Patent Publication Number: US-9848922-B2

Title: Systems and methods for performing spine surgery

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. patent application claiming the benefit of priority from commonly owned and U.S. Provisional Application Ser. No. 61/888,990 filed on Oct. 9, 2013 and entitled “Systems and Methods for Performing Spine Surgery”, the complete disclosure of which is hereby expressly incorporated by reference into this application as if set forth in its entirety herein. 
    
    
     FIELD 
     The present application pertains to spine surgery. More particularly, the present application pertains to systems and methods related to the planning, design, formation, and implantation of spinal implants. 
     BACKGROUND 
     The spinal column is a highly complex system of bones and connective tissues that provide support for the body and protect the delicate spinal cord and nerves. The spinal column includes a series of vertebral bodies stacked atop one another, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord is located behind the vertebral bodies. The spine has a natural curvature (i.e., lordosis in the lumbar and cervical regions and kyphosis in the thoracic region) such that the endplates of the upper and lower vertebrae are inclined towards one another. 
     There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), excess kyphosis (abnormal forward curvature of the spine), excess lordosis (abnormal backward curvature of the spine), spondylolisthesis (forward displacement of one vertebra over another), and other disorders caused by abnormalities, disease, or trauma (such as ruptured or slipped discs, degenerative disc disease, fractured vertebrae, and the like). Patients that suffer from such conditions often experience extreme and debilitating pain, as well as diminished nerve function. Posterior fixation for spinal fusions, decompression, deformity, and other reconstructions are performed to treat these patients. The aim of posterior fixation in lumbar, thoracic, and cervical procedures is to stabilize the spinal segments, correct multi-axis alignment, and aid in optimizing the long-term health of the spinal cord and nerves. 
     Screws, hooks, and rods are devices used to stabilize the spine during a spinal fixation procedure. Such procedures often require the instrumentation of many bony elements. The devices, for example rods, can be extremely challenging to design and implant into the patient. Spinal rods are usually formed of stainless steel, titanium, cobalt chrome, or other similarly hard metal, and as such are difficult to bend without some sort of leverage-based bender. Moreover, a spinal rod needs to be oriented in six degrees of freedom to compensate for the anatomical structure of a patient&#39;s spine as well as the attachment points (screws, hooks) for securing the rod to the vertebrae. Additionally, the physiological problem being treated as well as the physician&#39;s preferences will determine the exact configuration necessary. Accordingly, the size, length, and particular bends of the spinal rod depends on the size, number, and position of each vertebrae to be constrained, the spatial relationship amongst vertebrae, as well as the screws and hooks used to hold the rods attached to the vertebrae. 
     The bending of a spinal rod can be accomplished by a number of methods. The most widely used method is a three-point bender called a French Bender. The French bender is a pliers-like device that is manually operated to place one or more bends in a rod. The French bender requires both handles to operate and provides leverage based on the length of the handle. The use of the French bender requires a high degree of physician skill because the determination of the location, angle, and rotation of bends is often subjective and can be difficult to correlate to a patient&#39;s anatomy. Other methods of bending a rod to fit a screw and/or hook construct include the use of an in-situ rod bender and a keyhole bender. However, all of these methods can be subjective, iterative, and are often referred to as an “art.” As such, rod bending and reduction activities can be a time consuming and potentially frustrating step in the finalization of a complex and/or long spinal construct. Increased time in the operating room to achieve optimum bending can be costly to the patient and increase the chance of the morbidity. When rod bending is performed poorly, the rod can preload the construct and increase the chance of failure of the fixation system. The bending and re-bending involved can also promote metal fatigue and the creation of stress risers in the rod. 
     Efforts directed to computer-aided design or shaping of spinal rods have been largely unsuccessful due to the lack of bending devices as well as lack of understanding of all of the issues involved in bending surgical devices. U.S. Pat. No. 7,957,831, issued Jun. 7, 2011 to Isaacs, describes a rod bending system which includes a spatial measurement sub-system with a digitizer to obtain the three dimensional location of surgical implants (screws, hooks), software to convert the implant locations to a series of bend instructions, and a mechanical rod bender used to execute the bend instructions such that the rod will be bent precisely to custom fit within each of the screws. This is advantageous because it provides quantifiable rod bending steps that are customized to each patient&#39;s anatomy enabling surgeons to create custom-fit rods on the first pass, thereby increasing the speed and efficiency of rod bending, particularly in complex cases. This, in turn, reduces the morbidity and cost associated with such procedures. However, a need still exists for improved rod bending systems that allow for curvature and deformity correction in fixation procedures, provide the user with more rod bending options, and accommodate more of the user&#39;s clinical preferences including the ability to determine the spatial orientation of the tip of the rod and the tip of the rod pusher relative to one another. 
     SUMMARY 
     The present invention includes a system and methods for rod bending that enable the user (e.g., surgeon) to customize screw-based rod bend instructions to suit the desired correction of a patient&#39;s spinal condition. According to a broad aspect, the present invention includes a spatial tracking system for obtaining the three-dimensional position information of surgical implants, a processing system with software to convert the implant locations to a series of bend instructions based on a desired correction, and a mechanical rod bender for bending a surgical linking device to achieve the desired spinal correction. 
     According to another aspect of the present invention, the spatial tracking system includes an infrared (IR) position sensor and at least one IR-reflective tracking array attached to a digitizer pointer used to digitize the surgical implant location. The spatial tracking system is communicatively linked to the processing system such that the processing system may utilize the spatial position information to generate bend instructions. 
     According to another aspect of the present invention, the processing system is programmed to generate bend instructions based on one or more surgeon-prescribed clinical objectives. For example, the processing system may be programmed to create a custom bend, adjust one or more points to which the rod will be bent to, suggest a pre-bent rod option, provide spinal correction in the sagittal plane, provide spinal correction in the coronal plane, and provide correction to achieve global spinal balance, and as well as perform a plurality of predetermined functions. The processing system is further configured to preview and display the results of these clinical objectives and/or predetermined functions to the user in a meaningful way. 
     According to another aspect of the invention, the spatial tracking system is configured to track the three-dimensional position of a spinal rod relative to the implanted surgical implants during rod insertion. For example, the spatial tracking system may include at least one IR-reflective tracking array attached to a rod inserter instrument and another attached to a screw guide rod pusher to continuously digitize the rod location in real time during insertion of the rod. The processing system is programmed to generate and display the real time three-dimensional location of the rod tip relative to the tip of the rod pusher and the implanted surgical implants. 
