Abstract:
Embodiments of the present invention include products and methods for reducing fractures with the aid of image guidance. In one embodiment, products and methods are directed to reduction for the placement of an intramedullary nail.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to U.S. Provisional Application No. 60/355,886 entitled “Image-Guided Fracture Reduction” filed on Feb. 11, 2002, the contents of which are incorporated herein by reference. 

   TECHNICAL FIELD 
   The invention is directed to treating skeletal fractures. More specifically, products and methods for reducing fractures with the aid of image guidance are disclosed. 
   BACKGROUND OF THE INVENTION 
   Fracture fixation of long bones such as the femur, tibia, humerus, fibula, or other long bones is challenging because of the difficulty of properly aligning and then securing fractured bone segments in place to allow the bone to heal. One very effective means of securing such fractures is intramedullary nailing. Intramedullary nailing is well-known in the art and essentially entails aligning two or more segments of bone that result from a fracture about a rod or nail that fits down the medullary canal of the fractured bone. Various techniques for intramedullary nailing are discussed in U.S. Pats. Nos. 5,951,561 and 6,010,506, which are hereby incorporated by reference. 
   Whether fixation is by intramedullary nailing or by some other means, the repositioning of the segments of a long bone fracture (fracture reduction) is one of the most challenging aspects of fracture fixation. The contraction of soft tissue subsequent to a fracture tends to shorten the fractured limb and place the fractured segments of the bone out of alignment relative to each other. Repositioning these segments to restore anatomic alignment can be very challenging. 
   One technique for realigning fractured bones comprises the use of a fracture table to first distract a limb back to its original length. When a patient is positioned and secured on the fracture table, a surgeon may then manipulate segments laterally to realign the segments. However, fracture tables are expensive and many surgeons do not use them due to cost, availability, or the limitations of having a patient fixed in one position. Multiple intra-operative x-ray or fluoroscopic images may need to be taken to assure alignment of the segments in all planes. Additionally, a fracture may continue to shift out of alignment as fixation is applied. 
   Another method for fracture reduction includes the attachment of an external distraction device to the bone via bone pins that pass through soft tissue and attach to the bone. These types of devices allow a surgeon to turn a threaded knob or other actuator and pull a fracture apart. Once distracted, repositioning is then accomplished by manual, physical manipulation of the limb. Once again, multiple intra-operative x-ray or fluoroscopic images may be necessary to assure proper realignment of the segments, and segments may shift out of alignment as fixation is applied. 
   Another method for fracture repositioning or reduction is through the use of the Internal Fracture Reduction Device manufactured by Smith &amp; Nephew, Inc. This device is inserted into a portion of the fractured long bone and allows manipulation of a segment of the fractured bone. However, such a device must be inserted over a guide rod that has already been placed through the medullary canal of all fractured bone segments. Therefore, placement of the guide rod first requires at least adequate fracture alignment to place the guide rod through the realigned medullary canal. 
   Smith &amp; Nephew, Inc. also manufactures a reducer for use with its TRIGEN® brand intramedullary nailing system. As shown in  FIG. 1 , the reducer  100  is an elongated, cannulated tube  101  with a connector  102  for attaching a handle  103 . The inner diameter of the tube  101  is large enough to accommodate the passage of a guide rod (not shown). The outer diameter of the tube  101  is small enough to be inserted into a long bone without reaming the bone. The tube  101  is typically formed to the same shape as the intramedullary nail that will subsequently be implanted in the bone. The distal tip of the tube  101  includes a finger  104  that is bent up slightly. This finger  104  serves several purposes. 
   First, the finger  104  can be used to deflect a guide rod as the end of the guide rod passes the end of the tube  101 . Specifically, the guide rod may be deflected in a desired direction by rotating the reducer  100  such that the distal end of the finger  104  is pointed in the desired direction. Second, the finger  104  places resistance on a guide rod that passes into or through the finger  104 , thereby holding the guide rod in position relative to the reducer  100  by friction. Third, the curved tip of the finger  104  allows the reducer  100  to be pushed smoothly through the medullary canal of a proximal segment and into a distal segment. While the TRIGEN® system reducer has significant advantages, multiple intra-operative x-ray, fluoroscopic, or other such images must still be used to assure proper alignment of the segments in all planes as the reducer  100  is inserted. 
