramedullary insertion, however, may make a jig inaccurate for the distal screws. In fact, the positioning of the distal locking screws and alignment of the drill for the drilling of the distal screw holes is the most time consuming and challenging step of the implantation procedure. The two main reasons for failure in distal locking are (1) incorrect entry point on the bone and (2) wrong orientation of the drill. If either of these problems occurs, then the drill will not go through the nail hole. An inaccurate entry point also compounds the problem as the rounded end of the drill bit often slips, damaging healthy bone rendering it difficult to place another drill hole next to the inaccurate hole. Inaccurate distal locking may lead to premature failure with breakage of the nail through the nail hole, breakage of the screw, or the breaking of the drill bit within the bone. 
     Manual techniques are the most common and accepted techniques for sighting the distal screw holes. The majority of manual distal targeting techniques employ a guide bushing or cylindrical sleeve that guides the drill. The mechanisms of aligning the guide bushing and keeping it in place differ. There are cases where the surgeons use a guide bushing cut in half longitudinally or a full guide bushing to help steady the drill bit. In either situation, the surgeon will incise the patient and insert the drill through the incision. Manual techniques are based primarily on the surgeon&#39;s manual skill and make use of radiographic x-ray imaging and mechanical jigs. 
     Another method for achieving this on long nails is by using a technique called “perfect circles” with the aid of a C-shaped arm. This is where the patient and the C-arm are oriented such that when viewing the implant fluoroscopically the hole through which the screw is to pass appears to be in the shape of a circle. If the C-arm is not perpendicular to the hole then the hole appears oblong or even absent. 
     A need exists for an improved system and method for accurately and dependably targeting landmarks of a medical implant. Further, a need exists for accurately positioning the distal locking screws and aligning the drill for the drilling of the distal screw holes. Still further, a need exists for an improved system for targeting landmarks whereby the components may be easily sterilized or autoclaved and reused again. 
     SUMMARY 
     In a general aspect, a system for identifying a landmark includes a field generator for generating an electromagnetic field and a landmark identifier. The field generator and the landmark identifier are disposed in a common housing, and the field generator, the landmark identifier, and the common housing are autoclavable. The system also includes an orthopaedic implant located within the electromagnetic field, and the orthopaedic implant includes at least one landmark. A first magnetic sensor is spaced apart from the at least one landmark by a set distance, and a processor compares sensor data from the first sensor and landmark identifier and uses the set distance to calculate the position of the landmark identifier relative to the at least one landmark. 
     Implementations may include one or more of the following features. For example, the landmark is selected from the group consisting of a structure, a hole, a void, a boss, a channel, a detent, a flange, a groove, a member, a partition, a step, an aperture, a bore, a cavity, a dimple, a duct, a gap, a notch, an orifice, a passage, a slit and a slot. The orthopaedic implant may be an intramedullary nail. The orthopaedic implant has an outer surface and an inner surface forming a cannulation, and the first sensor is mounted to a distal portion of a probe that extends into the cannulation. The common housing in some implementations also accommodates a drill motor, the drill motor being coupleable to a drill bit. The housing may include a drill sleeve. The housing may be disk-shaped. The drill extends normally outward from the disk-shaped housing. The system can also include an insertion handle removably coupled to the orthopaedic implant. An adjustable stop can be coupled to the implant and includes a slot through which the probe extends. The adjustable stop includes a clamp mechanism to hold the probe in a fixed position. The probe may include a plurality of spaced apart markings, and the adjustable stop includes a clamp mechanism to hold the probe in a fixed position on a marking or between two markings. 
     In another general aspect, identifying a landmark includes providing an orthopaedic implant assembly having an orthopaedic implant having at least one landmark, implanting the orthopaedic implant assembly in a patient, and placing a probe in the implant. The probe includes an electromagnetic sensor. Identifying the landmark further includes generating an electromagnetic field that encompasses the sensor and landmark, identifying the at least one landmark using a landmark identifier, installing a transfixion element in the at least one landmark, and removing the probe. The landmark identifier is disposed in an autoclavable housing. 
     Implementations may include one or more of the following features. For example, the landmark is selected from the group consisting of a structure, a hole, a void, a boss, a channel, a detent, a flange, a groove, a member, a partition, a step, an aperture, a bore, a cavity, a dimple, a duct, a gap, a notch, an orifice, a passage, a slit and a slot. The orthopaedic implant may be an intramedullary nail. The orthopaedic implant has an outer surface and an inner surface forming a cannulation, and identifying a landmark further includes mounting the first sensor to a distal portion of a probe that extends into the cannulation. The field generator and landmark identifier are disposed in a common autoclavable housing and identifying the landmark also includes autoclaving the housing. The field generator and landmark identifier are disposed in a common autoclavable housing that may also accommodates a drill motor, the drill motor being coupled to a drill bit, and identifying a landmark further comprises autoclaving the housing and drill. The housing may include a drill sleeve. The housing may be disk-shaped. Identifying a landmark also includes removably coupling an insertion handle to the orthopaedic implant and/or clamping the probe in a fixed position. The probe comprises a plurality of spaced apart markings and the probe is clamped in a fixed position on a marking or between two markings. 
     In another general aspect, a system for identifying a landmark includes an autoclavable housing accommodating a field generator for generating an electromagnetic field, a landmark identifier, and a drill motor. An orthopaedic implant is located within the electromagnetic field and the orthopaedic implant has at least one landmark. A probe includes a first electromagnetic sensor and is placed within the orthopaedic implant and spaced apart from the at least one landmark by a set distance. A processor is also included for comparing sensor data from the first sensor and landmark identifier and for using the set distance to calculate the position of the landmark identifier relative to the at least one landmark. The first electromagnetic sensor is coupled to the processor via the probe. 
     In another general aspect, a kit for identifying landmarks on medical implants includes an autoclavable housing accommodating a field generator for generating an electromagnetic field, and a landmark identifier. A plurality of orthopaedic implants are also included, one of which is located within the electromagnetic field. Each orthopaedic implant includes at least one landmark. A plurality of probes, each including an electromagnetic sensor, is included. One of the probes selected based on a size of the implant disposed in the electromagnetic field. The selected probe is placed within the implant in the electromagnetic field and spaced apart from the at least one landmark by a set distance. A processor is included for comparing sensor data from the first sensor and landmark identifier and for using the set distance to calculate the position of the landmark identifier relative to the at least one landmark, wherein the first electromagnetic sensor is coupled to the processor via the probe. 
     In another general aspect, a system for targeting a landmark of an orthopaedic implant includes an autoclavable housing, a field generator disposed within the housing for generating an electromagnetic field, a first electromagnetic sensor for disposition at a set distance from the landmark that generates sensor data in response to the generated electromagnetic field, and an element removably coupled to the housing, the element defining a longitudinal axis that represents one axis of the generated magnetic field. The system is configured to use the one axis of the generated electromagnetic field to determine the position of the element relative to the landmark. 
     Implementations may include one or more of the following features. For example, the system can include a first probe having a proximal portion and a distal portion, the first electromagnetic sensor disposed on the distal portion of the probe, a retractable probe including the first electromagnetic sensor, or a retractable probe including the first electromagnetic sensor and a housing containing at least a portion of the retractable probe. A second electromagnetic sensor disposed on the proximal portion of the first probe can also be included. The system can include a second probe having a proximal and a distal portion and a third electromagnetic sensor disposed on the distal end of the second probe, where the second probe is longer than the first probe. The system can also include a processor for comparing the sensor data from the first electromagnetic sensor and the element and using the set distance to calculate the position of the element relative to the landmark. The system can include an adjustable stop that is connectable to the orthopedic implant. The adjustable stop can include a slot through which the first or the second probe extends and includes a clamping mechanism to hold the first or second probe in a fixed position. The first or the second probe can include a plurality of spaced apart indicators such that the clamping mechanism can be selectively set to hold the first or second probe in a fixed position at an indicator or between indicators. A handle can be removably coupled to the orthopedic implant. The autoclavable housing can be disk-shaped. The element can include one of a drill guide, a drill sleeve, a drill, a drill nose, a drill barrel, a drill chuck, and a fixation element. The orthopedic implant can include one of an intramedullary nail, a bone plate, a hip prosthetic, a knee prosthetic, a spinal prosthetic, and a shoulder prosthetic. The first or the second probe can be coiled or bent prior to placement into the orthopedic implant. The first electromagnetic sensor includes a proximal end and a distal end. The distal end of the first electromagnetic sensor is connected to a proximal end of the orthopedic implant such that the first electromagnetic sensor is spaced apart a set distance from at least one landmark disposed in a proximal region of the orthopedic implant. At least the housing and the element are reusable. The housing is made from one of ceramic, silicone, polypropylene (PP), polycarbonate (PC), polymethylpentene (PMP), PTFE resin, or polymethyl methacrylate (PMMA or acrylic). 
     In another general aspect, an apparatus for targeting a landmark of an orthopaedic implant includes an insertion handle removably attachable to the orthopaedic implant, an adjustable stop comprising an actuator, and a probe comprising a sensor and a plurality of markings to assist in placing the probe and sensor at a desired location with respect to the orthopaedic implant. 
     Implementations may include one or more of the following features. For example, the adjustable stop includes a mating portion such that when the stop is connected to the insertion handle, the stop is located or fixed within three degrees of freedom. The insertion handle is attached to the orthopaedic implant through use of a cannulated bolt. 
     In another general aspect, a kit for targeting a landmark of an orthopaedic implant includes a proximal targeting probe comprising a tape body and a sensor included within or on the tape body at a predetermined distance from a reference point of the tape body. The proximal targeting probe includes a first indicator that indicates that the proximal targeting probe is to be used for targeting proximal landmarks of an orthopaedic implant. The kit also includes a distal targeting probe that includes a tape body that is longer than the tape body of the proximal targeting probe and a sensor included within or on the tape body of the distal targeting probe at a second predetermined distance from a second reference point of the target body of the distal targeting probe. The distal targeting probe includes a second indicator that indicates that the distal targeting probe is to be used for targeting distal landmarks of the orthopaedic implant. 
     Implementations may include one or more of the following features. For example, the first indicator includes a color-coded grip and the second indicator includes a color-coded grip that is a different color than the first indicator. The first indicator includes a color-coded grip and the second indicator includes a color-coded grip that is a different color than the first indicator. The proximal targeting probe includes a cable for carrying a signal from the sensor included within or on the tape body of the proximal targeting probe to a control unit, and the distal targeting probe includes a second cable for carrying a second signal from the sensor included within or on the tape body of the distal targeting probe to the control unit. The sensors included within or on the tape bodies of the proximal and distal targeting probes are connected to one or more Programmable Read-Only Memory microchip that identifies whether the proximal and distal targeting probes are used for proximal or distal targeting. The tape bodies of the proximal and distal targeting probes include one or more bends to bias at least a portion of the tape bodies against a wall of the orthopaedic implant. 
     In another general aspect, a probe for use in targeting a landmark of an orthopaedic implant includes a housing, a retractable or extensible body disposed within the housing. The body is configured to form a generally straight shape when extended from the housing. A sensor is disposed within the body and is positionable at a first location for targeting a proximal landmark of the orthopaedic implant. The sensor is positionable at a second location for targeting a distal landmark of the orthopaedic implant. The body comprises one of layered, flexible stainless steel spring bands, resilient plastics, or rubber tubing or sheeting. The body includes a plurality of nested segments of tubing that can extend and retract by sliding within adjacent tubing segments. 
     In another general aspect, an apparatus for targeting a landmark located in a proximal end of an orthopaedic implant includes an insertion handle and a sensor disposed within or on the insertion handle at a predetermined distance from a proximal locking aperture formed in the orthopaedic implant when the insertion handle is attached to the orthopaedic implant. The sensor is passive or electrically powered. The sensor is mounted in a housing that is unitary or integral with the insertion handle. 
     The disclosed methods and apparatuses include several advancements. First, the disclosed methods and apparatuses can operate independently of fluoroscopy and eliminate the necessity of X-ray devices for targeting of transfixion elements, thereby reducing the exposure of users and patients to radiation. Second, disclosed methods and apparatuses allow a user to lock the driving-end of the implant before locking the non-driving end of the implant. In other words, the disclosed methods and apparatuses do not require use of an implant cannulation that requires proximal locking prior to distal locking. 
     Other advantages and features will be apparent from the following detailed description when read in conjunction with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a system for identifying a landmark. 
         FIG. 2  is a sectional view of an orthopaedic implant of  FIG. 1 . 
         FIG. 3  is a partial sectional of the implant of  FIGS. 1 and 2  illustrating the sensor mounting. 
         FIG. 4  is a partial sectional view of another sensor mounting in an implant. 
         FIG. 5  is a sectional view of the sensor and implant illustrated in  FIG. 4 . 
         FIG. 6  illustrates another orthopaedic implant assembly. 
         FIG. 7  is a partial plan view of a removable lead. 