     According to another aspect of the invention, one or more surgical procedures may be performed using various embodiments of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a surgical procedure setup depicting the components of a surgical bending system, according to one embodiment; 
         FIG. 2  is a perspective view of one embodiment of a digitizer array in the closed position comprising part of the system of  FIG. 1 ; 
         FIG. 3  is an exploded perspective view of the digitizer array of  FIG. 2 ; 
         FIG. 4  is a perspective view of the digitizer array of  FIG. 2  in the open position; 
         FIG. 5  is a front view of one embodiment of a digitizer pointer assembly comprising part of the system of  FIG. 1 ; 
         FIG. 6  is a perspective view of various surgical pointers compatible with the digitizer array of  FIG. 2 ; 
         FIG. 7  is a partial perspective view of the offset pointer of  FIG. 6  in a collapsed position; 
         FIG. 8  is a partial exploded view of the offset pointer of  FIG. 6 ; 
         FIG. 9  is a partial perspective view of the offset pointer of  FIG. 6  in an extended position; 
         FIG. 10  is a flowchart depicting the steps of the spatial tracking algorithm according to one embodiment; 
         FIG. 11  is a flowchart depicting the rod bending workflow according to one embodiment; 
         FIG. 12  is a flowchart depicting the steps in generating a rod solution according to a first embodiment; 
         FIG. 13  is a flowchart depicting the steps in generating rod solution according to a second embodiment; 
         FIG. 14  is a flowchart depicting the steps in generating a rod solution according to a third embodiment; 
         FIG. 15  is a flowchart depicting the steps of the rod bending process according to a first embodiment; 
         FIG. 16  is a screen shot depicting an example setup screen of the system of  FIG. 1 ; 
         FIG. 17  is a screen shot depicting an example IR positioning sensor setup screen of the system of  FIG. 1 ; 
         FIG. 18  is a screen shot depicting an example screw location digitization screen during a first step in the Acquire Screws step of  FIG. 15 ; 
         FIG. 19  is a screen shot depicting an example screw location digitization screen during a second step in the Acquire Screws step of  FIG. 15 ; 
         FIG. 20  is a screen shot depicting an example screw digitization screen during a third step in the Acquire Screws step of  FIG. 15 ; 
         FIG. 21  is a screen shot depicting an example bend instructions screen in the Bend Instructions step of  FIG. 15 ; 
         FIG. 22  is a flowchart depicting the steps of the rod bending process according to a second embodiment; 
         FIG. 23  is a screen shot depicting an example Advanced Options menu screen of the system of  FIG. 1 ; 
         FIG. 24  is a screen shot illustrating a first example screen of an Adjust Points feature according to one embodiment; 
         FIG. 25  is a screen shot illustrating a second example screen of the Adjust Points feature of  FIG. 24 ; 
         FIG. 26  is a screen shot illustrating a third example screen of the Adjust Points feature of  FIG. 24 ; 
         FIG. 27  is a screen shot illustrating a first example screen of a Pre-Bent Preview feature according to one embodiment; 
         FIG. 28  is a screen shot illustrating a second example screen of the Pre-Bent Preview feature of  FIG. 27 ; 
         FIG. 29  is a screen shot illustrating a third example screen of the Pre-Bent Preview feature of  FIG. 27 ; 
         FIG. 30  is a screen shot illustrating a first example screen of a Sagittal Correction feature according to one embodiment; 
         FIG. 31  is a screen shot illustrating a second example screen of the Sagittal Correction feature according to the first embodiment; 
         FIG. 32  is a screen shot illustrating a first example screen of the Sagittal Correction feature according to a second embodiment; 
         FIG. 33  is a screen shot illustrating an additional example screen of the Sagittal Correction feature according to the first and/or second embodiment; 
         FIG. 34  is a screen shot illustrating a first example screen of the Coronal Correction feature according to a first embodiment; 
         FIG. 35  is a screen shot illustrating a second example screen of the Coronal Correction feature according to a first embodiment; 
         FIG. 36  is a screen shot illustrating a third example screen of the Coronal Correction feature according to the first embodiment; 
         FIG. 37  is a screen shot illustrating a fourth example screen of the Coronal Correction feature according to the first embodiment; 
         FIG. 38  is a screen shot illustrating a first example screen of the Coronal Correction feature according to a second embodiment; 
         FIG. 39  is a screen shot illustrating a second example screen of the Coronal Correction feature according to the second embodiment; 
         FIG. 40  is a screen shot illustrating a third example screen of the Coronal Correction feature according to the second embodiment; 
         FIG. 41  is a flowchart illustrating the steps of the Global Spinal Balance feature according to one embodiment; 
         FIG. 42  is a screen shot illustrating a first example screen of the Global Spinal Balance feature in pre-operative mode; 
         FIG. 43  is a screen shot illustrating a first example screen of the Global Spinal Balance feature in target mode; 
         FIG. 44  is a screen shot illustrating a second example screen of the Global Spinal Balance feature in target mode; 
         FIG. 45  is a screen shot illustrating a first example screen of the Global Spinal Balance feature in intraoperative mode; 
         FIG. 46  is a screen shot illustrating a second example screen of the Global Spinal Balance feature in intraoperative mode; 
         FIG. 47  is a screen shot illustrating a third example screen of the Global Spinal Balance feature in intraoperative mode; 
         FIG. 48  is a screen shot illustrating a fourth example screen of the Global Spinal Balance feature in intraoperative mode; 
         FIG. 49  is a screen shot illustrating a fifth example screen of the Global Spinal Balance feature in intraoperative mode; 
         FIG. 50  is a perspective view of one embodiment of a mechanical rod bender comprising part of the surgical bending system of  FIG. 1 ; 
         FIG. 51  is a perspective view of a lumbar spine illustrating example spinal fixation anchors with attached extension guides and an example rod pusher instrument in use during implantation of a two level fixation construct, according to one example; 
         FIG. 52  is a perspective view of the example rod pusher of  FIG. 51 ; 
         FIG. 53  is a plan view of an example rod inserter configured for use in the implantation procedure of  FIG. 51 ; 
         FIG. 54  is a screen shot illustrating a first example screen of the Rod Tracking feature according to one embodiment; and 
         FIG. 55  is a screen shot illustrating a second example screen of the Rod Tracking feature according to one embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in development of any such actual embodiment, numerous implantation-specific decisions must be made to achieve the developers&#39; specific goals such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The systems and methods disclosed herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination. 
     With reference now to  FIG. 1 , there is shown, by way of example, one embodiment of a surgical bending system  10  including a spatial tracking system  12  to obtain the location of one or more surgical implants  14 , a control unit  16  containing software to convert the implant locations to a series of bend instructions, and a bending device  18  to execute the bend instructions. 
     Preferably, the spatial tracking system  12  includes an IR sensor  20 , a digitizer pointer  23 , as well as other components including Host USB converter  21 . The spatial tracking system  12  is in communication with control unit  16 . The control unit  16  has spatial relation software and is communicatively linked to the display  32  so that information relevant to the surgical procedure may be conveyed to the user in a meaningful manner. By way of example, the relevant information includes, but is not limited to, spatial positioning data (e.g., translational data in the x, y, and z axes and orientation/rotational data R x , R y , and R z ) acquired by the IR sensor  20 . According to one or more embodiments, the system  10  comprises a neuromonitoring system communicatively linked to the spatial tracking system  12  via the control unit  16 . By way of example only, the neuromonitoring system may be the neuromonitoring system shown and described in U.S. Pat. No. 8,255,045, entitled “Neurophysiologic Monitoring System” and filed on Apr. 3, 2008, the entire contents of which are hereby incorporated by reference as if set forth fully herein. 