   Several manufactures currently produce image-guided surgical navigation systems that are used to assist in performing surgical procedures. The TREON™ and iON™ systems with FLUORONAV™ software manufactured by Medtronic Surgical Navigation Technologies, Inc. are examples of such systems. Systems and methods for accomplishing image-guided surgery are also disclosed in U.S. S No. 60/271,818 filed Feb. 27, 2001, entitled Image Guided System for Arthroplasty, which is incorporated herein by reference as are all documents incorporated by reference therein. Further image-guided surgery devices, systems, and methods are disclosed in a provisional application entitled SURGICAL NAVIGATION SYSTEMS AND PROCESSES, Application Serial No. 60/355,899, filed on Feb. 11, 2002, hereby incorporated by this reference. 
   The Medtronic systems use fluoroscopic imaging to capture anatomical characteristics and infrared cameras that detect certain targets placed in the surgical field to track instruments and anatomy. As used herein, an infrared camera can be any type of sensor or detector that is capable of sensing or detecting light of an infrared wavelength. Any number and orientation of so-called targets, fiducials, frames, markers, indicia, or any other desired location-assisting functionality (“references”) can be used as targets to be detected by an imaging system or sensor. Other imaging or data capture systems such as CT, MRI, visual, sonic, digitized modeling, traditional x-ray equipment, or any other effective system or technique which has the capacity to image bone or other desired structures or tissue in the body can be used. Such systems generally include transducer functionality for emitting energy or otherwise performing sensing or location of objects and anatomical structure, a processor, mass memory storage, input/output functionality to control and direct operation of the system, and at least one monitor or other visual output functionality for rendering images that may be constructed by the system, whether or not in combination with images obtained from the transducer in real time. 
   Such systems typically combine processes and functionality for obtaining, storing, manipulating and rendering images of internal body structure with functionality that senses, stores, manipulates and virtually renders representations of components or objects such as instrumentation, trial components, surgical tools and other objects. The systems can then generate and display representations of the objects in combination with images of the body structure or tissue. Such combination renderings can be created using real time imaging of the body structure or tissue, or the system can obtain appropriate imaging of such structure or tissue and later computer generate and display renderings of it. The Medtronic systems, for instance, require the use of references attached to the anatomy, typically in a rigid fashion, such as to bone structure. The system tracks movement of the reference in three dimensions and then generates images of the bone structure&#39;s corresponding motion and location. 
   The references on the anatomy and the instruments either emit or reflect infrared light that is then detected by an infrared camera. The references may be sensed actively or passively by infrared, visual, sound, magnetic, electromagnetic, x-ray, or any other desired technique. An active reference emits energy, and a passive reference merely reflects energy. In some embodiments, the references have at least three, but usually four, markers that are tracked by an infrared sensor to determine the orientation of the reference and thus the geometry of the instrument, implant component or other object to which the reference is attached. References have been attached to surgical and implant devices such as instrumentation, trial instruments, and the like. For example, references have been attached to probes, instruments for placing acetabular cups and trial implants, drill guides, and cutting blocks. 
   The Medtronic imaging systems allow references to be detected at the same time the fluoroscopy imaging is occurring. Therefore, the position and orientation of the references may be coordinated with the fluoroscope imaging. Then, after processing position and orientation data, the references may be used to track the position and orientation of anatomical features that were recorded fluoroscopically. Computer-generated images of instrumentation, components, or other structures that are fitted with references may be superimposed on the fluoroscopic images. The instrumentation, trial, implant or other structure or geometry can be displayed as 3-D models, outline models, or bone-implant interface surfaces. 
   Current systems and techniques do not provide for effective image-guided reduction of fractures. Improved products and methods would include structures and techniques for guiding a reducer through the medullary canals of two or more bone segments that have been created by a fracture of a bone. Improved products and methods would also provide for reduced numbers of x-ray, fluoroscopic, or other images, and would not necessitate pre-operative imaging or surgical procedures prior to the primary procedure. Further, improved products and methods would allow alignment of bone segments to occur using images of at least one of the bone segments in combination with images of one or more implements, instruments, trials, guide wires, nails, reducers and other surgically related items, which are properly positioned and oriented in the images relative to the bone segments. Further, improved products and methods would provide for updated monitoring of bone segment positions, and therefore, rapid alignment of bone segments. 