         FIG. 8  is a top view of the orthopaedic implant assembly illustrated in  FIG. 6 . 
         FIG. 9  illustrates a landmark identifier that includes a drill sleeve. 
         FIG. 10  is a partial and sectional view illustrating two point contacts of an implant. 
         FIG. 11  is another partial sectional view illustrating point contacts in another implant. 
         FIG. 12A  is a partial and sectional view of an implant illustrating a crimp electrical connection. 
         FIG. 12B  is a partial exploded view illustrating the electrical connection in a disclosed implant. 
         FIG. 12C  is a side view of the electrical connection illustrated in  FIG. 12B . 
         FIG. 12D  is a partial exploded illustrating the electrical connection in another disclosed implant. 
         FIG. 13A  is a partial perspective and exploded view illustrating alternative mechanisms for aligning a disclosed orthopaedic implant and a disclosed insertion handle. 
         FIG. 13B  is a partial perspective and exploded view illustrating alternative mechanisms for aligning a disclosed orthopaedic implant and an electrical connection. 
         FIG. 14  is a partial side view illustrating a connection of the insertion handle to the orthopaedic implant. 
         FIG. 15  illustrates another system for identifying a landmark. 
         FIG. 16  is a schematic illustration of view selection criteria. 
         FIG. 17  is a flowchart illustrating view selection during a fixation surgery. 
         FIG. 18  is a schematic illustration of another method of aligning a landmark identifier. 
         FIG. 19  is a schematic illustration of another disclosed method of aligning a landmark identifier. 
         FIG. 20  illustrates a disclosed monitor with exemplary views. 
         FIG. 21  illustrates another disclosed landmark identifier. 
         FIG. 22  is a partial view another disclosed insertion handle. 
         FIG. 23  illustrates another disclosed system for identifying a landmark. 
         FIG. 24  is a partial view of yet another disclosed insertion handle. 
         FIG. 25  illustrates another disclosed system for identifying a landmark. 
         FIG. 26  is a partial cross-sectional view of an intramedullary nail. 
         FIG. 27  illustrates a packaging for a disclosed implant. 
         FIG. 28  illustrates a method of connecting a landmark identifier system to a network. 
         FIG. 29  illustrates yet another disclosed system for identifying a landmark. 
         FIG. 30  is a flow chart for using a disclosed landmark identifying system. 
         FIG. 31  is another flow chart for using a disclosed landmark identifying system. 
         FIG. 32  is a schematic illustration of tracking drill depth. 
         FIGS. 33A and 33B  are also schematic illustrations of tracking drill depth. 
         FIG. 34  is a partial illustration of a disclosed device for tracking drill depth. 
         FIG. 35  is a perspective view of another insertion handle. 
         FIG. 36  is a top perspective view of an adjustable stop. 
         FIG. 37  is a bottom perspective view of the adjustable stop illustrated in  FIG. 36 . 
         FIG. 38  is another illustrating system calibration. 
         FIG. 39  is a perspective view of another landmark identifier housing a field generator and a drill sleeve and that may be sterilized or subject to an autoclave procedure. 
         FIG. 40  is a side view of the landmark identifier/field generator/drill sleeve of  FIG. 39  making contact with a bone. 
         FIG. 41  is a perspective view of the landmark identifier/field generator/autoclavable housing of  FIG. 39  coupled to a screw driver attachment. 
         FIG. 42  is a plan view of an insertion handle, adjustable stop and probe. 
         FIG. 43  is a perspective view of an exemplary adjustable stop to hold a probe in a desired position. 
         FIG. 44  is a perspective view of another exemplary adjustable stop. 
         FIG. 45  is a perspective view of an intramedullary nail, an insertion handle, an adjustable stop, and a probe. 
         FIG. 46  is a perspective view of another intramedullary nail, an insertion handle, an adjustable stop, and a probe. 
         FIG. 47  is a perspective view of two probes for use in targeting landmarks of an implant. 
         FIG. 48  is a perspective view of another probe for use in targeting landmarks of an implant. 
         FIG. 49  is a sectional view of a retractable probe. 
         FIG. 50  is a perspective view of an intramedullary, an insertion handle, and an adjustable stop. 
         FIG. 51  is an illustration of a system for targeting a landmark of an implant. 
         FIG. 52  is an illustration of a device for use in calibrating the system of  FIG. 51 . 
         FIGS. 53-62  are illustrations of adjustable stops. 
     
    
    
     It should be understood that the drawings are not necessarily to scale and that the disclosed implementations are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of the disclosure or which render other details difficult to perceive may have been omitted. It should be understood, of course, that this disclosure is not limited to the particular implementations illustrated herein. 
     DETAILED DESCRIPTION 
     Referring to the accompanying drawings in which like reference numbers indicate like elements,  FIG. 1  illustrates one disclosed system  10  for identifying a landmark. The system  10  may include a processor  12 , a magnetic field generator  16 , a landmark identifier  18 , and an orthopaedic implant assembly  28 . The system  10  may also include a monitor  14  electrically connected to the processor  12  and an insertion handle  40  removably attached to the orthopaedic implant assembly  28 . The processor  12  is depicted as a desktop computer in  FIG. 1  but other types of computing devices may be used. As examples, the processor  12  may be a desktop computer, a laptop computer, a personal data assistant (PDA), a mobile handheld device, or a dedicated device. The magnetic field generator  16  is a device available from Ascension Technology Corporation of 107 Catamount Drive, Milton Vt., U.S.A.; Northern Digital Inc. of 103 Randall Drive, Waterloo, Ontario, Canada; or Polhemus of 40 Hercules Drive, Colchester Vt., U.S.A. Of course, other generators may be used. As examples, the field generator  16  may provide a pulsed direct current electromagnetic field or an alternating current electromagnetic field. The system  10  may also include a control unit (not shown) connected to the magnetic field generator  16 . The control unit controls the field generator  16 , receives signals from small mobile inductive sensors, and communicates with the processor  12 , either by wire or wirelessly. The control unit may be incorporated into the processor  12  either through hardware or software. 
     The system  10  is a magnetic position tracking system. For illustrative purposes, the system  10  may include a magnetic field generator  16  comprised of suitably arranged electromagnetic inductive coils that serve as the spatial magnetic reference frame (i.e., X, Y, Z). The system  10  may also include small mobile inductive sensors, which are attached to the object being tracked. It should be understood that other variants could be easily accommodated. The position and angular orientation of the small mobile inductive sensors are determined from its magnetic coupling to the source field produced by magnetic field generator  16 . 
     It is noted that the magnetic field generator  16  generates a sequence, or set, of here six, different spatial magnetic field shapes, or distributions, each of which is sensed by the small mobile inductive sensors. Each sequence enables a sequence of signals to be produced by the small mobile inductive sensors. Processing of the sequence of signals enables determination of position and/or orientation of the small mobile inductive sensors, and hence the position of the object to which the small mobile inductive sensor is mounted relative the magnetic coordinate reference frame which is in fixed relationship to the magnetic field generator  16 . The processor  12  or the control unit may use the reference coordinate system and the sensed data to create a transformation matrix comprising position and orientation information. 
     The landmark identifier  18  is used to target a landmark, such as a landmark on the orthopaedic implant assembly  28 . The landmark identifier  18  may include one or more small mobile inductive sensors or may include the field generator. The landmark identifier  18  has a second sensor  20 . The landmark identifier  18  may be any number of devices. As examples, the landmark identifier may be a device that includes a structure that provides a user with an understanding of the location and orientation of a hidden landmark. For example, the landmark identifier can include a drill guide, a drill sleeve, a drill, a drill nose, a drill barrel, a drill chuck, or a fixation element. In some implementations, the structure can be a housing having an opening, or other structure that indicates the location and orientation of a landmark. In  FIG. 1 , the landmark identifier  18  is a drill sleeve and includes a sensor  20 , whereas in  FIG. 39 , the landmark identifier  2016  includes a housing  2020  having a central aperture and includes a magnetic field generator (not shown) in the housing  2020 . The landmark identifier  18  may include one or more of a serrated tip  22 , a tube  24 , and a handle  26 . The tube  24  also may be referred to as a bushing, cylinder, guide, or drilling/screw placement guide. The second sensor  20  is oriented relative to an axis of the tube  24 . The tube  24  may receive a drill. This offset of the sensor  20  from the tube  24  allows the position and orientation of the tube to be located in space in six dimensions (three translational and three angular) relative to the magnetic field generator  16  and/or another sensor in the system. The processor  12  may need to be calibrated to adjust for the offset distance of the second sensor  20 . The landmark identifier  18  and the field generator  16  may be combined into a single component. For example, the field generator  16  may be incorporated within the handle  26 . 
     The orthopaedic implant assembly  28  may include an implant  30  and one or more small mobile inductive sensors. The orthopaedic implant assembly  28  includes a first sensor  32 . In  FIG. 1 , the implant  30  is in the form of intramedullary nail but other types of implants may be used. As examples, the implant may be an intramedullary nail, a bone plate, a shoulder prosthetic, a hip prosthetic, or a knee prosthetic. The first sensor  32  is oriented and in a predetermined position relative to one or more landmarks on the implant  30 . As examples, the landmark may be a structure, a void, a boss, a channel, a detent, a flange, a groove, a member, a partition, a step, an aperture, a bore, a cavity, a dimple, a duct, a gap, a notch, an orifice, a passage, a slit, a hole, or a slot. In  FIG. 1 , the landmarks are transfixion holes  31 . The offset of the first sensor  32  from the landmark allows the position of the landmark to be located in space in six dimensions (three translational and three angular) relative to the magnetic field generator  16  or another sensor in the system, such as the second sensor  32 . The processor may need to be calibrated to adjust for the offset distance of the first sensor  32 . 
     The first sensor  32  and the second sensor  20  are coupled to the processor  12 . This may be accomplished by wire or wirelessly. The first sensor  32  and the second sensor  20  may be a six degree of freedom sensor configured to describe the location of each sensor in three translational axes, generally called X, Y and Z and three angular orientations, generally called pitch, yaw and roll. By locating the sensor in these reference frames, and knowing the location and orientation of each sensor, the landmark identifier  18  may be located relative to the landmark on the implant  30 . In one particular implementation, the information from the sensors allows for a surgeon to plan the surgical path for fixation and properly align a drill with a blind fixation hole  31 . The sensors  32 , 20 are six degrees of freedom sensor from Ascension Technology Corporation of 107 Catamount Drive, Milton Vt., U.S.A.; Northern Digital Inc. of 103 Randall Drive, Waterloo, Ontario, Canada; or Polhemus of 40 Hercules Drive, Colchester Vt., U.S.A. Of course, other sensors may be used. 
     The first sensor  32  may be attached to the implant  30 . For example, the first sensor  32  may be attached to an outer surface  37 . In  FIG. 1 , the implant  30  may also include a groove  34  and a pocket  36  (best seen in  FIG. 2 ). The groove  34  and pocket  36  are located in a wall of the implant  30 . The first sensor  32  is intended to be attached to the implant  30  and installed in a patient for the service life of the implant  30 . Further, the orthopaedic implant assembly  28  may include a cover  38  to cover the pocket  36  and/or the groove  34 . The cover  38  may be substantially flush with the external surface  37  of the implant  30 . Accordingly, the implant  30  may include a second opening  39  (see  FIG. 2 ) to receive the cover  38 . 
     The first sensor  32  may be tethered to leads for communication and power. The leads, and the sensor, may be fixed to the implant  30 . A lead  50  may be used to connect the first sensor  32  to the processor  12  or the control unit. The lead  50  may be made from biocompatible wire. As an example, the lead  50  may be made of DFT wire available from Fort Wayne Metals Research Products Corp., 9609 Indianapolis Road, Fort Wayne, Ind. 46809. DFT is a registered trademark of Fort Wayne Metals Research Products Corp. A first connector  52  may be used to place the lead  50  relative to the implant  30 . A second connector  54  may be used to connect the lead  50  to another device, such as the processor  12 , the control unit, or the insertion handle  40 . 
     The first sensor  32  may be fixed in the pocket  36  using a range of high stiffness adhesives or polymers including epoxy resins, polyurethanes, polymethyl methacrylate, polyetheretherketone, UV curable adhesives, silicone, and medical grade cyanoacrylates. As an example, EPO-TEK 301 available from Epoxy Technology, 14 Fortune Drive, Billerica, Mass. 01821 may be used. The lead  50  may be fixed in the groove in a similar manner. These types of fixation methods do not adversely affect the performance of the electrical components. Thereafter, the cover  38  may be placed on the implant  30  and welded in-place. For example, the covers may be laser welded to the implant. 
     The monitor  14  may be configured to display the position and orientation of the first sensor  32  and the second sensor  20  so that the display may show a surgeon both sensor positions and orientations relative to one another. The processor  12  may send positional data, either by wire or wirelessly, to a user interface, which may graphically display the relative positions of the landmark identifier and the implant on the monitor. The view displayed on the monitor  14  may be oriented relative to the landmark identifier so that the surgeon may visualize the user interface as an extension of the landmark identifier. The user interface also may be oriented so that the surgeon may view the monitor simultaneously with the surgical field. 