       FIGS. 2-9  depict the various components of one or more digitizer pointers  23  for use with the present invention.  FIGS. 2-4  detail an example IR-reflective tracking array  22  component of the digitizer pointer  23 . Array  22  includes a housing  34 , bilateral shutters  36 , and a plurality of IR-reflective spheres  38  arranged in a calculated manner at various locations on the array  22  such that their position information is selectively detectable by the IR sensor  20 . Housing  34  comprises a top housing  40 , bottom housing  42 , and a distal threaded aperture  56  configured to threadably receive the threaded end  78  of a stylus (e.g., stylus  24 ,  26 ,  28 , and/or  30 ). Top housing portion  40  is further comprised of upper portion  44 , underside  46 , and sides  48 . A plurality of sphere apertures  52  extend between upper portion  44  and underside  46  and are sized and dimensioned to receive reflective spheres  38  within recessed pockets  54 . Each side  48  includes cutout  50  sized and dimensioned to receive tongue  70 . Bottom housing  42  is comprised of a first face  58  and a second face  60 . The first face  58  includes nesting platforms  62  and bullet posts  64 . Each shutter  36  includes handle portion  66 , cover portion  68 , tongue  70 , interdigitating gear teeth  72 , and channel  74  for receiving bullet posts  64 . A spring  76  extends between the two shutters  36  and is held in place via spring posts  71 . 
     In an assembled state, each IR-reflective sphere  38  is nested on a platform  62 . Top housing  40  is placed over bottom housing  42  in a snap fit configuration such that each IR-reflective sphere  38  fits within a recessed pocket  54  within its respective sphere aperture  52 . According to one implementation, bilateral shutters  36  are positioned over the housing  34  with tongues  70  sliding into cutouts  50  such that each shutter cover  68  obscures exactly one half of the IR-reflective sphere  38  (for example, the middle IR-reflective sphere  38 ) as depicted in  FIG. 2 . 
     As depicted in  FIG. 5 , the IR-reflective tracking array  22  mates with one or more surgical objects (for example styluses  24 ,  26 ,  28 ,  30 ). Each stylus  24 ,  26 ,  28 ,  30  includes a threaded proximal end  78  for mating with the threaded distal aperture  56  of the IR-reflective tracking array  22 , elongate shaft  80 , and shaped distal tip  82 . Shaped distal tip  82  may be any shape that is complimentary to, and fits securely within, the shape of a particular screw head. For example,  FIG. 6  shows styluses  24 ,  26 ,  28 , and  30  each with a different shaped distal tip designed to mate with different open screw systems, minimally-invasive screw systems, and closed tulip, iliac, and offset connector systems. The distal tip  82  is preferably inserted into each screw while orienting the digitizer pointer coaxial to that screw (or other fixation device). 
     According to some implementations (for example, the implementations shown with respect to styluses  24 ,  26 , and  28 ), the length of the elongate shaft  80  is fixed relative to the array  22  such that all digitized points are a consistent length from the geometry of the IR-reflective markers  38  and position information may be obtained from this relationship. According to other implementations (for example, the implementation shown with respect to offset pointer  30 ), the length of the elongate shaft  80  is adjustable relative to the array  22  such as that shown with stylus  30 , effectively elongating the distance from the digitized point and the IR-reflective markers. This longer distance translates to digitization of a point above the actual screw head based on the distance the user adjusted the elongate shaft  80 . As will be appreciated in conjunction with the discussion below, the resulting bend instructions would shape a rod that traverses that point above the screw allowing the user to reduce the screw to the rod. 
     As shown in  FIGS. 6-8 , offset pointer  30  includes an elongate tubular member  84  and an inner piston  86 . Elongate tubular member  84  is comprised of a milled helical slot  104  and a plurality of offset depth slots  106  located around the helix that correspond to a plurality of offset distances as will be described below. Inner piston  86  includes shaft  88 , T-shaped cap  92 , springs  94 , and bushing  96 . The T-shaped cap  92  is positioned over the proximal end of the shaft  88  and is preferably welded to the proximal end  105  of the elongate tubular member  84 . Springs  94  are slideably positioned along the length of the shaft  88  between the distal end  93  of the T-shaped cap  92  and bushing  96 . Bushing  96  is positioned over the distal end of the shaft  88 . Pin  100  is travels through, and protrudes laterally from, slots  90 ,  98  on the inner shaft  88  and bushing  96 , thereby securing the bushing  96  to the inner shaft  88 . The pin  100  is sized and dimensioned such that it travels through the helical slot  104  and be positioned within each of the offset depth slots  106 . 
     The offset pointer  30  gives the user the ability to execute planned screw movement by a specific amount. The user inserts the offset pointer  30  into the screw head. Keeping the distal tip  82  engaged to the screw head, the user then selects an offset amount to be added to the screw and angles the offset pointer  30  in the direction he or she wishes to apply the offset to. To adjust between offset depth slots  106 , the shaft  88  is pulled away from the array  22  and twisted until the pin  100  falls into the desired offset slot  106 . As the shaft  88  is pulled, it telescopes in and out of the elongate tubular member  84  such that the distance between the shaped distal end  82  and the array  22  is increased. For purposes of illustration,  FIG. 8  shows the offset pointer  30  with the pin  100  in the 16 mm offset slot  106  corresponding to a 16 mm offset between the pointer  30  length and the IR-reflective array  22 . Offset options may be provided, by way of example only from 0 mm to 16 mm offsets in 2 mm increments. The system  10  will then acquire position information at that 
     The digitizer pointer  23  may be used to acquire positional information about some or all screw locations. According to a preferred embodiment, the shaped distal tip  82  is coaxially aligned into the screw head and the array  22  is triggered to register the screw point. Screw locations may be digitized in a superior-inferior or inferior-superior direction. According to some implementations, the first screw location digitized correlates to the rod insertion direction component of the bend instructions (described below). Squeezing handles  66  activates the spring mechanism and permits the shutters  36  to open equally via the interdigitating gear teeth  72  ( FIG. 4 ). Opening the shutter covers  68  exposes the middle IR-reflective sphere  38  and allows the IRtracking array  22  to be “seen” by the IR sensor  20  and the position of the digitizer pointer  23  to be digitized. In this way, the IR sensor  20  only recognizes the digitizer pointer  23  once the middle sphere  38  is exposed which allows for point-by-point tracking and obviates the sensing and digitization of one or more unnecessary data points which may occur with prior art systems that continually track surgical objects. Further, use of the gear mechanism allows the passive IR-reflective sphere  38  to be “seen” symmetrically by the IR sensor  20 , thereby enabling a more accurate calculation of position information by the system  10 . According to some implementations, the control unit  16  emits an audible sound to notify the user that the middle sphere  38  is recognized by the IR sensor  20  and the screw point is acquired. Once a point has been registered, the shutter handles  66  may be released, thereby closing the bilateral shutters  36 . This process is then repeated for all screw locations to be digitized. 