   SUMMARY 
   An embodiment according to certain aspects of the invention is a method of aligning segments of a fractured bone. The method involves attaching references to at least two segments of a fractured bone and to a reducer. The position and orientation of at least two of the references are recorded, and the position and orientation of one or more of the segments of the fractured bone and in some embodiments, the reducer, are recorded. Each of the respective segments or reducer is located relative to a respective reference. The reducer is inserted into a medullary canal of one of the segments, and the reducer is aligned with a representation of another of the segments. The reducer is then inserted into a medullary canal of that segment. 
   Another embodiment according to certain aspects of the invention is a method of enabling reduction of a fractured bone by virtually representing at least one fractured segment of the bone and virtually representing an instrument for aligning two or more segments. The position and orientation of a first segment of the bone is recorded and that first segment is tracked. The position and orientation of the instrument for aligning the segments is recorded and tracked as well. If alignment has been achieved such that the instrument may be engaged with the first segment and a second segment, an indication is provided to a user through a virtual representation. 
   Still another embodiment according to certain aspects of the invention is an instrument operable with an image-guided surgical navigation system for aligning fractured segments of a bone. The instrument may include at least an elongated body and a reference coupled to the elongated body for enabling the instrument to be located by the image-guided surgical navigation system. The reference may have a predefined physical relationship with the elongated body such that by observing the position and orientation of the reference relative to at least one of the fractured segments, the position and orientation of the elongated body relative to at least one of the fractured segments can be determined. 
   Yet another embodiment according to certain aspects of the invention is a system for enabling reduction of a fractured bone. The system is operable to virtually represent at least one fractured segment of the bone and virtually represent an instrument for aligning the at least one fractured segment. The system includes a first reference coupled to the at least one fractured segment, and a second reference coupled to the instrument. This embodiment includes a detector operable to collect position and orientation information regarding the at least one fractured segment and the instrument, and a data processing device operable to store position and orientation information about the at least one fractured segment and the instrument, and to calculate virtual positions of the at least one fractured segment and the instrument based upon inputs from the detector. An indicator device for notifying a user of the relative positions of the at least one fractured segment and the instrument is also provided. 
   Yet a further embodiment according to certain aspects of the invention includes methods, instruments, and systems as described above, wherein the instrument enabling reduction or alignment of a fractured bone is a flexible reducer. The flexible reducer may be an elongated body with an at least partially flexible portion having one or more location elements associated with the flexible body. The one or more location elements can be provided on the flexible portion in order to assist determining the physical relationship of at least certain parts of the flexible portion with respect to a reference, a bone segment, or the surgical table. The at least partially flexible portion may further be provided with a feature or features that impart at least partial rigidity to the reducer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a prior art reducer. 
       FIG. 2  is a perspective view of an embodiment of a navigated reducer and a segment of bone with an attached reference according to certain aspects of the invention. 
       FIG. 3  is a perspective view of a segment of bone with an attached reference according to certain aspects of the invention, where the segment of bone contains an intramedullary nail. 
       FIG. 4  is an elevation view of an embodiment of a navigated reducer according to certain aspects of the invention. 
       FIG. 5  is a side elevation view of the navigated reducer of  FIG. 4 . 
       FIG. 6  is an elevation view of a bracket according to certain aspects of the invention used in embodiments of the navigated reducer. 
       FIG. 7  is a side elevation view of the bracket of  FIG. 6 . 
       FIG. 8  is a cross-section view taken though the bracket of  FIG. 7 . 
       FIG. 9  is an enlarged elevation view of the proximal end of the navigated reducer of  FIG. 4 . 
       FIG. 10  is a side elevation view of the proximal end of  FIG. 9 . 
       FIG. 11  is a cross-section view taken through the proximal end of  FIG. 10 . 
       FIG. 12  is an elevation view of an embodiment of a navigated reducer according to certain aspects of the invention being used to reduce a fractured humerus bone. 
       FIG. 13  is an elevation view of an embodiment of a navigated reducer according to certain aspects of the invention being used to reduce a subtrocanteric fracture in a femur. 
       FIG. 14  is an elevation view of an embodiment of a navigated reducer according to certain aspects of the invention with a curved elongated body and an adjustable bracket for supporting a reference. 
       FIG. 15  is an elevation view of an embodiment of a navigated reducer according to certain aspects of the invention with a straight elongated body and an adjustable bracket for supporting a reference. 