     The insertion handle  40  may be used for installation of the orthopaedic implant assembly  28  and also may be used to route the leads from the first sensor  32 . For example, the insertion handle  40  may route both communication and power leads between the implant  30  and the processor  12 . 
     In  FIG. 1 , the landmark identifier  18  and the insertion handle  40  each include a communications module  21 ,  25  for wirelessly transmitting data from the sensor  20 ,  32  to the processor  12 , but those skilled in the art would understand that other methods, such as by wire, may be used. The second connector  54  plugs into the communications module  25 . Alternatively, and as is explained in greater detail below, the implant  30  and the insertion handle  40  may have mating electrical contacts that form a connection when the components are assembled such that the first sensor  32  is connected to the communications module  25 . 
     The implant  30  may include a communications circuit and an antenna for wireless communication. Power for the first sensor  32  and/or the communications circuit may be positioned within the insertion handle  40 . For example, a battery may be placed within the insertion handle  40  for transferring power to the first sensor  32  and/or other electronics. Alternatively, the communications circuit, the antenna, and the battery may be located within the insertion handle  40  and each of these may be tethered to the first sensor  32 . In yet another implementation, the implant  30  may include a coil to inductively power the communications circuit and communicate data from the first sensor  32 . The power source may be a single source mode or may be a dual mode AC/DC. 
     In use, the orthopaedic implant assembly  28  is installed in a patient. For example, in the case of internal fixation, the intramedullary nail is placed within an intramedullary canal. Optionally, the user may use transfixion elements, such as screws, to first lock the proximal end of the intramedullary nail. An operator uses the targeting device  18  and the first sensor  32  to identify the landmarks. For example, in the case of intramedullary nail fixation, a surgeon uses the targeting device  18  to identify the blind transfixion holes  31  and drill through the holes  31  for placement of a transfixion element. 
       FIG. 2  further illustrates the implant  30  as illustrated in  FIG. 1 . The implant  30  may include the first sensor  32 , the longitudinal groove  34 , the pocket  36 , the cover  38 , and the second opening  39 . As examples, the cover  38  may be comprised of gold or titanium foil. The implant  30  may include an inner surface  35  that forms a cannulation  33 . The outer surface of the implant  30  is shown at  37 . 
       FIG. 3  illustrates an implementation of the first sensor  32 . The first sensor  32  may include two coils cross-layered to one another and having an angle α. 
       FIGS. 4 and 5  illustrate another implementation of the first sensor  32 . The first sensor may include two coils generally orthogonal to one another in order to establish the orientation and position in the six degrees of freedom. A first coil may be oriented along the length of the implant  30 . The second coil may be oriented either wrapped around the circumference of the implant, for example in a groove, or along the radius of the implant  30 . In addition, while the coils may be perpendicular to one another, other orientations may be used, although the mathematics may be more complex. Further, the coils may be oriented spirally around the implant  30 . Such an orientation may allow two coils to be placed perpendicular to each other with both coils placed along both the length of the implant and along the circumference of the implant  30 . 
       FIGS. 6-8  illustrate a second implementation of the orthopaedic implant assembly  60 . The orthopaedic implant assembly  60  may include the implant  30 . In  FIG. 6 , the implant  30  includes landmarks in the form of transfixion holes  31 . The implant  30  may include a longitudinal internal groove  66  and a removable lead  64 . In  FIG. 8 , a diameter of the longitudinal groove  66  is shown as intersecting with the cannulation  33 ; however, in other implementations, the diameter of the longitudinal internal groove is contained between the outer surface  37  and the inner surface  35 . The removable lead  64  may include the first sensor  32  at its distal end portion  65 . The first sensor  32  is located a known offset from the landmarks  31 . The implant in  FIGS. 6-8  is comprised of biocompatible material, and may be a metal alloy or a polymer. The longitudinal groove  66  may be machined or molded in place. 
     In use, the implant  30  with the removable lead is installed in a patient. For example, in the case of internal fixation, the intramedullary nail is placed within an intramedullary canal. Optionally, the user may use transfixion elements, such as screws, to first lock the proximal end of the intramedullary nail. Because of the location of the longitudinal groove  66 , the removable lead  64  does not interfere with locking the proximal end of the intramedullary nail. An operator uses the targeting device  18  and the first sensor  32  to identify the landmarks  31 . For example, in the case of intramedullary nail fixation, a surgeon uses the targeting device  18  to identify the blind transfixion holes  31  and drill through the holes  31  for placement of a transfixion element. After the implant  30  is secured, the operator removes the removable lead  64  and it may be discarded. 
     A method for identifying a landmark is disclosed. The method may include providing an orthopaedic implant assembly having an orthopaedic implant with a longitudinal groove and a removable lead or probe having an electromagnetic sensor attached thereto situated within the longitudinal groove. The orthopaedic implant includes a proximal end portion, a distal end portion, and at least one landmark on the distal end portion. The method includes implanting the orthopaedic implant assembly in a patient. Then, transfixion elements in the proximal end portion are installed. At least one distal landmark is identified using a landmark identifier. A transfixion element is installed in the at least one distal landmark. The removable lead or probe may then be removed. The situation of the removable lead or probe within the longitudinal groove allows for proximal locking of the implant prior to distal locking. 
       FIG. 9  illustrates the landmark identifier  18  of  FIG. 1 . The landmark identifier  18  may include the sensor  20 , the serrated tip  22 , the tube  24 , and the handle  26 . A drill  90  has markings  92  that interact with a marking sensor  19  adjacent the tube  24 . The interaction is similar to a pair of digital measuring calipers in that the position between the markings  92  and the sensor  19  equate to a distance. This distance can be used to determine the depth of the drill into the bone and ultimately the length of the bone screw that will be inserted into the drilled hole. Distance, or drill depth, readings are only obtainable when the markings  92  and the sensor  19  are in close proximity to each other, i.e. the drill  90  is inside the tube  24 . Exemplary measurement devices are illustrated in U.S. Pat. No. 6,675,491 and U.S. Pat. No. 7,253,611. The marking sensor  19  is connected to the communications module  21 . Alternatively, the marking sensor  19  may be connected by wire to the processor  12 . In FIG.  9 , the communications module  21  may include a third connector  23  for electrical connection to the processor  12 . 
       FIGS. 10-12  illustrate exemplary methods of electrically connecting the implant  30  to the insertion handle  40 , which has corresponding electrical contacts. In  FIG. 10 , biasing elements  72  bias contacts  70  toward the insertion handle  40 . In  FIG. 11 , the implant  30  has elastomeric electrical contacts  74 . In  FIG. 12A , wires extending between the lead  50  and another component are crimped together at junction  76 . In one method, the wires are torn free and separated at the junction  76  after installation of the orthopaedic implant assembly  28 . In yet another method, the wires are cut above the junction  76  after installation of the orthopaedic implant assembly  28 . In  FIGS. 12B  and C, two flex boards  53  are soldered together one or more pads  57  to connect a wiring harness  55  to the sensor. The wire harness  55  may be mounted to the insertion handle  40  or within a cannulation of the insertion handle  40 . In the depicted implementation, four pads  57  are soldered together. Locking tabs  59  are sandwiched between the implant  30  and the insertion handle  40  to withstand abrasion and tension associated with the implant insertion. Once the insertion handle  40  is removed, the wire harness  55  can be pulled such that all non-biocompatible materials are pulled with it. In  FIG. 12D , rings  61 ,  63  are connected during manufacturing. After implantation, both rings  61 ,  63  are removed by pulling on a jacketed wire  67 . 
     Referring now to  FIGS. 13A and 13B , the implant  30  and/or the insertion handle  40  may include one or more alignment features  44  and mating notch  80  or alignment pin  46  and mating hole  82 . The insertion handle may be configured to align with an upper surface of the implant. In one implementation, the insertion handle may have a key configured to mate to a slot on the implant. Other alignment guides may be used. In addition, the guide may have an electrical connector configured to mate to an electrical connector on the implant. The connection between the guide and the implant may be spring loaded to ensure electrical contact between the electrical connectors. In order to avoid shorting the connection between the guide and the implant, the electrical connector may be insulated. As another example of electrically connecting the insertion handle to the implant, the electrical connectors may include a post and slip rings. The rings may be located on the implant, and the posts located on the insertion handle. The posts are biased to contact the rings. In such an implementation, the angular location of the insertion handle  40  relative to the axis of the implant is not fixed. This would allow the insertion handle  40  to be positioned to the implant irrespective of angular position. 
     In another implementation shown in  FIG. 13B , the implant  30  and/or the insertion handle  40  may include one or more alignment pin  47  and mating hole  83 . The alignment pins  47  may be spear tip pins designed to engage a single time and when removed, the pins grip portion of the implant to remove all non-biocompatible materials with them. 
     Any of the electrical connectors above may include a memory storage device (not shown) for storing offset values for sensor calibration. 
     Referring now to  FIG. 14 , the implant  30  and the insertion handle  40  may be sized such that space remains available for the first connector  52  even when the components are assembled or mated. As an example, the system for identifying a landmark may be used to target blind screw holes of an implanted intramedullary nail. The intramedullary nail is implanted in the patient. The electromagnetic field generator is activated. The processor receives signals from the sensor mounted to the intramedullary nail and from the sensor mounted to the landmark identifier, such as a drill sleeve. A computer program running on the processor uses the information of the at least two sensors and graphically display them in relative position on the monitor. A surgeon moves the landmark identifiers into position using feedback provided by the processor. When the landmark identifier is in the proper location, the surgeon drill through bone and the intramedullary nail to create a screw hole. The processor may provide feedback as to the depth of the drilled hole. The surgeon may then place a screw through the drilled hole to affix the blind hole of the intramedullary nail. 
     Provided feedback information may be selected from the group consisting of audible, visual, and tactile. The audible feedback may be output through a speaker, headphones, ear buds, or an ear piece. The audible feedback signal may be transmitted over wire or wirelessly using radio frequency or terrestrial data transmission. The visual feedback may be output through a cathode ray tube, a liquid crystal display, or a plasma display. Visual feedback devices may include, as examples, a television monitor, a personal digital assistant, or a personal media player. The visual feedback signal may be transmitted over wire or wirelessly using radio frequency or terrestrial data transmission. The tactile feedback may be output through gloves, instruments, or a floor mat. The tactile feedback signal may be transmitted over wire or wirelessly using radio frequency or terrestrial data transmission. 
       FIG. 15  illustrates a system  110  for identifying a landmark in another implementation. The system  110  may include a processor  112 , a landmark identifier  118 , and an orthopaedic implant assembly  128 . The system  110  may also include a monitor  114  and an insertion handle  140 . 
     The landmark identifier  118  is used to target a landmark. The landmark identifier  118  may include a second sensor  120 . In  FIG. 15 , the landmark identifier  118  is a drill sleeve with a serrated tip  122 , a tube  124 , and a handle  126 . The second sensor  120  is oriented relative to an axis of the tube, which may receive a drill. This offset of the sensor from the tube allows the position of the tube to be located in space in six dimensions (three translational and three angular) relative to the transmitter or another sensor in the system. The processor may need to be calibrated to adjust for the offset distance of the second sensor  120 . 
     The orthopaedic implant assembly  128  may include an implant  130  and a magnet  132 . The magnet may be a permanent magnet or an electromagnet. The magnet  132  is oriented in a predetermined position relative to a landmark on the orthopaedic implant  130 . This offset of the magnet from the landmark allows the position of the landmark to be located in space in six dimensions (three translational and three angular) relative to the transmitter or another sensor in the system, such as the second sensor. The processor may need to be calibrated to adjust for the offset distance of the magnet  132 . As with the implant  30  of  FIG. 1 , the implant  130  may also include a pocket  136  and a cover  138 . In the case of an electromagnet, a lead  150  connects to the magnet  132  and is contained within a groove  134 . 
     As an example, the system for identifying a landmark may be used to target blind screw holes of an implanted intramedullary nail. The intramedullary nail is implanted in the patient. The processor receives signals from the sensor mounted to the landmark identifier, such as a drill sleeve. A computer program running on the processor uses the information of the sensor and graphically displays the sensor in relative position to the magnet on the monitor. A surgeon moves the landmark identifiers into position using feedback provided by the processor. When the landmark identifier is in the proper location, the surgeon drill through bone and the intramedullary nail to create a screw hole. The processor may provide feedback as to the depth of the drilled hole. The surgeon may then place a screw through the drilled hole to affix the blind hole of the intramedullary nail. 
       FIG. 16  illustrates a method for selecting views corresponding to landmark identifier position. The view displayed on the monitor is dependent upon the location of the landmark identifier relative to the implant. The diameter of the implant is broken into sectors or fields. In  FIG. 16 , the diameter is broken down into three fields: (A) 135° to 225°; (B) 0° to 135°; and (C) 225° to 360°. The initial view is based upon landmark identifier orientation relative to the implant. As the user moves landmark identifier toward or away from the implant, the monitor display zooms in or out on the selected field. 