     In accordance with the present invention, there are provided a plurality of algorithms for achieving rod bends. The surgical bending algorithms may be divided into two smaller sub-systems: (1) the spatial location algorithms that acquire, collect, and digitize points in space and (2) the bending algorithms that analyze the points and calculate the bend instructions and rod length needed to bend a rod with the mechanical bending device  18 . 
     As set forth above, the spatial tracking system  12  measures the six degrees of freedom (6 DOF) information for the tracked IR-reflective spheres  38 . These data provide the full pose (position and orientation) of each screw of interest which may then be made available to the algorithm library to calculate the bend instructions.  FIG. 10  is a flow chart indicating the steps of the spatial location data acquisition process according to one embodiment. The system  10  initializes the sensor objects from configuration to connect to, control, and read data from the IR sensor  20  (step  140 ). The system  10  then inspects all devices connected to it and finds the device with a device ID that corresponds to the IR sensor  20  (step  141 ). At step  142 , if an IR sensor  20  is found at step  141 , the system  10  continues to establish a connection with the IR sensor  20  (step  143 ). However, if not the system  10  continues to search. After the system  10  connects to the IR sensor  20 , it then loads a tool file that defines the array  22  (step  144 ). After initialization and tool file loading, the IR sensor  20  must prepare for taking data. At step  145 , the IR sensor  20  is enabled and ready to generate positional data but is left idle until tracking is enabled. By way of example and as described with reference to  FIG. 17 , selecting the position of the IR sensor  20  with respect to the patient&#39;s body causes the control unit  16  to send the IR sensor  20  a command to begin tracking. With tracking enabled (step  146 ), the IR sensor  20  may be polled to for data (step  147 ). Preferably, new data is requested twenty times per second from the IR sensor  20 . At step  148 , the data generated from polling the IR sensor  20  is checked to ensure that it is reporting valid data. The data may be considered valid if all of the IR-reflective spheres  38  are visible to the IR sensor  20 , the digitizer pointer  23  is fully inside the IR sensor&#39;s  20  working volume, there is no interference between the IR sensor  20  and the digitizer pointer  23 , and both the location and rotation information reported are not null. At step  149 , if the data is not deemed valid, then the digitized point is not used by the system  10  and polling is resumed. If the fifth IR-reflective sphere  38  (i.e. the middle sphere) is visible on the digitizer pointer  23  (step  150 ), the process of collecting positional data for the bend algorithm commences. If the middle sphere  38  is not visible, then the data is available to the system  10  only to show proximity of the IR sensor  20  and IR-reflective tracking array  22  (step  151 ). Points used by the bend algorithm are preferably an average of several raw elements (step  152 ). Normally, five points are collected at this step before the points are processed and made available to the bend algorithm. The position data is averaged using a mean calculation. The directions are averaged in the quaternion representation (raw form) then converted to a unit direction vector. The data is rotated from the spatial tracking system  12  coordinate from into the system  10  coordinate frame using a rotation matrix. At step  153 , after all processing, the data is available for the bend algorithm to collect and process further as will be described in greater detail below. 
     The surgical bending software takes the location and direction data of the screw locations as described above and uses one or more geometry-based algorithms to convert these relative screw locations into a series of bend instructions.  FIG. 11  is a flow chart indicating the steps of the surgical bending process according to a first embodiment. At the input validation step  154 , the system  10  may validate the system inputs to ensure the rod overhang is greater than zero, validate the sensor setup to ensure that the IR sensor  20  location has been set, and validate each of the acquired points. By way of the example, the validation of each of the acquired points ensures, for example, that there are at least two screw points digitized, no two screw locations are too far apart, no two screw locations are too close together, and the span between the superior-most and inferior-most screw locations is not longer than the longest available rod. 
     At the transformation step  155 , the data may be centered and aligned such that the first data point acquired is set at the system  10  coordinate&#39;s origin and all data is aligned to the x-axis of the system&#39;s coordinates thereby reducing any potential misalignment of the IR sensor  20  relative to the patient&#39;s spine. 
     At the rod calculations step  156 , the system  10  may perform rod calculations for a straight rod solution, a pre-bent rod solution, and a custom-bend solution. For a straight rod solution, the system  10  first determines the length of a straight rod that will span all of the screw locations. This length may be calculated to accommodate each of the screw heads, hex and nose lengths of the rods chosen, and the user&#39;s selected rod overhang length. The system  10  then fits the data to a straight line, if the screw data is within tolerance of the straight line, then the bend instructions will return a straight rod, otherwise it will return no rod solution and proceed to look for a pre-bent rod solution. By way of example only, the tolerance may be 2 mm in each of the sagittal and coronal planes. 
     For a pre-bent rod solution, the system  10  first determines the length of the shortest pre-bent rod from the available rod from the available rods (as will be described in greater detail below) that will span all of the screw locations. This length may be calculated to accommodate each of the screw heads, hex and nose lengths of the rods chosen, and the user&#39;s selected rod overhang length. Next, the system  10  fits the digitized screw data to a circular arc in 3-dimensional space. If the screw data is within the tolerance of the arc, then the bend instructions will return a pre-bent rod solution, otherwise it will return no rod solution and proceed to look for a custom-bend rod solution. By way of example, this tolerance may be 2 mm in each of the sagittal and coronal planes. 
       FIG. 12  depicts a flow chart of a custom bend algorithm according to one embodiment. At step  158 , screw location and direction data is generated by the spatial tracking system  12  as set forth above. The data is then projected into two planes: the x-y plane (coronal view) and the x-z plane (sagittal view). Each projection is then handled as a 2D data set. At step  159 , a fixed size loop is generated over small incremental offsets for the first bend location for the end of the rod which optimizes the ability of the bend reduction step  162  to make smooth solutions. At step  160 , the system  10  creates a spline node at each screw location and makes a piecewise continuous 4 th  order polynomial curve (cubic spline) through the screw points. At step  161 , the smooth, continuous spline is sampled at a regular interval (e.g., every 1 cm) along the curve to generate an initial set of proposed bend locations. At step  162 , as many bends as possible are removed from the initial set of proposed bend locations from step  161  as possible to reduce the number of bends the user must execute on a rod in order to fit it into a screw at each digitized screw point. According to one embodiment, no bend is removed if eliminating it would: (1) cause the path of the bent rod to deviate more than a predefined tolerance limit; (2) cause any of the bend angles to exceed the maximum desired bend angle; and (3) cause the rod-to-screw intersection angle to exceed the maximum angulation of the screw head. Once the number of bends has been reduced, the 2D data sets are combined and handled as a 3D data set. The 3D line segments are then evaluated based on distance between each line segment interaction (Location), the angle between two line segments (Bend Angle), and the rotation (Rotation) needed to orient the bend into the next bend plane using the following calculations:
 