       FIG. 16  is an elevation view of an embodiment of a navigated reducer according to certain aspects of the invention with a straight elongated body and a fixed bracket for supporting a reference. 
       FIG. 17  is a perspective view of an active navigated drill guide and a segment of bone with an attached reference according to certain aspects of the invention. 
       FIG. 18  is an elevational view of various fingers or ends of embodiments of a navigated reducer according to certain aspects of the invention. 
       FIG. 19  is a perspective view of a segment of bone with an attached reference according to certain aspects of the invention. 
       FIG. 20  is a perspective view of a segment of bone with an attached reference according to certain aspects of the invention, where the segment of bone contains an intramedullary nail. 
       FIG. 21  is a plan view of an at least partially flexible reducer according to certain aspects of the invention with location elements positioned on the elongated body. 
       FIG. 22  is a top view of  FIG. 6 . 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows an instrument  10  according to certain aspects of the invention operable with an image-guided surgical navigation system. As described above, an image-guided surgical navigation system can be any of a variety of systems that capture anatomical characteristics and/or other references connected to the body and/or other surgical devices and/or other structures associated with a reference. Such a system then tracks parts of the body and the surgical devices relative to one another. Generally, reference to a system as “image-guided” means that the system produces images by which surgical navigation information is conveyed to the user. For example, a computer display showing virtual representations of an instrument and its relationship with a bone is considered one example of an image-guided system. As shown in  FIG. 2 , the position and orientation of instrument  10  are being recorded by placing finger  14  on a portion of a bone reference  15 . As shown, the bone reference  15  is connected to an upper proportion  16  of a femur. The instrument  10  may be used to align fractured segments of a bone such as the upper portion  16  of a femur and a lower portion  20  of the femur shown in  FIG. 3 . 
   As illustrated in  FIG. 2 , instrument  10  includes an elongated body  11  and a reference  12  that is coupled to the elongated body  11 .  FIG. 4  shows elongated body  11  in one embodiment of the invention. As shown, the elongated body  11  is tubular or in similar terminology, cannulated. In other embodiments, the elongated body  11  may be solid.  FIGS. 2 and 12  illustrate an elongated body  11  greater than half the greatest length of the fractured bone  25 . In other embodiments such as shown in  FIG. 13 , the elongated body  11  may be less than or equal to half the greatest length of the fractured femoral bone  26 . The condition of being less than or equal to half the greatest length is not limited to association with the femur, but can be with regard to any bone. The elongated body  11  may be curved as shown in  FIG. 14  or substantially straight as shown in  FIGS. 15 and 16 . The elongated body  11  shown in  FIGS. 2 and 4  has substantially the same curvature as an implant  21  (for example, the implant shown in  FIG. 3 ). Implant  21  may be used to fix bone segments such as upper portion  16  and lower portion  20  in place. The intramedullary reduction device may be bent to follow the shape of whatever implant is used for fixation: IM nails, IM rods, IM hip screws, etc. This has the benefit of positioning the segments in the same position as the subsequent fixation device. However, less exact bends in the elongated body  11  may also be beneficial. 
   The reference  12  enables the instrument  10  to be located by an image-guided surgical navigation system. As illustrated in  FIG. 2 , reference  12  is coupled to the elongated body  11  in a predefined physical relationship.  FIGS. 4–11  show one embodiment of a structure for coupling the reference  12  to the elongated body  11 . As illustrated, a bracket  30  is rigidly affixed near a proximal end  17  of the elongated body  11 . As best seen in  FIGS. 6–8 , a dovetail mount  31  is located at one end of the bracket  30 . The dovetail mount  31  is designed to be received by a reference  12  that has a mating dovetail opening (not shown). 
   Bracket  30  is shown as adapted to slide over proximal end  17  of elongated body  11 . Although not shown, it is understood that the bracket may alternatively be a clamp that opens and closes to secure elongated body  11  or any other attachment device or structure suitable for attaching components to each other. Those skilled in the art will understand that any member that can rigidly attach reference  12  to instrument  10  is considered a “bracket” within the scope of this invention. 
   Another embodiment of this invention provides a reference  12  having an integral attachment structure (not shown). Attachment structure may be a bracket integrally formed with reference  12  or any other connection element that will achieve securement of reference  12  to instrument  10 . 