       FIG. 17  is a flowchart for view selection and display of one landmark. The process may be repeated for multiple landmarks. The processor  12  uses the transformation matrix in the following process steps. In step  200 , landmark identifier position is computed relative to the implant based upon the positions of the relevant sensors, and the landmark closest the landmark identifier is selected for display. In step  210 , a global view is defined showing the whole implant with the selected landmark oriented for proper viewing. A global view is analogous to viewing the implant at a distance. In step  220 , there is a decision whether there are multiple landmarks having the same orientation. If yes, then in step  230 , the processor calculates which landmark is nearest to the landmark identifier position and selects it for viewing. If no, in step  240 , a local view is defined and centered upon the selected landmarks. A local view is analogous to viewing the implant in close proximity. In some implementations, it may be desirable to hide the landmark identifier when the local view is defined. In steps  250 ,  260 , and  270 , the processor  12  identifies the distance from landmark identifier to the landmark and depending upon the decision made, either hides or renders the landmark identifier. In step  250 , the distance from landmark identifier to the landmark and a comparison is made between the calculated distance D and set variables T Global  and T Local . If D&gt;T Global , then the global view is selected in step  260  and the processor proceeds to step  285 . If D&lt;T Local , then the local view is selected and centered upon the landmark in step  270 . Thereafter, the processor proceeds to step  275 . In optional step  275 , the landmark identifier is hidden. Otherwise, an intermediate camera position is calculated based upon the distance D to enable a smooth transition from global view to a local view in step  280 . In step  285 , the landmark identifier is shown. In step  290 , the scene with selected camera position is rendered. 
       FIG. 18  is a schematic illustrating a first alternative method of aligning the landmark identifier. A computer program running on the processor may be used to take the information of the at least two sensors and graphically display them in relative position (the second sensor relative to the first sensor) on the monitor. This allows the user to utilize the system to guide the placement of the landmark identifier. In the case of drilling a blind intramedullary nail hole, the system guides the user in placement of the drill sleeve and subsequently drilling accurately thru the hole in the intramedullary nail. The graphical user interface may include an alignment guide for each of the degrees of freedom. A minimum alignment level may be set such that the surgeon continues to orient the landmark identifier until each of the degrees of freedom meets the minimum alignment level for an effective placement of the landmark identifier. The example of  FIG. 18  shows an instance where the placement in the Y-direction meets the minimum required tracking placement. However, none of the other translational or rotational meets the minimum requirements. While the magnitudes of tracking are illustrated as bar graphs, other graphical representations, such as color coding, may be used. 
       FIG. 19  is a schematic illustrating a second alternative method of aligning the landmark identifier. In this implementation, a graphical interface using a plurality of LEDs to position the drill may be placed upon the landmark identifier, such as a drill sleeve. By using the LEDs to trajectory track the drill, the surgeon may align the drill with the blind fixation hole. The trajectory may additionally use secondary displays to add more information to the system. For example, for affecting the magnitude of adjustment, the trajectory may include flashing LEDs so that high frequency flashing requires larger adjustments while low frequency flashing may require smaller adjustments. Similarly, colors may add information regarding adjustments to alignment. 
       FIG. 20  illustrates a monitor with exemplary views. A first portion  500  indicates the distance the drill is on each side of the implant. This may provide the user with a better understanding of drill depth and alert the user when to stop when appropriate drill depth has been achieved. The second portion  510  provides the user with alignment information. As an example, drill depth data may be obtained using the implementation illustrated in  FIG. 9 . 
       FIG. 21  illustrates an alternative implementation of the landmark identifier. The landmark identifier is configured to display, with LEDs, the position and trajectory information for proper alignment. The size of the LEDs may display additional information regarding the magnitude of required adjustment. The trajectory light may display a simple on/off toggle between an aligned trajectory and a mal-aligned trajectory. As another example, the trajectory LED may be color coded to suggest the magnitude of necessary adjustment for proper alignment. 
       FIG. 22  illustrates a first alternative implementation of the insertion handle  700 . The insertion handle  700  may include an arcuate slot  710 . The arcuate slot limits the movement of the landmark identifier  18 ,  118  within the operating space. In the case of identifying a blind screw hole, the arcuate slot limits the movement of the drill sleeve for fine adjustment of its position. The insertion handle  700  may include a carriage  712  that receives the landmark identifier and rides in the slot  710 . 
       FIG. 23  illustrates the system for identifying a landmark in a third implementation. In this implementation, the orthopaedic implant  800  is a bone plate and the insertion handle  810  is a little guide affixed to the bone plate. The inductive sensor is placed on the surface of the orthopaedic implant  800  relative to one or more landmarks. The guide  810  may allow a landmark identifier  818  to translate and/or rotate relative to the guide to properly align the landmark identifier with a landmark  802 , such as a fastener hole. In addition, where multiple fixation holes are on the implant, then additional guide holes  812  on the guide  810  may help approximate the position of the additional fixation holes. 
       FIG. 24  illustrates a second alternative implementation of the insertion handle. The insertion handle  900  may include fine adjustment in landmark identifier  918  positions through the use of small servomotors  920 ,  922 ,  924 . The servomotors  920 ,  922 ,  924  may adjust the orientation and position of the landmark identifier  918 . Control of the servos may be automatic or may be controlled by a surgeon. 
       FIG. 25  illustrates a bone  100  and another system  1010  for identifying a landmark. The system  1010  may include a control unit  1012 , a field generator  1014 , a landmark identifier  1016 , an intramedullary nail  1024 , and a probe  1029 . The landmark identifier  1016  also may be referred to as a targeter. The control unit  1012  may be included as part of the processor described above or may be a separate unit. The intramedullary nail  1024  is inserted into the bone  100 , and the intramedullary nail  1024  has a hole or landmark  1028 . The field generator  1014  is electrically connected to the control unit  1012 . An insertion handle  1022  is removably attached to the intramedullary nail  1024 . The insertion handle  1022  and/or the intramedullary nail  1024  may be formed with a cannulation. The insertion handle  1022  may include a third sensor  1032 . 
     The landmark identifier  1016  may include a second sensor  1020 . The landmark identifier  1016  may guide a drill bit  1018 , and the drill bit  1018  may be connected to a drill (not shown). The second sensor  1020  may be connected to the control unit  1012 , either by wire or wirelessly. The field generator  1014  may be included in or on the landmark identifier  1016 , in which case, the second sensor  1020  may be omitted. 
     The probe  1029  may include a wire  1030 , a tape  1034 , and a stop  1036 . The tape  1034  may be about 0.125 inch wide by about 0.060 inch thick 300 series stainless steel fish tape available from Ideal Industries, Inc. of Sycamore, Ill. However, those of ordinary skill in the art would understand that other materials and other sizes may be used. For example, any narrow band of polymer, composite material, or metal may be used as the tape  1034 , but it may be preferred to use a non-ferrous metal. The tape  1034  may be coiled before placement into the intramedullary nail  1024 . Coiling of the tape  1034  may cause it to have a natural curvature. The tape  1034  may have, in some implementations, a rectangular geometry that assists in orienting the tape as it is placed into a cannulation of the intramedullary nail  1024 . An oval, square, or circular geometry also may be used. The wire  1030  may be operatively connected to the tape  1034 . For example, this may be accomplished through the use of an adhesive or fastener. The tape  1034  may include graduations or detents to indicate a depth of the tape as it is inserted into the implant. 
     A first sensor  1026  is connected to the control unit  1012 , either by wire or wirelessly. The first sensor  1026  is connected through the use of the wire  1030  and a connector  1038 . The connector  1038  may be omitted. The first sensor  1026  may be connected to a distal end of the tape  1034 , and the stop  1036  may be connected to a proximal end of the tape  1034 . 
     The probe  1029  may include a sensor housing (not shown) to house the first sensor  1026 . The sensor housing may be attached to the tape  1034 . The sensor housing may be made of a non-ferrous material, such as a polymer, a composite, or a metal. The sensor housing may include an appropriate strain relief to shield the wire  1030  from stresses. The sensor housing may be constructed and arranged to be large enough to hold the first sensor  1026  but small enough to fit through the cannulation of the insertion handle or the implant. Further, the sensor housing may be constructed and arranged to be long enough to allow passage through intramedullary nail bends, intramedullary nail bow, and/or bends in relevant instrumentation. Geometry of the leading and trailing faces of the sensor housing may be designed such that the sensor housing does not catch or snag on the cannulation of the instrumentation or implant. 
     The stop  1036  may be used to control the placement of the sensor  1026  and probe  1029 . If the tape  1034  is a fixed length and the distance is known from the end of the insertion handle to the hole  1028 , repeatable placement of the first sensor  1026  may be achieved. The tape  1034  may be of sufficient length such that the sensor  1026  is aligned with the hole  1028 , adjacent the hole  1028 , or offset from the hole  1028 . As discussed below, the probe  1029  may be used to position the sensor with the hole  1028  or other landmark. 
     The insertion handle  1022  may be omitted. In such a case, a different tape length may be selected such that the stop  1036  engages a portion or end of the nail  1024 . 
       FIG. 26  is a partial detailed view of the intramedullary nail  1024 , the sensor  1026 , and the hole  1028 . The sensor  1026  may be aligned with the hole  1028 , adjacent the hole  1028 , or offset from the hole  1028 . The sensor  1026  is generally adjacent to the hole  1028 . 
     In use, the intramedullary nail  1024  is placed into the bone  100 . The insertion handle  1022  may be attached to the intramedullary nail  1024 . The probe  1029  is fed through the cannulation of the insertion handle  1022  and into the cannulation of the intramedullary nail  1024  until the stop  1036  engages the insertion handle  1022 . In one particular implementation, the wire  1030  is connected to the control unit  1012 , and the sensors  1026 ,  1020 , and  1032  are calibrated using the control unit  1012 . The probe  1029  may be removed after calibration. If so, the third sensor  1032  and a transformation matrix may be used to identify the relative position of the second sensor  1020  and hence landmark identifier  1016 . Optionally, the user may use transfixion elements, such as screws, to first lock the proximal end of the intramedullary nail. An operator uses the landmark identifier  1016  and the first sensor  1026  to identify the landmarks  1028 . For example, in the case of intramedullary nail fixation, a surgeon uses the landmark identifier  1016  to identify the blind transfixion holes and drill through the holes for placement of a transfixion element. 
       FIG. 27  illustrates a packaging implementation. In general, intramedullary nails must be sterilized before implantation. If the sensor is installed in the intramedullary nail prior to serialization, the sensor may lose its calibration during the serialization process, particularly if the sterilization process involves radiation. For example, gamma radiation may be used to sterilize hermetically sealed components, such as the sensor. The implementation depicted in  FIG. 27  illustrates a way to maintain the sterilization of the intramedullary nail while allowing for recalibration of the sensor. The package  FIG. 27  may include a first package  1040 , a second package  1042 , a first connector  1044 , a second connector  1046 , and a cable  1048 . In the depicted implementation, a sensor (not shown) and intramedullary nail  1024  are located within the first package  1040 . Alternatively, the probe  1029  and the sensor are located within the first package  1040 . In yet another example, only the sensor is located within the first package  1040 . A memory device (not shown) may be connected to the sensor. The memory device may be used to store a calibration transformation matrix (x 1 , y 1 , z 1 , x 2 , y 2 , z 2 ) as well as other data, such as length and size of the intramedullary nail or the probe. The memory device may be mounted to or placed on the intramedullary nail  1024  or the probe  1029 . The first connector  1044  is electrically connected, but removably attached, to the second connector  1046 . The first connector  1044  is also electrically connected to the sensor or the memory device. The first package  1040  maintains the sterilization of the device held within. The cable  1048  is electrically connected to the second connector  1046  and a storage device (not shown). The calibration for the sensor is downloaded from the storage device and transmitted through the connectors  1044 ,  1046  to the sensor or the memory device. The calibration step may be performed during manufacturing of the system or immediately prior to implantation of the implant. 
       FIG. 28  illustrates a method of connecting the system  1010  to a network.  FIG. 28  illustrates a network  1060 , a computing device  1050 , the cable  1048 , the second connector  1046 , the first connector  1044 , and the intramedullary nail  1024 . In the depicted implementation, a sensor (not shown) is located within the intramedullary nail  1024 . Alternatively, the sensor may be attached to the probe  1029  or freestanding. The intramedullary nail  1024  may be wrapped in packaging, such as the first package  1040  and/or second package  1042  but this is not always the case. A memory device (not shown) may be connected to the sensor. The memory device may be used to store a calibration transformation matrix (x 1 , y 1 , z 1 , x 2 , y 2 , z 2 ) as well as other data, such as length and size of the intramedullary nail or the probe. The memory device may be mounted to or placed on the intramedullary nail  1024  or the probe  1029 . The network  1060  maybe a local area network or a wide area network. The computing device  1054  is connected to the network  1060 . The network communication may be encrypted. The cable  1048  connects the computing device  1054  to the sensor or the memory device through the use the connectors  1044 ,  1046 . In this way, the sensor calibration may be downloaded from the computing device  1054  and/or the network  1060 . While the depicted implementation illustrates the sensor within the intramedullary nail, this is not always the case. The sensor may be attached to the probe or freestanding. The memory device may be located within the control unit, and the control unit is connected to the network to download the calibration data. 