Location: (( X   2   −X   1 ) 2 +( Y   2   −Y   1 ) 2 +( Z   2   −Z   1 ) 2 ) 1/2  
 
Bend Angle: arc-cosine( V   12   ·V   23 )
         where · is the dot product and V is a vector between 2 points
 
Rotation: arc-cosine( N   123   ·N   234 )
   where · is the dot product and N is the normal vector to a plane containing 3 points.
 
These calculated numbers are then tabulated to the physical design of the rod bender  18  and the selected rod material and diameter. Bend angles account for the mechanical rod bender&#39;s  18  tolerance and will account for the rod&#39;s material and diameter based on previous calibration testing performed with mechanical rod bender  18  and the specific kind of rod. Calibration testing quantifies the amount of spring-back that is expected when bending a certain rod material and diameter. By way of illustration, a 5.5 mm diameter titanium rod&#39;s spring-back can be characterized by a 1 st  order linear equation:
 
 BA   A =0.94* BA   T −5.66
 
where BA T  is the theoretical bend angle needed that was calculated from the 3D line segment and BA A  is the actual bend angle needed to bend the rod to so it can spring back to the theoretical bend angle. Thus, using this equation, when 20 degrees of bend is calculated from the 3D line segment above, the “spring-back” equation for that rod will formulate that a 25 degree bend needs to be executed in order for it to spring-back to 20 degrees. The length of the final rod is the total of all the calculated distances plus the selected rod overhang.
       

     Once all of the rod solutions have been generated, the loop is completed (step  163 ). At step  164 , from all of the rod solutions generated in the loop above, the system  10  may output the rod solution having the smallest maximum bend angle (i.e., the smoothest bent rod). It is to be appreciated that the system  10  may choose the rod solution displayed based on any number of other criteria. At step  169 , the system  10  then generates the three-dimensional locations of the bends in space. 
     Referring back to the flow chart of  FIG. 11 , from the geometric bend locations and/or pre-bent rod output of the rod calculations step  156  above, the system  10  generates instructions for the user to choose a straight rod, a pre-bent rod, or to custom bend a rod (step  157 ). All of the output instructions are human-readable strings or characters. In all cases, the length of the required rod is calculated as described above and is displayed to the user as either a cut rod or standard rod. For custom bend solutions, rods are loaded into the bender with the “inserter end” (e.g., one pre-determined end of the rod) into the bender collet  126 . If, due to geometric constraints, the rod cannot be bent from the inserter end, then the instructions are flipped, and the cut (or nose) end of the rod is instructed to be put into the bender collet  126 . The bend instructions are generated from the geometric bend locations and are given as “Location”, “Rotation”, and “Bend” values as will be described in greater detail below. These values correspond to marks on the mechanical bender  18 . 
       FIGS. 13-14  depict a flow chart of a second embodiment of a custom bend algorithm. In accordance with this second embodiment, the custom bend algorithm includes a virtual bender used to render a virtual rod. The following calculations and the flowcharts of  FIGS. 13-14  highlight the steps of this embodiment. 
     The 3D vector s i =[s i   x , s i   y , s i   z ] T  denotes the i&#39; th  screw digitized by the user such that the set of N acquired screws that defines a rod construct may be denoted as
 
 s=[s   0   , . . . ,s   N−1 ]ε   3×N   (1)
 
It may be assumed that the screws have been collected in order (e.g. superior-most screw to inferior-most screw or inferior-most screw to superior-most screw) so the index i can also be thought of as the index through time.
 