     FIGS. 14 and 15  show articulating brackets  32  releasably movably coupled with elongated body  11 . With such a feature, the instrument  10  can be effectively used on either side of the patient by moving the articulating bracket  32  among two or more predetermined positions. In some embodiments, the instrument  10  is releasably movable between positions located at ninety degree intervals around the elongated body. In other words, viewing elongated body from one end, instrument  10  may be positioned at a first ninety degree position, a second ninety degree position, a third ninety degree position, or a fourth ninety degree position. 
   It is advantageous in some embodiments of the invention to limit the number of positions to which the articulating bracket  32 , and thereby the reference  12 , may be positioned. This is because a predefined physical relationship must be maintained between the elongated body  11  and the reference  12 . By limiting the number of positions, the number of predefined relationships may be more easily defined and tracked. 
     FIG. 5  illustrates an embodiment of the bracket  30 A that is asymmetrically coupled to the elongated body  11 . Consequently, if a reference  12  were coupled to the bracket  30 A, the reference  12  would also be asymmetrically fixed relative to the elongated body  11 . Such an arrangement may be beneficial to more effective use on a particular side of a patient and is sometimes preferred. In other instances, a reference may be symmetrically coupled to the elongated body  11 . Preferences are typically dictated by the ability of the image-guided surgical navigation system to effectively detect a reference in a particular application. In some embodiments, the system&#39;s detector is a line-of-site device. 
   The reference  12  may also include energy-reflecting surfaces  13  that are detectable by a sensor.  FIG. 2  shows four such energy-reflecting surfaces  13  mounted on the reference  12 . As illustrated, the energy-reflecting surfaces  13  reflect energy in at least the visible and infrared ranges. However, as discussed in the background section above, various types of energy detectors may be employed. Energy-reflecting surface  13  is considered a passive device because it does not internally generate or convert energy to emit.  FIG. 17  illustrates an active energy emitting component  23  that is incorporated into an active reference  22 . Note that the active reference  22  includes a wire  24  through which electricity is supplied to the active energy emitting components  23 . As shown, there are four active energy emitting components  23 . As with the passive device, the active energy emitting components  23  may be operable with various types of energy detectors. 
   In some embodiments of the invention, the instrument  10  may include a handle  40  (shown in  FIGS. 12 ,  13 ,  21 ) connected at proximal end  17 . Such a handle  40  would be useful in manipulating the instrument  10  during a surgical procedure. The handle  30  may or may not be removable from instrument  10 . If handle  40  is not removable, bracket  30  may be a clamp or other device or structure suitable for attaching components to each other. 
   Some embodiments of the invention also include a finger  14 , shown for example in  FIG. 4 .  FIG. 18  shows a variety of finger shapes that may also be advantageous in various embodiments of the invention, although different finger shapes may be preferred for various procedures. Note that each of these shapes may be placed on the end of a solid or cannulated elongated body and may themselves be solid or cannulated. 
   The invention may also be embodied in a system for enabling reduction of a fractured bone. The system is operable to virtually represent at least one fractured segment of the bone and virtually represent an instrument for aligning the at least one fractured segment. The system includes a first reference coupled to the at least one fractured segment, and a second reference coupled to the instrument. The first reference may be coupled to a bone segment through which the instrument is inserted. In this case, position and orientation of another segment of the bone would have to be determined as well, which could be accomplished in any technically effective way. 
   Alternatively, the first segment could be coupled to a segment of bone toward which the instrument was being directed. In any case, the system also includes a detector operable to collect position and orientation information regarding the at least one fractured segment and the instrument. As discussed in the background section above, the detector could be an infrared camera, visual camera, or any of a variety of sensors capable of detecting any kind of reference or characteristic. The system also includes a data processing device operable to store position and orientation information about one or more fractured segments and the instrument. The data processing device calculates virtual positions of the at least one fractured segment and the instrument based upon inputs from the detector. Such calculations could involve matrix transformations, table look-up functionality, or any other operation effective in calculating the respective virtual positions. An indicator device for notifying a user of the relative positions of the at least on fractured segment and the instrument is also provided. Such an indicator could be a visual cue on a computer screen such as color changes or alignment of articulating lines, sounds, flashes of light, or any device for showing a changeable condition, or some combination of any of these. 