       FIG. 29  illustrates a system  1110  for identifying a landmark in a fourth implementation. The system  1110  may include a control unit  1112 , a field generator  1114 , a landmark identifier  1116 , an intramedullary nail  1124 , a drop  1136 , and a probe  1129 . The control unit  1112  may be included as part of the processor described above or may be a separate unit. The intramedullary nail  1124  is inserted into the bone  100 , and the intramedullary nail  1124  has a hole or landmark  1128 . The field generator  1114  is connected to the control unit  1112 , either by wire or wirelessly. In the depicted implementation, an insertion handle  1122  is removably attached to the intramedullary nail  1124 . The insertion handle  1122  and/or the intramedullary nail  1124  may be formed with a cannulation. The insertion handle  1122  may include a third sensor  1144 . The drop  1136  may include a fourth sensor  1139 . 
     The landmark identifier  1116  may include a second sensor  1120 . The landmark identifier  1116  may guide a drill bit  1018 , and the drill bit  1018  may be connected to a drill (not shown). The second sensor  1120  may be connected to the control unit  1112 , either by wire or wirelessly. The field generator  1114  may be included in or on the landmark identifier  1116 , in which case, the second sensor  1120  may be omitted. 
     The probe  1129  may include a wire  1130 , a tape  1134 , and a stop  1136 . As shown below, the probe may be more unitary in structure as well. The tape  1134  may have, in some implementations, a rectangular geometry that assists in orienting the tape as it is placed into a cannulation of the intramedullary nail  1124 . The wire  1130  may be operatively connected to the tape  1134 . For example, this may be accomplished through the use of an adhesive or fastener. A first sensor  1126  is connected to the control unit  1112 , either by wire or wirelessly. The first sensor  1126  is connected through the use of the wire  1130 . In some implementations, a detachable connector may be used. The first sensor  1126  may be connected to a distal end of the tape  1134 , and the stop  1136  may be connected to a proximal end of the tape  1134 . The stop  1136  may be used to control the placement of the sensor  1126 . If the tape  1134  is a fixed length and the distance is known from the end of the insertion handle to the landmark  1128 , repeatable placement of the first sensor  1126  may be achieved. The tape  1134  may be of sufficient length such that the sensor  1126  is aligned with the landmark  1128 , adjacent the landmark  1128 , or offset from the landmark  1128 . 
     In use, the intramedullary nail  1124  is placed into the bone  100 . The insertion handle  1122  may be attached to the intramedullary nail  1124 . The probe  1129  is fed through the insertion handle  1122  and into the intramedullary nail  1124  until the stop  1136  engages the insertion handle  1122 . In one particular implementation, the wire  1130  is connected to the control unit  1112 , and the sensors  1126 ,  1120 , and  1132  are calibrated using the control unit  1112 . The probe  1129  may be removed after calibration. If so, the third sensor  1132  and/or the fourth sensor  1139  and a transformation matrix may be used to identify the relative position of the second sensor  1120  and hence targeter  1116 . Optionally, the user may use transfixion elements, such as screws, to first lock the proximal end of the intramedullary nail. An operator uses the landmark identifier  1116  and the first sensor  1126  to identify the landmarks  1128 . For example, in the case of intramedullary nail fixation, a surgeon uses the landmark identifier  1116  to identify the blind transfixion holes and drill through the holes for placement of a transfixion element. 
       FIG. 30  illustrates a first method for using the system to identify a landmark. The method begins at step  1210 . In step  1212 , the sensor is placed in the nail. In step  1214 , the insertion handle is connected to the nail, and the drop is attached to the insertion handle. In step  1216 , the control unit is connected to the sensor. In step  1218 , the sensor is calibrated. In step  1220 , the sensor is aligned with the hole. In step  1222  the sensor position is recorded through the use of the control unit. In step  1224 , the sensor is removed from the nail. In step  1226 , the nail is implanted into the bone. In step  1228 , the hole is drilled using the targeter. The method stops in step  1230 . 
       FIG. 31  illustrates a second method for using the system to identify a landmark. In step  1310 , the tracking system is turned on. In step  1312 , the intramedullary nail is inserted into bone. In step  1314 , the probe  1129  is inserted into the intramedullary nail canal at a predetermined location and orientation using the stop  1136  and detents spaced along a length of the probe  1129 . In step  1316 , there is a decision whether the intramedullary nail needs to be locked proximally before distally. If yes, then in step  1326  the drop is attached to the nail. In step  1328 , an offset is calculated between the probe and the drop. In other words, a transformation matrix is created. Alternatively, the drop is not connected to the intramedullary but instead a sensor mounted in the insertion handle is used to calculate an offset. In step  1330 , the probe is removed from the nail. In step  1334 , the nail is locked proximally. This may be accomplished through the use of the landmark identifier, a mechanical jig, or by manual operation. In step  1336 , the landmark identifier is used to target the drill. In step  1338 , the hole is drilled for the distal screw. In step  1340 , the intramedullary nail is locked distally. On the other hand, if the decision is to lock distally first, then in step  1318  the landmark identifier and probe are used to target the drill bit. In step  1320 , the hole is drilled for the distal screw. In step  1322 , the intramedullary nail is locked distally. In step  1324 , the probe is removed from the intramedullary nail. In step  1324 , the intramedullary nail is locked proximally. This may be accomplished through the use of the landmark identifier, a mechanical jig, or by manual operation. 
       FIG. 32  illustrates a system for measuring depth of drill bit placement. The system  1400  may include a stator  1410  and a slider  1412 . The stator  1410  and the slider  1412  form a capacitive array that can sense relative motion. Moving the stator  1410  and the slider  1412  in a linear relation relative to one another causes a voltage fluctuation that can be interpreted and used to determine the distance traveled. In some implementations, an electronic measuring circuit (not shown) and the slider  1412  may be housed inside the landmark identifier, and the drill bit may be specially constructed to have the stator  1410  along outer surface so that the stator  1410  and the slider  1412  are in very close linear proximity to each other. The linear movement of the drill bit stator  1410  induces a voltage in the receiving slider  1412  which is interpreted by the electronic measuring circuit as a distance measurement. The distance measurement may be sent to the control unit and/or displayed on the monitor. Capacitive sensors are highly susceptible to moisture, and so some implementations may be made to prevent liquids, such as bodily fluids, from traveling between the stator  1410  and the slider  1412 . 0-rings or some other similar form of wipes can be incorporated within the landmark identifier in order to keep the drill bit substantially moisture free. 
       FIGS. 33A and 33B  illustrate another system for measuring depth of drill bit placement. The system  1500  may include a reflective code wheel or strip  1510 , a lens  1512 , and an encoder  1514 . The lens  1512  focuses light onto bar of the code strip  1510 . As the code strip  1510  rotates, an alternating pattern of light and shadow cast by the window and bar, respectively, falls upon photodiodes of the encoder  1514 . The encoder  1514  converts this pattern into digital outputs representing the code strip linear motion. The encoder is an Avago Technologies AEDR-8300 Reflective Optical Encoder available from Avago Technologies of 350 W Trimble Road, San Jose, Calif. Alternatively, the Avago Technologies ADNS-5000 One Chip USB LED-based Navigation System may be used. The encoder and its supporting electronics may be mounted inside the landmark identifier so that its input region is oriented toward a “window” in the landmark identifier cannulation. Markings, such as dark colored concentric rings or bright reflective rings, may be added to the drill bit in order to enhance the visibility of the bit to the encoder. These markings could also be used to denote the starting zero point for measurement. As the drill bit moves linearly within the landmark identifier, the encoder measures the movement of the drill bit. The distance measurement may be sent to the control unit and/or displayed on the monitor. 
       FIG. 34  illustrates yet another system for drill depth measurement. The system  1600  utilizes a Linear Variable Differential Transformer (LVDT)  1612 . An LVDT is a type of electrical transformer used to measure linear displacement. The LVDT  1612  may include a plurality of solenoid coils  1618  placed end-to-end around a tube  1610 , which is the landmark identifier in the depicted implementation. In  FIG. 34 , the center coil is the primary coil and the outer two coils are the secondary coils. A cylindrical ferromagnetic core  1610 , such as the drill bit, slides along the axis of the tube. An alternating current  1614  is driven through the primary coil, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary. A pickup sensor  1616  measures the magnitude of the output voltage, which is proportional to the distance moved by the core (up to its limit of travel). The phase of the voltage indicates the direction of the displacement. Because the sliding core does not touch the inside of the tube, it can move without friction, making the LVDT a highly reliable device. The absence of any sliding or rotating contacts allows the LVDT to be completely sealed against the environment. The distance measurement may be sent to the control unit and/or displayed on the monitor. 
       FIGS. 35-37  illustrate an insertion handle  1700  ( FIG. 35 ) and an adjustable stop  1800  ( FIGS. 37-38 ). The insertion handle  1700  a stem  1710  that connects to an implant, such as an intramedullary nail (not shown), at an end portion  1712 . The insertion handle  1700  may include a quick connect  1716  for attachment to a drop, proximal targeting device, or some other instrument or apparatus. The insertion handle may include a top portion  1714 , which may include a hole and/or an alignment feature. The adjustable stop  1800  may include a slot  1810 , an alignment member  1812 , and a fastener hole  1814 . 
     In  FIGS. 35-37 , the adjustable stop  1800  may be removably attached to the top portion  1714  of the handle  1700 . The adjustable stop may be integrally formed with the insertion handle  1700 . In yet other implementations, the adjustable stop may be permanently attached to the insertion handle  1700 . The alignment member  1812  fits within an alignment feature of the top portion to prevent rotation of the adjustable stop. A fastener (not shown) may be placed through the fastener hole  1814  to attach the adjustable stop to the insertion handle  1700 . The tape  1034 ,  1134  may be placed through the slot  1810 , through the stem  1710 , and into the intramedullary nail cannulation. The slot  1810  may have a shape to match the geometry of the tape and/or probe  1129  to aid in its insertion or to prevent rotation of the tape. The tape  1034 ,  1134  or probe  1129  may include markings, graduations, or detents to indicate an appropriate depth for the given nail length. The adjustable stop  1800  may include a locking mechanism (not shown) to temporarily lock the tape  1034 ,  1134  at a particular depth. In its simplest form, the locking mechanism may be a fastener that frictionally engages the tape  1034 ,  1134 . 
       FIG. 38  illustrates a method for calibrating the system for identifying a landmark. Calibration is necessary for accuracy. The method begins at step  1900 , which may include powering up the system. In step  1910 , the probe and the landmark identifier are removed from packaging, if any, and scanned. The drop is also scanned. Scanning may include reading a bar code using a bar code reader. Scanning causes the system to retrieve offset sensor values that correspond to the bar code from a look up table in step  1912 . The look up table may be local or accessed over a network, such as the Internet. Alternatively, the probe and the landmark identifier may include a serial number or other unique identifier, and the unique identifier is used in conjunction with the look up table to retrieve offset sensor values. The offset sensor values are stored in local memory of the system in step  1914 . In step  1916 , the user places the probe relative to the implant and attempts to track a landmark using the landmark identifier in step  1916 . In step  1918 , there is a decision whether the calibration is correct. If so, the method ends in step  1920 . Otherwise, new offset values are retrieved in step  1912 . 
       FIG. 39  illustrates an implementation combining a landmark identifier, a field generator and a drill sleeve. The handheld landmark identifier  2016  houses an electromagnetic field generator (not shown) which may include one or more induction coils or other elements to create a suitable electromagnetic field or fields. The electromagnetic field generator is mounted in or on an autoclavable material and encapsulated in a silicone housing body  2018  that may be easily sterilized, and which is removably engageable with a tool. The relative orientation and position of the induction coils or elements in the landmark identifier  2016  may be selected to optimize the balance between the qualities and strength of the electromagnetic field or fields and their interaction with the sensor and the weight, size, form factor and ergonomics of the identifier  2016 . At least three induction coils (not shown) may be mounted in or on the autoclavable material. 