     A virtual rod (R) of length L r  given in mm is broken down into Nr uniformly distributed points, R=[r 0 , . . . , r Nr-1 ]. Each rod point r i  is composed of two components, a spatial component and a directional component r i ={r i   s ,r i   d }, where r i   s , r i   d ε   3 . The segments between rod points is constant and defined by
 
δ i   =|r   i+1   s   −r   i   s |. Let Δ d =Σ i=0   d δ i , then Δ N     r     −1   =L   r .
 
     A virtual bender (B) consists of a mandrel (M) of radius M r  (mm). Preferably, though not necessary, the key assumption when bending the virtual rod around M is the conservation of arc length. For illustrative purposes only, if a 90° bend is introduced to an example rod R of length 100 mm around a mandrel with radius 10 mm to produce a rod {circumflex over (R)}, then
 
∫ dR=∫d{circumflex over (R)}.   (2)
 
     The virtual rod, R, is bent according to a list of instructions. Each instruction consists of a location (I l ), rotation (I r ), and bend angle (I θ ). The location is the position of the rod in the bender and corresponds to the point directly under the mandrel M. The rotation is given in degrees (0°-360°) and corresponds to the amount the rod is rotated from 0 in the collet. The bend angle is given by a single letter that corresponds to a specific angle in degrees. There is a corresponding notch on the bender with the same letter for the user to select. 
     The rod is initialized (step  166 ) such that the spatial component r i   s =[Δ i ,0,0] T ┘ i=0   N     r     −1 , and the direction component r i   d =[0,1,0] T | i=0   N     r     −1  which effectively orients the virtual rod to be at zero rotation in the virtual bender. For each bend instruction (step  167 ), the system  10  rotates the virtual rod around the x-axis by I r  (step  168 ). The system  10  finds the point {circumflex over (r)} i  that matches I l . The virtual rod is translated by −{circumflex over (r)} i . Next, each rod point from i to i+M r *I θ  is projected onto the mandrel M while preserving segment length (step  169 - 170 ). The virtual rod is then rotated around the x-axis by angle −I r . Next, the system  10  checks that r 0   d =[0,1,0] T  to verify that the virtual rod in the collet has the correct direction vector (step  171 ). At this point, R has approximated the geometry of the rod as it would be bent in the physical mechanical bender  18 . 
     The next step is to align the bent virtual rod to the acquired screw positions (step  172 ). According to one embodiment, the alignment process has two stages—first, the system  100  finds the optimum rotation coarse scale (step  174 ). Second, the system performs the iterative closest point iteration algorithm fine scale. 
     Preferably, the system first initializes the result close to a global minimum (step  173 ). In the rod alignment algorithm, this initialization follows the approach described below: 
     Using the arc length of the custom rod and the arc length of the screws, putative matches from the screws to the rod are produced. This produces two 3D point sets of equal size. Given two 3D mean centered point sets Σ=[σ 0 , . . . , σ N−1 ] and Γ=[γ 0 , . . . , γ N−1 ], then in the least squares sense, it is desirable to minimize 
                   E   =       1   N     ⁢       ∑     i   =   0       N   -   1       ⁢           ⁢         (       σ   i     -     T   ⁢           ⁢     γ   i         )     T     ⁢     (       σ   i     -     T   ⁢           ⁢     γ   i         )                   (   3   )               
Where T denotes the rotation matrix. Let {circumflex over (T)} denote the optimum 3D rotation matrix, then
 
                   T   =       arg   T     ⁢           ⁢   min   ⁢     1   N     ⁢       ∑     i   =   0       N   -   1       ⁢           ⁢         (       σ   i     -     T   ⁢           ⁢     γ   i         )     T     ⁢     (       σ   i     -     T   ⁢           ⁢     γ   i         )                   (   4   )               
It turns out that {circumflex over (T)}=UV T , where
 
 C=SVD ( H )= UΣV   T   (5)
 
and
 
     
       
         
           
             
               
                 
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                       ⁢ 
                       
                         
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                               ( 
                               
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                                 174 
                               
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                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
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     Due to error potentially introduced by differences in arc length, the proposed solution may not be the global minimum. Thus, the following are repeated until convergence (step  175 ): 
     For each s i , find the closest r j  (step  176 ) 
     Calculate the residual vector e i =s i −r j    
     Calculate the average residual vector 
     
       
         
           
             
               e 
               ^ 
             
             = 
             
               
                 1 
                 N 
               
               ⁢ 
               
                 
                   ∑ 
                   
                     i 
                     = 
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                 ⁢ 
                 
                     
                 
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                       step 
                       ⁢ 
                       
                           
                       