   Another embodiment of the invention is a method of aligning segments of a fractured bone. As shown in  FIG. 3 , one method includes attaching a first reference, such as a distal reference  18 , to a first segment of a fractured bone, such as a lower portion  20 . The position and orientation of distal reference  18  may then be recorded relative to a first datum. As used herein, the term “recording” includes without limitation capturing or storing in computer memory or on a tangible medium such as film. Any such acquisition of information associated with position or orientation, regardless of how transiently maintained in a system, medium, or component is within the definition of recording as used herein. In some embodiments of the invention, recording may include the use of an infrared camera that registers the positions of energy-reflecting surfaces  13 . 
   Alternatively, a reference may not be coupled with a segment of bone, but may be attached to a probe. Such a probe may be recorded at a predetermined anatomical position and orientation. Therefore, by knowing the position of the reference attached to the probe, and the probe&#39;s position and orientation on the anatomy, the position of the anatomy can be calculated. In either case, a position and orientation of the first segment of the bone relative to a second datum is recorded. Such a recording may be accomplished by capturing fluoroscopic images of the first segment. As discussed in the background section, the imaging may be through processes other than fluoroscopic imaging, such as CT, MRI, or other effective technologies. The first datum may be the same as the second datum, or information relating the first datum and the second datum may be stored such that transforms relating their relative positions may be calculated. As a result, the first segment will be located relative to the first reference. 
   The term “datum” as used herein is generally a coordinate system to which three-dimensional association can be made. As such, a number of datums can be defined and then associated to one another by use of three-dimensional transforms, matrix calculations, or the like. Such calculations are well-suited to implementation on computing devices. Similarly, objects being tracked can be positioned and oriented relative to a single datum. In any case, to effectively track objects&#39; positions and orientations, association among the objects must be established and maintained. A strength of the current system is that sensor or camera positions and orientations and patient and instrument positions and orientations may change relative to one another, but through the tracking that embodiments of the invention provide, accurate location and bone segment alignment can be accomplished. 
   As shown in  FIG. 2 , a second reference, such as bone reference  15 , may be attached to a second segment, such as upper portion  16 . As with the first segment and reference, the positions and orientations of the second reference and the second segment are recorded relative to respective third and fourth datums, and the second segment is located relative to the second reference. 
   A third reference is attached to an instrument  10 , such as a reducer. As described above, the reducer is operable to align segments of a fractured bone through the medullary canal of the segments. The term “reducer” as used herein may refer more generally to any instrument used to assist with the alignment of bones. As with the first and second references, a position and orientation of the third reference relative to a fifth datum is recorded. In the case of a reducer or other instrument, locating the reducer relative to the third reference is simplified because there is a predetermined relationship between the reducer and the third reference. As discussed in association with the bracket  30 , a single or at least finite number of predetermined relationships between portions of the instrument and the associated reference may be defined. Given a predetermined setting of the instrument relative to the reference, tracking of the reference is effective to track the instrument. Recording of the third reference position and orientation may be accomplished inter-operatively or prior to the beginning of an operation. 
   Once all of the references, segments, and instrument (or instruments) have been located, they may all be continuously or intermittently tracked without the use of fluoroscopy for as long as desired. As used herein, “continuously” shall mean at a rate that appears substantially continuous to a user, but could include tracking accomplished at a standard electronic sampling rate such as a rate greater than one sample per second. Typically, this tracking is accomplished by use of a computer system that is interfaced with an infrared camera or other device, the computer also calculating transforms regarding each datum and its relationship to each other datum. 
   Insertion of the instrument  10  may be accomplished prior to, during, or after the process of recording and locating described above. With each of the first segment, the second segment, and the reducer being tracked, the reducer can be aligned with a representation of the second segment. For instance, a surgeon could hold and manipulate a first segment of fractured bone with an inserted reducer while observing a representation of the second segment on a computer screen. The image on the computer screen may also include representations of other bone segments or instruments, such as the reducer. When an indication is received that alignment has been achieved, the surgeon inserts the reducer into the medullary canal of the second segment. The upper portion  16  of a femur shown in  FIG. 2  and the lower portion  20  of a femur shown in  FIG. 3  are merely examples of the first and second segments. 
   As previously discussed, the fractured bone need not be a femur. Additionally, the first and second segments may be either the lower or upper portions of bone, depending upon surgeon preference. In many orthopedic procedures, entry can be made from two or more possible approaches. 