     The autoclavable material allows the landmark identifier  2016  to be sterilized or autoclaved multiple times without degradation of the autoclavable material, internal components, or operational performance. For example, the housing  2018  includes an internal body or mounting structure (not shown) on which the coils and/or other electromagnetic field generating components are mounted. The internal body is formed of a material that does not adversely interfere with a generated electromagnetic field and which can be subjected to sterilization processes, including autoclaving. For example, the internal body can be formed from a glass-reinforced epoxy laminate, such as a NEMA grade G-11 glass reinforced epoxy laminate (VETRONITE G11) or equivalent. The internal body is surrounded by a first covering  2018   a  formed from a first material, such as an overmolding of VMQ silicone material #71385C available from Minnesota Rubber &amp; Plastics, 1100 Xenium Lane N., Minneapolis, Minn. 55441. The housing  2018  also includes a second covering  2018   b  that may provide an additional layer of protection or insulation at an outer edge of the housing  2018 . The second covering  2018   b  may be formed from a second material, such as an overmolding of VMQ silicone material #71325C available from Minnesota Rubber &amp; Plastics, 1100 Xenium Lane N., Minneapolis, Minn. 55441. The housing  2018  also includes a coupling member  2018   c  that passes through the internal body and that engages one or more attachable components. The coupling member  2018   c  may be formed from polysulfone, such as a GEHR PPSU polyphenylsulfone RAL 9005 Black (Solvay Radel R-5500) or equivalent, and can be at least partially covered by the first covering  2018   a.    
     The particular landmark identifier  2016  illustrated in  FIG. 39  may also include a removable drill sleeve attachment  2020  and a drill sleeve  2022  with a serrated tip  2024 , though different components and constructions can be included as mentioned elsewhere. The sleeve attachment  2020  and drill sleeve  2022  can be formed as a single unit or as separate units connected to each other by adhesives or other connection means known to one skilled in the art. For illustration purposes, the sleeve  2022  as illustrated in  FIG. 39  is a drill sleeve, but it can also be a larger size sleeve such as a screw driver sleeve or other sleeves as selected by the surgeon, or other components as disclosed herein. To change sleeves or other components, the surgeon unscrews the sleeve attachment and replaces it with another sleeve attachment of choice and its corresponding sleeve. 
     Unlike the landmark identifier  18  illustrated in  FIG. 9 , the landmark identifier  2016  depicted in  FIGS. 39-40  does not require the second sensor  20  illustrated in  FIG. 9  because the origin of the global space (the area in which the electromagnetic field is generated) can be defined within the landmark identifier  2016 . One axis of the global space co-ordinate system can be the longitudinal axis of the drill sleeve or other component  2022 . In that situation, the other two axes of the global space co-ordinate system can be defined by planes orthogonal to that longitudinal axis and to each other. Advantages of incorporating the field generator into the landmark identifier  2016  include a smaller size field generator because it can be brought into the local working space (area which may include the landmarks such as implant holes that are to be targeted for screw placement) therefore requiring a smaller electromagnetic field. The global space and local working space become the same or at least correspond more closely spatially when the landmark identifier  2016  with its field generator brought into the vicinity of the landmark, implant or probe sensor (not shown). Because the electromagnetic field size requirement is smaller, the induction coils within the field generator can be smaller which therefore reduces the size and weight of the handheld field generator to make it more manageable for handheld usage. A light may be provided in an area of the landmark identifier/field generator/drill  2016 , such as the area  2025  to indicate to the user that power is being supplied to the landmark identifier/field generator/drill  2016 . In  FIG. 41 , the drill sleeve  2022  has been removed and the landmark identifier  2016  has been engaged with a screw driver  2100  for fixing the implant to the bone. As shown in  FIG. 41 , the housing  2018  of the landmark identifier  2016  may include one or more indentations  2018   d  for finger placement to allow the user to comfortably place his or her hand around the landmark identifier  2016 . In the implementation depicted in  FIG. 41 , there are six indentations. Additionally, the texture and dimension of an exterior surface of the housing  2018  can be configured allow a user to comfortably and securely grip the housing  2018 . Additionally, or alternatively, the landmark identifier  2016  can be attachable to a tool, such as a housing of the screw driver  2100 . 
     The material selection for the landmark identifier  2016  of  FIG. 39  which houses the field generator can be optimized for weight and stability after multiple autoclave cycles. Any autoclavable material can be used, and the materials are preferably non-magnetic or weak magnetic to avoid or minimize interference with the electromagnetic fields. Exemplary materials include ceramic, autoclavable polymers such as polypropylene (PP), polypropylene copolymer (PPCO), polycarbonate (PC), polymethylpentene (PMP), polytetrafluoroethylene (PTFE) resin, polymethyl methacrylate (PMMA or acrylic), ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethlyene (ECTFE), fluoro ethylene propylene (FEP), polyether imide (PEI), perfluoroalkoxy (PFA), polyketone (PK), polyphenylene oxide (PPO), polysulfone (PSF), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), silicone, or thermoplastic elastomers (TPE), and still other autoclavable materials which will be apparent to those skilled in the art, including combinations of the above. 
       FIG. 42  illustrates another insertion handle  2122 , an adjustable stop  1801 , a probe  2129  and a sensor  2126  located within or on a body  2129   a  of the probe  2129 . The insertion handle  2122  is removably attached to an orthopaedic implant, such as an intramedullary nail. The probe  2129  includes a cable  2130  and a connector  2131  for connection to a processor for use in targeting landmarks of the intramedullary nail, or other implant. The probe  2129  also includes a grip  2132  that secures the cable  2130  to the body  2129   a  to prevent tension forces applied to the cable  2130  from damaging the sensor  2126  and/or the electrical connection between the sensor  2126  and the cable  2130 . The adjustable stop  1801  and an alternative stop  1803  are also illustrated in  FIGS. 43 and 44 . The stop  1801  and the stop  1803  each include a push button actuator  1802 . The stop  1801  includes a thumb wheel  1806  that is used to turn a threaded bolt  1807  for attaching the stop  1801  to an insertion handle while the stop  1803  includes a knob clamp  1804  that is also connected to a threaded bolt  2531  ( FIG. 58 ). The probe  2129  may be equipped with detents or markings to assist the user in placing the probe  2129  and sensor  2126  in the correct position. 
     The system may include different stops, such as stops  1801 ,  1803 , depending upon the particular surgical approach contemplated. For example, the surgical approach in the case of retrograde placement of an intramedullary nail may utilize the stop  1801 , whereas antegrade place placement of an intramedullary nail may favor use of the stop  1803 . Other surgical approaches may yet require other variations. 
     Now referring to  FIG. 45 , the adjustable stop  1803 , an insertion handle  2123 , and the probe  2129  are illustrated in an assembled configuration attached to an intramedullary nail  2125 . A distal portion  2124  of the insertion handle  2123  is attached to a proximal head  2126  of the intramedullary nail  2125 . The insertion handle  2123  is connected to the intramedullary nail  2125  through the use of a cannulated bolt (not shown). Alternatively, the insertion handle  2123  can be connected to the intramedullary nail  2125  using a quick-connect mechanism, or other attachment device. 
     The adjustable stop  1803  is attached to a proximal surface  2127  of the insertion handle  2123 . The adjustable stop  1803  has a complimentary mating portion such that when the stop  1803  is connected to the insertion handle  2123 , the stop  1803  is located or fixed relative to the insertion handle  2123  within three degrees of freedom. The probe  2129  is inserted through a hole  1805  of the adjustable stop  1803 , through the distal portion  2124  of the insertion handle  2123 , through the cannulated bolt, and into a cannulation (not shown) of the intramedullary nail  2125 . The probe  2129  includes the sensor  2126  ( FIG. 42 ) located proximate a distal end (not shown). The sensor  2126  ( FIG. 42 ) is electrically connected to the cable  2130  that includes the connector  2131  for transmitting signals from the sensor  2126  ( FIG. 42 ) to a control unit (not shown). 
       FIG. 46  shows the adjustable stop  1801 , the insertion handle  2122 , and the probe  2129  in an assembled configuration attached to an intramedullary nail  2155 . The adjustable stop  1801  is mounted to a proximal surface  2147  of the insertion handle  2122  by rotating the thumb wheel  1806  to thread the bolt  1807  ( FIG. 43 ) into a threaded connection (not shown) formed in the insertion handle  2122 . The adjustable stop  1801  has a complimentary mating portion such that when the stop  1801  is connected to the insertion handle  2122 , the stop  1801  is located or fixed relative to the insertion handle  2122  within three degrees of freedom. A distal portion  2144  of the insertion handle  2122  is attached to a head  2156  of the intramedullary nail  2155 . For example, the insertion handle  2122  is connected to the intramedullary nail  2125  through the use of a cannulated bolt (not shown). The probe  2129  is inserted through a hole  1808  of the adjustable stop  1801 , through the distal portion  2144  of the insertion handle  2122 , through the cannulated bolt, and into the head  2156  of the intramedullary nail  2155 . 
     Now referring to  FIG. 47 , a proximal targeting probe  2161  and a distal targeting probe  2171  are illustrated. The proximal targeting probe  2161  includes a tape body  2163  and a sensor  2165  disposed within or on the tape body  2163  at a predetermined distance D 1  from a reference point R 1  of the body  2163 . The proximal targeting probe  2161  also includes a color-coded grip  2167  that indicates that the probe  2161  is to be used for targeting proximal landmarks of an orthopaedic implant, such as the intramedullary nail  2155  of  FIG. 46  and a cable  2169  for carrying a signal from the sensor  2165  to a control unit (not shown). The distal targeting probe  2171  includes a tape body  2173  that is longer than the body  2163  of the proximal targeting probe  2161 . A sensor  2175  is included within or on the tape body  2173  at a second predetermined distance D 2  from a reference point R 2  of the body  2173 . The distal targeting probe  2171  also includes a color-coded grip  2177  that is a different color than the grip  2167  and that indicates that the probe  2171  is to be used for targeting distal landmarks of an orthopaedic implant, such as the intramedullary nail  2155  of  FIG. 46 . A cable  2179  is included to transmit a signal from the sensor  2175  to a control unit (not shown). The tape body  2163  of the proximal targeting probe  2161  and/or the tape body  2173  of the distal targeting probe  2171  may have, in some implementations, a rectangular geometry that assists in orienting the tape body as it is placed into a cannulation of the intramedullary nail. An oval, square, or circular geometry also may be used. In some implementations, the tape body  2163  and the tape body  2173  may be a hollow metal tube. Instead of color-coded grips, in some implementations, each of the sensors  2165  and  2175  is connected to a Programmable Read-Only Memory (PROM) microchip that stores the calibration offset values and also stores an identifier that identifies whether the probe is used for proximal or distal targeting. In that way, when the sensors  2165  and  2175  are both connected to a processor, such as the processor  2327  of  FIG. 51 , the processor automatically identifies the type of targeting contemplated and may display such information on a display device, such as the display  2326 . 
     The tape body  2163  and the tape body  2173  may include one or more bends to bias at least a portion of the tape body  2163  and the tape body  2173  against the wall of the cannulation of the orthopaedic implant. Biasing a portion of the tape body against the wall of the cannulation increases the repeatability of locating the sensors  2165  and  2175  relative to landmarks. Alternatively, the probes  2161  and/or  2171  could be formed having dimensions approximately equal to dimensions of the cannulation of the intramedullary nail or other implant with which they are intended to be used so that the proper location of the sensor within the cannulation can be repeatably achieved. 
     With reference to  FIG. 48  and as an alternative to the pair of probes illustrated in  FIG. 47 , a targeting probe  2181  can be used to target both distal and proximal landmarks of an orthopaedic implant. The probe  2181  is tubular and made from a non-magnetic metal, such as stainless steel. The probe  2181  includes a tape body  2183 , a first sensor  2185  disposed within a distal portion of the tape body  2183  and a second sensor  2186  disposed within a proximal portion of the tape body  2183 . The first sensor  2185  is located at a distance D 3  from a reference point R 3  of the body  2183 , which may be formed as a notch or detent, and the second sensor is located at a second distance D 4  from the reference point R 3 . 
     In use, the first sensor  2185  is used to target a distal landmark of an orthopaedic implant, such as a distal locking aperture of an intramedullary nail, and the second sensor  2186  is used to target a proximal landmark of the orthopaedic implant, such as a proximal locking aperture of the orthopaedic nail. In some implementations, the construction of the probe  2181  can be similar to the distal targeting probe  2171  ( FIG. 47 ), with the addition of the second sensor  2186 . The probe  2181  also includes a grip  2187 , which can be color-coded to be distinguishable from the distal targeting probe  2171 , and to indicate that the probe  2181  can be used to target both distal and proximal landmarks of an implant. As discussed above, each of the sensors  2185  and  2186  can be connected to a PROM or other storage device that stores reference values for use in determining a position of a landmark identifier, such as the landmark identifier  2016 , relative to a landmark of an implant. The PROMs also store identifiers that allow a processor to determine whether a received signal was generated by the distal sensor  2185  or the proximal sensor  2186 . 