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                       177 
                     
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     Translate the rod by ê (step  178 ) 
     Verify the error is reduced (step  179 ). 
     Next the virtual rod is rendered at step  180 . The curve may be simplified for rendering purposes by traversing each triad of rod points and calculating the angle between the two vectors. If the first triad is {r 0 , r i , r2}, the two vectors are formed as v=r 1 −r 0  and w=r 2 −r 0 . If |v×w|=0, then the middle point of the triad (in this case r 1 ) is redundant, provides no new information to the geometry of the rod and may be removed. 
     It will be appreciated that, in accordance with this embodiment of the rod bending algorithm, the virtual bender may be capable of bending a rod at any location of any angle perfectly to observe arc length. Using a virtually bent 3D rod to determine problem screws (i.e. screw locations with a high screw-rod fit error) may give an accurate fit between the actual screws and actual rod before the actual rod is bent. This may be particularly advantageous in certain surgical applications where it is desirable to quantify the amount of offset between a rod solution and the digitized screw locations as well as input one or more surgical parameters into the rod bending calculation. 
     In accordance with the present invention, there is described a third embodiment of an algorithm for generating a custom bend which may be utilized in conjunction with the second embodiment. The approach is directed to one or more algorithms that sample from probability distributions and employ random sampling to obtain a numerical result. A Markov chain is a sequence of random variables, X 0 , X 1  . . . , such that the current state, the future and past states are independent.
 
 p ( X   n+1   =x|X   0   =x   0   ,X   1   =x   1   , . . . ,X   n   =x   n )= p ( X   n+1   =x|X   n   =x   n )  (1)
 
     Given an ordered set of screws that define a construct
 
 S=[s   0    . . . ,s   N−1 ]ε   3×N   (2)
 
where s i =[s i   x , s i   y , s i   z ] T  denotes the i th  3D screw digitized by the user, the system  10  finds the set of bend instructions that define a rod that fits the screws in an optimum way defined by an error function. It is to be appreciated that the search of the bender space is quite complex as there are several constraints that must be observed for the algorithm to produce valid bend instructions (e.g., the bend locations cannot be in close proximity to the screws, the bend locations must be in multiples of 5 mm apart, the bend angles must be in multiples of 5°, no bend angle can be greater than 60°, etc.).
 
     In accordance with the second embodiment, the likelihood or error function may be constructed based on how well the virtual rod fits the data. Here, the rod is fit to the data in the least squares sense. In this way, a likelihood function is defined that incorporates, for example, a prior to prefer fewer bend instructions: 
                   L   =       ⁢       ∏     i   =   0         N   s     -   1       ⁢           ⁢       1       σ   s     ⁢     π         ⁢     ⅇ       -       (       s   i     -     r   i       )     2         σ   s   2         ⁢     ⅇ       -     N   b       α                       =       ⁢         (     1       σ   s     ⁢     π         )       N   s       ⁢     ⅇ       ∑     i   =   0         N   s     -   1       ⁢           ⁢       -       (       s   i     -     r   i       )     2         σ   s   2           ⁢     ⅇ       -     N   b       α                     
such that the log-likelihood function may be defined as
 
                     log   ⁡     (   L   )       =         -     N   s       ⁢     log   ⁡     (     σ   s     )         -       ∑     i   =   0         N   s     -   1       ⁢           ⁢       -       (       s   i     -     r   i       )     2         σ   s   2         -       -     N   b       α               (   3   )               
Where N b  denotes the number of bends in the rod, N s  denotes the number of screw locations, s i  is the i&#39;th screw, r i  is the i&#39;th rod point, and α is the control hyper-parameter for the number of bends (e.g. α=0.05).
 