   In some embodiments of the invention, a representation of alignment may include only a representation that the first segment and the second segment, each of which is being tracked, are aligned. In other embodiments, the key to a representation of alignment may be the reducer that is being tracked. 
   In some embodiments of the invention, only two of a first segment, a second segment, and an instrument may need to be recorded, located, and tracked. For example, if two segments are being tracked, alignment of those segments could be indicated to the user. Given the fact that the user knows that the reducer is located in the medullary canal of one of the segments, the user would know that the reducer could be pushed into the medullary canal of the other segment. Similarly, if only the reducer and the segment into which the reducer is to be inserted second are being tracked, the locations of only that second segment and the reducer could be represented to the user. In this embodiment, the reducer is located in the medullary canal of the other segment. Therefore, by aligning the reducer with the segment into which the reducer is to be inserted second, the user has adequate information to accurately complete the procedure. 
   In other embodiments and for some procedures, an at least partially flexible reducer  50 , as shown in  FIG. 21 , may be beneficial. For instance, a surgeon may desire to use a flexible reducer if the bone fracture to be aligned or reduced is so misaligned that a rigid reducer is not workable or would be particularly difficult to use. For example, two bone segments of a fracture may be so offset from one another that a rigid reducer would not appropriately engage the second segment. In these instances, the at least partially flexible reducer  50  of the present invention could be used. (For the purposes of this document, “at least partially flexible” and “flexible” mean capable of being even slightly flexed or bent, turned, bowed, or twisted, without breaking; or pliable; or yielding to pressure, whether strong pressure or slight pressure.) The flexible reducer  50  is at least partially flexible to allow the surgeon to more easily manipulate the flexible reducer  50  in order to properly guide it into the second segment. It should be understood that there may be other instances in which a flexible reduced  50  may be preferred. 
   Flexible reducer  50  according to the particular embodiment shown in  FIG. 21  features an at least partially flexible elongated portion or shaft  52 . The at least partial flexibility may be provided by a shaft that is hollow, cannulated, or solid. The shaft may have a spiral or helical configuration, a laser cut shaft, a shaft of a material that becomes flexible when subjected to heat (for example, nitinol), a shaft of a thin material that permits flexibility, a shaft with a plurality of flexible elements joined by a connection, a shaft having a series of inter-engaged links, a shaft with a plurality of slots (provided in any configuration) cut at an angle relative to the shaft, a plastic tube (or any other material that provides at least partial elasticity), or any other design that provides a reducer of a flexible nature. Examples of flexible shafts are provided in U.S. Pat. No. 6,053,922, which is incorporated herein by this reference. 
   Once flexible reducer  50  has been positioned with respect to both bone segments, the surgeon may wish to impart at least partial rigidity to the flexible reducer  50  in order to more properly align the bone segments. In this instance, flexible reducer  50  can be provided with a separate rigid member (not shown), a feature or features on the flexible reducer  50  itself that imparts rigidity to the flexible reducer (also not shown), or any structure or mechanism that imparts at least partial rigidity to reducer  50 . 
   For example, the flexible reducer  50  may be provided with a rigid member with an outer diameter smaller than the inner diameter of the flexible reducer  50 , such that inserting the rigid member through the flexible reducer adds rigidity at the desired point in the procedure. Alternatively, the flexible reducer  50  itself can be provided with a cable or wire disposed through the flexible reducer  50  such that when the cable or wire is pulled taut, the flexible shaft  52  is forced to undertake at least partial rigidity. Flexible reducer  50  may alternatively be provided with a trigger, such that once the trigger is activated, the flexible portions become rigid. The flexible portions may be made rigid by a magnetic force, by a mechanical force, or any other mechanism that imparts at least partial rigidity to the flexible reducer  50  at a specified time during the surgery. It should be understood that any feature that provides an at least partially flexible reducer  50  with at least partial rigidity is considered a feature that imparts at least partial rigidity to the reducer within the scope of this invention. 
   One challenge presented with the use of a flexible reducer  50  is the fact that, by its very nature, it is flexible, and thus, does not retain a rigid position from tip  54  to end  56  in relation to reference  12 . This presents a challenge to the use of the image-guided systems and methods described herein, because the flexible elongated portion  52  will not necessarily remain in a fixed position with respect to the reference  12  (or any other reference point being used, such as a bone segment, another instrument, a surgical table, etc.) in order to provide the surgeon with accurate cues about its physical position. Thus, there is also a need to provide a way to determine the position of the flexible elongated portion  52  when it is flexed in a particular direction. 