     Another alternative probe  2191  is illustrated in  FIG. 49 . The probe  2191  includes a housing  2192  and a retractable/extensible body  2193  that can be coiled within the housing  2192 . A sensor  2195  disposed within the body  2193  can be positioned at a first position P 1  for targeting a proximal landmark of an implant, and can be positioned at a second position P 2  for targeting a distal landmark of an implant. In some implementations, the body  2193  can be formed as a concave metal strip that tends to maintain a generally straight shape when extended from the housing, but can also be coiled within the housing  2192 . For example, the body  2193  may consist of layered, flexible stainless steel bi-stable spring bands that, when straightened, create tension within the springy metal bands to maintain a generally straight orientation, but coil within the housing  2192  when the tension is relieved. However, other types of materials can be used to form the body, including resilient plastic or rubber tubing or sheeting. Another alternative is to form the body  2192  from nested segments of tubing that can extend and retract by sliding within adjacent tube segments. The probe  2191  can include a rotary encoder, an optical device, or other measuring device or method to determine a length of the body  2193  that is currently extending from the housing  2192 . The determined length can be used to determine whether the sensor  2195  is positioned at a desired position, such as the first position P 1  or the second position P 2 . 
     Now referring to  FIG. 50 , the adjustable stop  1801  is mounted to an alternative insertion handle  2210 . The insertion handle  2210  includes a body  2211  that is engaged with the head  2156  of the intramedullary nail  2155  (also shown in  FIG. 46 ). As an example, a cannulated bolt (not shown) may be used to connect the insertion handle  2210  to the head  2156 . The insertion handle  2210  includes a sensor  2213  for use in targeting a proximal landmark of the nail  2155 , such as a proximal locking aperture  2157 . The sensor  2213  is located within or on the body  2211  at a predetermined distance D 5  from the proximal locking aperture  2157  when the insertion handle is attached to the nail  2155 . The sensor can be passive, or electrically powered by an internal battery (not shown) or an external power supply (not shown). The sensor  2213  may be mounted in a compartment that is unitary or integral with the body  2211 , such as the exterior compartment  2216 . Alternatively, the sensor  2213  can be located in an internal compartment  2213   a,  shown in  FIG. 51 . The insertion handle  2210  is made of plastic, but other materials could alternatively be used. 
     The adjustable stop  1801  is used with a probe, such as the probe  2129  ( FIG. 46 ), for targeting distal landmarks of the nail  2155 , such as a distal aperture  2159  ( FIG. 51 ). As described above, the adjustable stop  1801  can be attached to the insertion handle  2210  by a bolt  1807  ( FIG. 43 ) that engages a threaded bore  2215  that is aligned with a longitudinal through hole of the insertion handle  2210  (not shown) that allows the probe to pass through the insertion handle  2210  and into the cannulation  2155   a  ( FIG. 51 ) of the nail  2155 . The adjustable stop  1801  also includes an arm  1809  that engages the body  2211  to prevent rotation between the adjustable stop  1801  and the insertion handle  2210 . 
     Although not illustrated, the adjustable stop  1803  (shown in  FIG. 45 ) can also be used with an insertion handle that includes an embedded sensor for targeting proximal landmarks of an implant. 
     In use, an orthopaedic implant, such as the intramedullary nail  2155 , is implanted into bone. The insertion handle  2210  may be connected to the orthopaedic implant before or after implantation. Thereafter, a landmark identifier can be used for targeting of the proximal landmarks of the orthopaedic implant. 
     In some implementations, the distal landmarks are targeted prior to the proximal landmarks. As before, the insertion handle  2210  may be connected to the orthopaedic implant before or after implantation. The stop  1801  or the stop  1803  is connected to the insertion handle  2210 . A probe is inserted into the stop, through the insertion handle  2210 , and into the orthopaedic implant, such as the nail  2155 . The distal landmarks are targeted, transfixion elements are placed in the distal landmarks to hold the orthopaedic implant, the probe is removed, and then the proximal landmarks are targeted. 
     Now referring to  FIG. 51 , a system  2300  for targeting a blind landmark of an orthopaedic implant is illustrated. The system  2300  includes the adjustable stop  1801 , the probe  2171 , and the insertion guide  2210  assembled and connected to the intramedullary nail  2155 , which is implanted in a bone B that includes a fracture F. The intramedullary nail  2155  includes a distal aperture  2159  that extends through the intramedullary nail  2155  and is configured to receive a locking fastener (not shown). The probe  2171  is received through an aperture or the adjustable stop  1801 , through a cannulation  2210   a  of the insertion handle  2210 , and within a cannulation  2155   a  of the intramedullary nail  2155 . The probe  2171  is received within the stop  1801  such that the sensor  2175  is positioned at a known distance from the distal aperture  2159 . The known distance may range from zero to about  102  millimeters from the sensor  2175  to the distal aperture  2159  or other landmark. In other implementations, the known distance may range from about two millimeters to about twenty-five millimeters or from about three millimeters to about ten millimeters. In the depicted implementation, the known distance is about five millimeters. 
     The system  2300  also includes a tool, such as a drill  2310  that includes a drill bit  2311 . The landmark identifier  2016  is engageable with the drill  2310  and/or the drill bit  2311  such that a position and orientation of the landmark identifier  2016  can be used to determine a position and orientation of the drill  2310  and/or the drill bit  2311 . For example, the housing of the landmark identifier  2016  can include a friction fit engagement with the drill  2310 , a strap, or other securing mechanism to at least temporarily secure the landmark identifier  2016  to the drill  2310 . In other implementations, the landmark identifier  2016  can be integrated with the drill  2310 , or other tool. In some implementations, the drill sleeve (not shown) may telescope to allow the user to place the tip of the drill sleeve against a patient and also allow the user to move the drill bit in a longitudinal direction for drilling. 
     A targeting system  2320  is operable to provide an indication to a user, such as a surgeon, of the relative position of a tool, such as a drill  2310  that includes a drill bit  2311 , relative to the distal aperture  2159 . The targeting system  2320  includes a housing  2321 , a first sensor port  2322 , a second sensor port  2323 , a field generator port  2324 , a display device  2325 , and a processor  2327 . The first sensor port  2322  is configured to receive a connector of the cable  2179  of the probe  2171  such that the targeting system  2320  receives signals generated by the sensor  2175 . The second sensor port  2323  is configured to receive a connector of a cable  2214  that is connected to the sensor  2213  of the insertion handle  2210  such that the targeting system  2320  receives signals generated by the sensor  2213 . The field generator port  2324  is configured to receive a connector of a cable  2019  of the landmark identifier  2016  such that the targeting system  2320  transmits signals via the cable  2019  to control the operation of the field generator of the landmark identifier  2016 . The display device  2325  is operable to output a display of a graphical user interface  2326  that includes a representation of the position and orientation of the drill  2310  relative to a location and orientation of a landmark of the intramedullary nail  2155 , such as the distal aperture  2159 , the proximal aperture  2157  ( FIG. 50 ), or another landmark. 
     The processor  2327  is operable to receive signals from the distal sensor  2175  and/or the proximal sensor  2213 , and to determine, based on the received signal(s), a current position and orientation of the landmark identifier  2016  relative to a selected landmark of the intramedullary nail  2155 . For example, a feature of a signal received from the distal sensor  2175 , such as one or more induced electrical currents, can be used by the processor  2327  to determine a distance of the landmark identifier  2016  from the sensor  2175 , as well as an orientation of a magnetic moment of a field generated by the landmark identifier  2016 . For example, the sensor  2175  can transmit a signal indicative of a current value and an identifier that indicates which of a plurality of induction coils produced the associated current value. The processor  2327  can compare the received current values with reference values associated with each of the induction coils to determine differences between the received values and the reference values. The reference values can be values of induced current associated with a reference field generation signal, a reference position, and a reference orientation of the landmark identifier  2016 . The processor  2327  uses these determined differences between the received and reference values to determine a difference in position and orientation of the landmark identifier  2016  from the reference position and orientation based on any determined difference in the magnetic field generated by the landmark identifier  2016  from the reference field. Based on the difference in position and orientation of the landmark identifier  2016  and the reference position and orientation, a current position and orientation of the landmark identifier  2016  relative to the sensor  2175  can be determined by the processor  2327 . 
     The current distance and orientation of the landmark identifier  2016  relative to the sensor  2175  are used by the processor  2327  to determine the current distance of the landmark identifier  2016  from the distal aperture  2159  and the current relative orientation of the magnetic moment of the generated magnetic field relative to a central through-axis of the distal aperture  2159 . For example, the processor  2327  determines the current distance and relative orientation of the landmark identifier  2016  relative to the distal aperture  2159  based on a known position and orientation of the distal aperture  2159  relative to the distal sensor  2175 . The processor  2327  also determines a current position of the drill  2310 , including the drill bit  2311 , from the distal aperture  2159  as well as a current orientation of the drill  2310  and the drill bit  2311  relative to the central through-axis of the distal aperture  2159  based on a known position and orientation of the drill  3210  and the drill bit  2311  relative to the location of the landmark identifier  2016  and the magnetic moment of the field generated by the landmark identifier  2016 . In the case of the landmark identifier  2016 , a longitudinal axis of the drill bit  2311  is coaxial with the magnetic moment of the magnetic field generated by the landmark identifier  2016 . 
     The graphical user interface  2326  is generated by the processor based on the determined current position and orientation of the drill  2310  and the drill bit  2311  relative to the distal aperture  2159 , or based on a current position and orientation of another tool relative to another landmark. The graphical user interface  2326  includes a first portion  2326   a  that includes an intramedullary nail image  2155   b  that represents the intramedullary nail  2155  and includes a distal aperture image  2159   a  that represents the distal aperture  2159 . The first portion  2326   a  of the graphical user interface  2326  also includes an orientation indicator  2330  that includes a first circle  2331 , a second circle  2333 , and a line  2335  that intersects the centers of each of the first circle  2331  and the second circle  2333 . The line  2335  provides an illustration to the user of the current orientation of the drill bit  2311  relative to the central through axis of the distal aperture  2159 . Particularly, when the first circle  2331  and the second circle  2333  are both disposed entirely within the distal aperture image  2159   a,  then the longitudinal axis of the drill bit  2311  is co-axial with the central through axis of the distal aperture  2159 , as shown in  FIG. 51 . The graphical user interface  2326  also includes a second portion  2326   b  that includes intramedullary nail image  2155   b  and a drill bit image  2331   b.  The current position and orientation of the drill bit  2311  relative to the intramedullary nail  2155  is illustrated in the second portion  2326   b  of the graphical user interface  2326 . 
     In use, the probe  2155 , the insertion handle  2210 , the adjustable stop  1801 , the landmark identifier  2016 , the drill  2310 , and the drill bit  2311  can be sterilized, such as by autoclaving, if one or more of the components is not sterile. When sterile, the probe  2155  is connected with the first sensor port  2322  of the targeting system  2320  and the insertion handle  2210  is connected with the second sensor port  2323  of the targeting system  2320 . The processor  2327  detects the connection of the distal sensor  2175  and the proximal sensor  2213  and can optionally cause a display of an indication of the proper (or improper) connection of the probe  2155  and the insertion handle  2210  and/or an indication of the proper (or improper) operation of the distal sensor  2175  and the proximal sensor  2213 . Similarly, the landmark identifier  2016  is connected with the field generator port  2324 , and the processor can detect the connection of the landmark identifier  2016  and cause a display of the proper (or improper) connection of the landmark identifier  2016  and/or the proper (or improper) operation of the field generator of the landmark identifier  2016 . The sensor  2175  is connected to a Programmable Read-Only Memory (PROM) microchip that stores the calibration values and also stores an identifier that identifies the sensor  2175  as a distal targeting sensor. When the sensor is connected to the processor  2327 , the processor  2327  automatically identifies the type of targeting contemplated and may display an indication on graphical user interface  2326  that a sensor of the identified type is connected. 
     The insertion handle  2210  is engaged with the intramedullary nail  2155  and the adjustable stop  1801  is engaged with the insertion handle  2210 . The probe  2155  is then inserted in the adjustable stop  1801  and positioned at a desired location. The button  1802  is manipulated to allow the probe  2155  to be adjusted, and the button  1802  is released to clamp the probe  2155  in a desired position. For example, the probe  2155  can be inserted until a reference mark, such as a printed mark or a detent or other structure of the probe  2155  is correctly positioned relative to a reference portion of the adjustable stop  1801 . The positioning of the probe  2155  locates the distal sensor  2175  in the correct position relative to the distal aperture  2159 . 
     A drill sleeve  2022  is selected and engaged with the drill sleeve attachment  2020  of the landmark identifier  2016 . For example, one of a short drill sleeve and a long drill sleeve is selected. An indication of the selection is input to the targeting system  2320 , such as by interaction with a menu  2326   c  of the graphical user interface  2326 . Additionally, an indication of the specific intramedullary nail  2155 , insertion handle  2210 , adjustable stop  1801 , and/or probe  2171  is input to the targeting system  2320 , if not automatically recognized by the targeting system  2320  and/or to confirm the specific intramedullary nail  2155 , insertion handle  2210 , adjustable stop  1801 , and/or probe  2171 . 