   Flexible reducer  50  is consequently provided with one or more location elements  75 . One or more location elements  75  assist the determination of at least portions of the physical relationship of the flexible elongated portion  52  with respect to reference  12 . A location element  75  may be provided at or near the tip  54  of flexible elongated portion  52 , at or near the middle of flexible elongated portion  52 , at multiple positions along the flexible elongated portion  52 , or any combination of these positions. The location elements may be spaced as close together or as far apart as necessary. The more location elements  75  provided, the more trackability is provided to flexible elongated portion  52 . 
   Location element  75  may be any component or device that permits the physical position of flexible elongated portion  52  to be sensed, detected, imaged, or mapped with respect to reference  12 . For example, location elements  75  may be sensed actively or passively by one or more of the following methods: infrared, visual, reflective, sound, ultrasound, radio waves, mechanical waves, magnetic, electromagnetic, electrical, x-ray, GPS systems or chips, transponder, transducer, or any other desired technique. This list is not intended to be inclusive, and any way in which the location of flexible elongated portion  52  can be relayed to a component that can track, sense, image, or map flexible elongated portion  52  for the surgeon to view is considered within the scope of this invention. It should be understood, however, that the flexible elongated portion  52  will be positioned within patient tissue in use, so the location method chosen should be able to sense location element  75  through various tissues, such as bone, muscle, blood, and skin. 
   Location elements  75  are preferably configured to sense, track, image, and map the physical position of reducer  50  in any plane, location, and/or orientation. In other words, in addition to sensing and tracking the medial-to-lateral movement of flexible reducer  50 , location elements  75  are also preferably adapted to sense and track anterior-to-posterior movement. 
   Location elements may be provided in any configuration or any shape. It is possible for location elements  75  to sense 2-dimensional movement for a rough view of the reducer&#39;s location and orientation. In other aspects of the invention, the location elements  75  sense 3-dimensional movement and provide a finer ability to sense and track the location and orientation of reducer  50 . Location elements  75  may be provided in any shape or configuration, such as the square-like elements  75  shown in  FIG. 21 , oval or round-like elements, cross-shaped elements, band-shaped elements, indented elements, bead-shaped elements, and so forth. 
   Location elements may be located along only one side of flexible elongated portion  52 , wrapped around elongated portion  52 , positioned in specific increments from one another, or scattered in various, unequal positions about elongated portion  52 . As previously mentioned, embodiments according to various aspects of this invention may include only a single location element  75 . 
   A single location element  75  may be used to track and sense the location and orientation of elongated portion  52  with respect to reference  12 . To the extent that any other reference point is being used, such as another instrument, a bone segment, or another reference point, it is preferred that two or more location elements  75  be provided. 
   Location elements  75  may operate in conjunction with systems which are preferably connected to other systems according to various aspects of the invention which sense and track references  12 , body portions, instruments, components of other devices, and so forth. 
   Embodiments of the invention are directed toward enabling reduction of a fractured bone by virtually representing at least one fractured segment of the bone and virtually representing an instrument for aligning two or more segments of bone. As described above, positions and orientations of a segment of bone and an instrument may be recorded and tracked in three-dimensional space with the use of cameras or sensors, imaging devices, and a digital computer. Then, through the use of a sound, visualization, or other stimulation, an indication that alignment has been achieved is provided to a user. Alternatively or in addition, indications regarding the progress of alignment may be provided to the user. “Tracking” as defined for use in this embodiment can include both detecting distinguishing characteristics, such as references or instrument configurations, and processing information regarding changes in position and orientation. 
   Therefore, embodiments of the invention provide for the location and tracking of bone segments and instruments such that the instruments may be aligned to assist with fixation or therapy. This is accomplished with reduced numbers of x-ray, fluoroscopic, and other such energy-intense imaging devices. There is no requirement for pre-operative imaging or any surgical procedures prior to the primary procedure. With various embodiments of the invention, continuous or nearly continuous monitoring of bone segment and instrument positions is accomplished. Therefore, rapid alignment of bone segments and instruments is facilitated using images of at least one of the bone segments in combination with images of one or more implements, instruments, trials, guide wires, nails, reducers, other surgically related items, or other bone segments which are properly positioned and oriented in the images.