     The accuracy of the targeting system  2320  is checked before implantation of the intramedullary nail  2155  by placing the landmark identifier  2016  directly over the distal aperture  2159  of the intramedullary nail  2155 , which can be done by inserting the tip  2024  of the drill sleeve  2022  within the distal aperture  2159 . If the second circle  2333  is shown within the distal aperture image  2159   a,  and if the orientation of the line  2335  corresponds to the orientation of the drill sleeve  2022 , then the targeting system  2320  is accurate. If the targeting system is not accurate, the input indications of selected components, and/or the position of the probe  2171  are checked. If no errors are found, then the targeting system  2320  is recalibrated, as described below with reference to  FIG. 52 . 
     When the components are assembled and checked as described above, the intramedullary nail  2155  is implanted in the bone B. When the intramedullary nail  2155  is located in the desired position, the tip  2024  of the drill sleeve  2022  is placed over the distal aperture  2159 . When the landmark identifier  2016  is brought near the sensor  2175 , a signal generated by the sensor  2175  is received by the processor  2327 , and one or more signal feature, such as a current value, and an identifier are used by the processor to determine that distal targeting is being attempted, and the targeting system  2320  enters a distal targeting mode. Locating the tip  2024  relative to the distal aperture  2159 , which is hidden within the bone B, is performed by a user by making reference to the graphical user interface  2326  in the distal targeting mode, and is confirmed when the first circle  2331  and the second circle  2333  are located within the distal aperture image  2159   a.    
     An incision is made in the skin at the location of the distal aperture  2159 . The drill sleeve  2022  is then inserted into the incision down to the bone B. The landmark identifier  2016  is then manipulated by a user to arrange both the first circle  2331  and the second circle  2333  completely within the distal aperture image  2159   a  and, while maintaining the position and orientation of the landmark identifier  2016 , the drill bit  2311  is inserted through the drill sleeve  2022  and a user drills through the bone B, through the distal aperture  2159 , to the cortex on the far side of the bone B. A desired drill depth can be achieved by the user by referring to the second portion  2326   b,  or by comparing one or more reference marks included on the drill bit  2311  to a reference portion of the landmark identifier  2016 . 
     The drill bit  2311  is then removed and a locking fastener (not shown) is engaged with the bone B and the distal aperture  2159  through the drill sleeve  2022 , again maintaining the first circle  2331  and the second circle  2333  within the distal aperture image  2159   a.  A desired depth of insertion of the locking fastener can be achieved by a user by referring to the second portion  2326   b  of the graphical user interface  2326 , or by comparing a reference marking on a fastener driving tool (not shown) to a reference portion of the landmark identifier  2016 . 
     In addition to engaging the locking fastener with the distal aperture  2159 , the targeting system  2320  can be used to target a proximal landmark of the intramedullary nail  2155 . For example, before or after engaging the locking fastener with the distal aperture  2159  and the bone B, a user can select the sensor  2213  from the menu  2326   c  or move the landmark identifier  2016  within a predetermined distance of the sensor  2213 , which causes the targeting system  2320  to enter a proximal targeting mode and output a display of the relative position and orientation of the drill  2300  and/or the drill bit  2016  relative to a proximal landmark of the intramedullary nail  2155 , such as the proximal aperture  2157  ( FIG. 50 ). A user can then engage a fastener or other tool or implant with the proximal landmark in a mariner similar to that described above with respect to drilling through the distal aperture  2159  and/or engaging the locking fastener with the distal aperture  2159 . 
     As mentioned above, a proximal landmark can be targeted using the targeting system  2320  and the sensor  2213  before or after targeting a distal landmark, such as the distal aperture  2159 . Particularly, a proximal landmark can be targeted before insertion of the probe  2171  within the adjustable stop  1801 , the insertion handle  2210 , and/or the intramedullary nail  2155 . The proximal landmark can also be targeted after removal of the probe  2171 , or while the probe  2171  is inserted within the adjustable stop  1801 , the insertion handle  2210 , and/or the intramedullary nail  2155 . For example, as discussed above, the probe  2171  can be inserted through a portion of the intramedullary nail  2155  that does not interfere with engagement of the drill bit  2311  or the fastener with a proximal aperture or other proximal landmark. Additionally, if the probe  2171  is inserted into the cannulation  2155   a  and the proximal aperture also passes through the cannulation  2155   a,  the cannulation  2155   a  can be large enough to simultaneously accommodate both a fastener or the drill bit  2311  and the probe  2171 . For example, the probe  2171  can be dimensioned to be disposed in a gap between the drill bit  2311  and an inner wall of the intramedullary nail  2155  that defines the cannulation  2155   a.  Similarly, the probe  2181  ( FIG. 48 ), which has both the distal sensor  2185  and the proximal sensor  2186 , can be inserted in the cannulation  2155   a  and both distal and proximal landmarks of the intramedullary nail  2155  can be targeted without removal or adjustment of the probe  2181 . 
     Alternatively, a proximal landmark of the intramedullary nail  2155  can be targeted using the targeting system  2320  and either the sensor  2175  of the probe  2171  or the sensor  2165  of the probe  2161  ( FIG. 47 ). For example, after engaging the locking faster with the distal aperture  2159 , the probe  2171  can be adjusted using the adjustable stop  1801  to secure the sensor  2175  in a predetermined location relative to one or more proximal landmarks of the intramedullary nail  2155 . The menu  2326   c  can then be used to select a proximal targeting mode such that the targeting system  2320  is operable to display a position and orientation of the drill  2310  and/or the drill bit  2311  (or other tool or implant) relative to the proximal landmark(s). Similarly, and particularly where it is undesirable to have a portion of the probe  2171  extending a distance from the adjustable stop  1801 , the probe  2161  can be connected to the targeting system  2320  and inserted in the adjustable stop  1801  such that the sensor  2165  is located in a know location relative to one or more proximal landmarks of the intramedullary nail  2155 . In either case, one or more proximal landmarks of the intramedullary nail can then be targeted using the targeting system  2320 , as described above. In other implementations, the proximal landmark(s) can be targeted before the distal aperture  2159  using the prone  2171  or the probe  2161 . 
     Now referring to  FIG. 52 , a calibration member  2340  is attached to the landmark identifier  2016  and the intramedullary nail  2155  for use in calibrating the targeting system  2320 . For example, if the accuracy of the targeting system  2320  is checked before inserting the intramedullary nail  2155  and errors are found, the targeting system  2320  can be re-calibrated. In use, the calibration member  2340  is engaged with the landmark identifier  2016 . Then a tip  2341  is inserted into the distal aperture  2159  until a reference portion (not shown) of the calibration member  2340  abuts the intramedullary nail  2155 . Re-calibration of the targeting system  2320  can then be achieved by interaction with the menu  2326   c  of the graphical user interface  2326 . For example, a “re-calibrate” option may be selected from the menu  2326   c,  which causes the targeting system  2320  to transmit a driving signal to the field generator of the landmark identifier  2016  and to store as reference values any current values received from the sensor  2175  of the probe  2171 . The graphical user interface  2326  can display an indication of a successful re-calibration of the targeting system  2320 . 
     Now referring to  FIGS. 53-57 , details of the adjustable stop  1801  are illustrated. The adjustable stop  1801  includes a housing  2401  that includes a clamp member slot  2402 . A clamp member  2411  is received within the clamp member slot  2402  and is biased by a spring  2413 . The clamp member  2411  is retained within the housing  2401  by pins  2415 . The clamp member  2411  also includes an actuator slot  2417 , a linkage aperture  2418 , and a probe aperture  2419 . 
     The button  1802  includes an actuating shaft  2421  and an actuating slot  2423 . The actuating shaft  2421  is received within an aperture  2405  of the housing  2401  and is biased against insertion into the housing by a spring  2425 . When assembled, the actuating shaft  2421  is received in the actuator slot  2417  of the clamp member  2411  and is retained in the housing  2401  by a linkage pin  2427  that is inserted into the actuating slot  2423  of the actuating shaft  2421  through an opening  2403  of the housing  2401  and through the linkage aperture  2418  of the clamp member  2411 . In use, when the button  1802  is depressed against the biasing force of the spring  2425 , the linkage pin  2427  is moved within the actuating slot  2423  which pushes the clamp member  2411  against the spring  2413  to allow a probe to be inserted into the hole  1808  and through the probe aperture  2419 . When the probe is inserted and the button  1802  is released, the springs  2413  and  2425  cause the clamp member  2411  to bear against the probe to maintain the position of the probe within the hole  1808 . 
     The thumb wheel  1806  is received within a thumb wheel slot  2407  of the housing  2401  and the bolt  1807  is threaded into a threaded aperture  2431  of the thumb wheel  1806  through a bolt aperture  2409  of the housing  2401 . After the bolt  1807  is threaded into the bolt aperture  2431 , a pin  2433  is inserted through an aperture  2435  ( FIG. 56 ) of the thumb wheel  1806  and into a slot  2437  ( FIG. 56 ) of the bolt  1807  to retain the bolt  1807  in engagement with the thumb wheel  1806 . 
     Now referring to  FIGS. 58-62 , details of the adjustable stop  1803  are illustrated. The adjustable stop  1803  includes a housing  2501  that includes a clamp member slot  2502 . A clamp member  2511  is received within the clamp member slot  2502  and is biased by a spring  2513 . The clamp member  2511  is retained within the housing  2501  by pins  2515 . The clamp member  2511  also includes an actuator slot  2517 , a linkage aperture  2518 , and a probe aperture  2519 . 
     The button  1802  includes an actuating shaft  2421  and an actuating slot  2423 . The actuating shaft  2421  is received within an aperture  2405  of the housing  2401  and is biased against insertion into the housing by a spring  2525 . When assembled, the actuating shaft  2421  is received in the actuator slot  2517  of the clamp member  2511  and is retained in the housing  2501  by a linkage pin  2527  that is inserted into the actuating slot  2423  of the actuating shaft  2421  through an opening  2503  of the housing  2401  and through the linkage aperture  2418  of the clamp member  2511 . In use, when the button  1802  is depressed against the biasing force of the spring  2525 , the linkage pin  2527  is moved within the actuating slot  2423  which pushes the clamp member  2511  against the spring  2513  to allow a probe to be inserted into the hole  1805  and through the probe aperture  2519 . When the probe is inserted and the button  1802  is released, the springs  2513  and  2525  cause the clamp member  2511  to bear against the probe to maintain the position of the probe within the hole  1805 . 
     A threaded bolt  2531  of the clamp knob  1804  is threaded into a bolt aperture  2509  of the housing  2501  to secure the adjustable stop  1803  to an insertion handle. 
     System calibration may be accomplished during manufacturing, after distribution, or immediately preceding implant implantation. The calibration step is analogous to registration in computer assisted surgery. Calibration may be needed for different reasons. For example, sensor calibration may be needed to correct for manufacturing tolerances. The system may be designed based upon a computer-aided-design model, and calibration is used to accurately place the sensors relative to one another. The processor or the control unit may include software to generate X, Y, Z, pitch, yaw, and roll offset values to locate the sensors in a global coordinate system or simply placement relative to one another. The system may be manufactured and calibrated during manufacturing and assigned a unique identifier, such as a serial number, color code, bar code, or RFID tag. If the system needs to be re-calibrated, the unique identifier may be used to retrieve the offset values, either locally or over a network. Further, the unique identifier may be used to retrieve other data, such as the size of the intramedullary nail or the length of the intramedullary nail and/or the probe. 
     The systems for identifying a landmark may be used for other purposes beyond targeting blind screw holes of an implanted intramedullary nail. These include, but are not limited to, targeting blocking screws and aligning guide pins. In one procedure, blocking (poller) screws can be inserted into the bone directly outside and tangent to the nail or rod. Targets are shown as two lines on the screen on opposing sides of the nail, such as anterior-posterior or medial-lateral, and offset from the nail at a distance, for example, 2.5 mm. The surgeon aligns the landmark identifier to one of the lines as determined by anatomical side where he or she wishes to place the blocking screw. Other symbols or indicia such as dots, bull&#39;s-eyes or combinations thereof can be used as targets shown on the screen. For this application, devices that are insertable in the medullary canal and instrumented with a sensor or sensors can be used as a means to target blocking screws, including but not limited to, a probe, a reducer or an awl. The depicted systems for identifying a landmark can also be used to align or center a guide pin in both A-P and M-L planes for placement of a lag screw in the proximal portion of a femoral nail. An exemplary implementation of this system may include a sensor placed with known orientation and location relative to and in the insertion handle and/or drill guide and/or alignment jig which is removably attached to the proximal portion of the femoral nail. 
     While  FIG. 1  illustrates a pocket for affixing the first sensor to the implant, other structure and/or methods may be used to affix these items together. For example, probes of varying length may be used to place the first sensors in the appropriate position as illustrated in  FIG. 42 . The adjustable stops  1801 ,  1803  of  FIGS. 41-42  may be used to precisely position the sensor  2126  in the implant  30 . 
     While only certain implementations have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.