     As can be seen from equation (3), there has been introduced a prior to control the number of bends introduced into the rod. This probabilistic approach to bend instruction generation allows for tailoring of constraints, for instance, a prior on the severity of the bends could also be introduced. Further, a prior could be introduced on how to define how close to the screws the bends may be located. This prior may have a “preferred” value, but probabilistically, there may be an optimal solution away from this idealized value. By way of example, some hypothesized rules that may be applied to this algorithm include, but are not limited to: birth move: add a bend to the current solution; death move; remove a bend from the current solution; update move: translate rod points along the rod. Use of this embodiment may provide more potential rod solutions to the user. 
     Details of the surgical bending system  10  are discussed in conjunction with a first embodiment of a method for obtaining a custom-fit rod. The system  10  is typically utilized at the end of a posterior or lateral fixation surgical procedure after screws, hooks or other instrumentation have been placed, but prior to rod insertion. As shown in the flowchart of  FIG. 15 , the surgical bending system  10  obtains position information of the implanted screw positions and outputs bend instructions for a rod shaped to custom-fit within those implanted screws. At step  190 , pertinent information is inputted into the system via a setup screen. At step  192 , the user designates which side (left or right) rod will be created. At step  194 , the system  10  digitizes the screw locations. At step  196 , the system  10  outputs bend instructions. At step  198 , the user bends the rod according to the bend instructions. Steps  190 - 198  may then be repeated for the other rod. 
       FIG. 16  illustrates, by way of example only, one embodiment of a screen display  200  of the control unit  16  capable of receiving input from a user in addition to communicating feedback information to the user. In this example (though it is not a necessity), a graphical user interface (GUI) is utilized to enter data directly from the screen display  200 . As depicted in  FIG. 16 , the screen display  200  may contain a header bar  202 , a navigation column  204 , device column  206 , and a message bar  208 . 
     Header bar  302  may allow the user to view the date and time, alter settings, adjust the system volume, and obtain help information via date and time display  210 , settings menu  212 , volume menu  214 , and help menu  216  respectively. Selecting the settings drop-down menu  212  allows the user to navigate to system, history, and shutdown buttons (not shown). For example, choosing the system button displays the rod bending software version and rod bender configuration file; choosing the shutdown option shuts down the rod bending software application as well as any other software application residing on the control unit  16  (e.g. a neuromonitoring software application); and choosing the history option allows the user to navigate to historical bend points/instruction data in previous system sessions as will be described in greater detail below. Selecting the help menu  216  navigates the user to the system user manual. As will be described in greater detail below, navigation column  204  contains various buttons (e.g., buttons  218 ,  220 ,  222 ,  224 ,  226 ) for navigation through various steps in the rod bending process. Pressing button  204  expands/minimizes the details of the navigation column. Devices column  206  contains various buttons indicating the status of one or more devices associated with the surgical bending system  10 . By way of example, devices column  206  may include buttons  228  and  230  for the digitizer  23  and IR sensor  20  components of the system  10 , respectively. Pressing button  206  expands/minimizes the details of the devices column. Furthermore, pop-up message bar  208  communicates instructions, alerts, and system errors to the user. 
       FIGS. 16-17  depict an example setup screen. Upon selecting setup button  218  on the display screen  200 , the surgical bending system  10  automatically initiates the setup procedure. The system  10  is configured to detect the connection status of each of its required components. By way of example only, icons  228 ,  230  indicate the connectivity and activity status of the digitizer  23  and IR sensor  20 , respectively. If one or more required components are not connected or are connected improperly, the display  200  may alert the user to address the issue before proceeding via textual, audio, and/or visual means (e.g., textual messages, audible tones, colored icons or screens, blinking icons or screens, etc.). According to one embodiment, the digitizer icon  228  is a status indicator for the active acquisition and/or recognition of the digitizer and the presence and background color of the icon  228  may change to indicate the digitizer tracking status. By way of example, the icon  228  may be absent when the system  10  is not acquiring screws and does not recognize the digitizer, gray when the system  10  is not acquiring screws and recognizes the digitizer, green when the system  10  is in screw acquisition mode and recognizes the digitizer, and red when the system  10  is in screw acquisition mode and does not recognize the digitizer. Pressing button  206  expands minimizes the details of the device column  206 . Depending on the type of surgery, type of patient deformity, etc., it may be advantageous for the user to choose a digitizer from a selection of different digitizers. According to one embodiment, pressing icon  228  expands a pull-out window for the different stylus options available with the rod bending system  10  (e.g., styluses  22 ,  24 ,  26 ,  30  as described above). According to another embodiment, the IR sensor graphic icon  230  is a status indicator for the IR sensor  20 . The presence and background color of the icon  230  may change to indicate the status of the IR sensor  20 . By way of example, the icon  230  may be absent when the system  10  does not recognize the IR sensor  20 , gray when the system  10  recognizes the IR sensor  20  is connected to the system  10 , and red when the system  10  senses a communication or bump error for the IR sensor  20 . Preferably, the IR sensor  20  should be recognized if it is connected after initialization of the bending application. 
     With all of the required components properly connected to the surgical bending system  10 , the user may then input one or more pieces of case-specific information from one or more drop-down menus. By way of example, drop-down menus for rod system  234 , rod material/diameter  236 , rod overhang  238 , procedure type (not shown), and surgical levels) may be accessed from the setup selection panel  232  of the screen display  200 . The rod system drop-down menu  234  allows the user to choose the rod system he/she plans to use. This selection drives choices for the rod material/diameter  236  drop-down menus. By way of example, under the rod system drop-down menu  234 , the system  10  may be programmed with numerous fixation options from one or more manufacturers. Alternatively, it may be programmed with the fixation system selections for one manufacturer only (e.g. NuVasive® Precept®, Armada®, and SpherX® EXT). The user may also choose the combination of rod material (e.g. titanium, cobalt chrome, etc.) and rod diameter (e.g. 5.5 mm diameter, 3.5 mm diameter, etc.). The drop-down menu  238  for material and diameter options may preferably be dependent upon the choice of rod system. Because the geometry and sizes can vary between manufacturers and/or rod systems, programming the system  10  with these specific inputs can aid in outputting even more accurate bend instructions. The user may also choose the amount of overhang from the rod overhang pull-down menu  238 . By way of example, the amount of overhang may be selectable in 0 mm, 2.5 mm, 5 mm, 7.5 mm, and 10 mm lengths. According to one embodiment, this function prescribes a symmetric overhang on both the superior and inferior ends of the rod. According to another embodiment, this function also prescribes different overhang lengths on either end of the rod based on user preference and patient anatomical considerations. Although not shown, the system  10  also contains functionality for accommodating multiple rod diameters and transitional rods as used, for example in Occipital-Cervical-Thoracic (OCT) fusion procedures. 
     After the setup inputs have been inputted into the setup selection panel  232 , the surgical bending system  10  aids the user in setting up the IR sensor  20  in an optimal position for positional data acquisition. It is to be appreciated that any visual (textual, graphic) indicator may be used to indicate the IR sensor placement instructions. According to some implementations, an active graphic directs the user to position the IR sensor  20  relative to the digitizer array  22  held static within the patient&#39;s body. As shown in  FIG. 17 , the user first selects the side of the patient the IR sensor  20  is located on by selecting the left side sensor position button  242  or right side sensor position button  244  in the IR sensor setup panel  240 . Choosing the left or right side sensor position button  242 ,  244  activates a the IR sensor positioning panel  246  such that IR sensor graphic  248  and a tracking volume box graphic  250  appear on the display screen  200 . Tracking volume box  252  that moves with the IR sensor graphic  248  as the IR sensor  20  is moved. Next, the user positions the digitizer array  22  into the body of the patient. Once recognized by the system  10 , a target volume box  252  (which may be displayed as white in color) is positioned over the patient graphic  254 . Next, the user moves the IR sensor  20  relative to the digitizer array  22  until the tracking volume box  250  matches up to the position of the target volume box  252 . According to some implementations, the IR sensor graphic  248  increases in size if it is moved superior to the target tracking volume and decreases in size if it is moved inferior to the target volume. According to some other implementations, the tracking volume box  250  may be color-coded to depict the relative distance to the target volume. By way of example, the tracking volume box  250  may be depicted in red if the distance to the target volume is outside of a certain distance in one or more axes (e.g., outside ±8 cm in all 3 axes.) and green if within or equal to ±8 cm in all 3 axes. Once the optimal position of the IR sensor  20  has been ascertained, the setup process is complete. 
     Once the user has completed all of the required steps in the setup screen, a graphic (e.g., a check) may appear on setup button  218  to indicate such a completion and the system  10  proceeds to step  192  in the flowchart of  FIG. 15 . Using the GUI, the user designates which side of the patient&#39;s spine to acquire digitized positional information from by selecting either the Left “L” toggle/status button  220  or Right “R” toggle/status button  222 . The user then selects the Acquire Screws button  224  which navigates the display screen  200  to an Acquire Screws (left or right) screen shown by way of example in  FIGS. 18-20 . In Acquire Screws mode, the display screen  200  includes a sagittal view panel  256  and a coronal view panel  258  with spine graphics  260 ,  262  in each of the sagittal and coronal views, respectively. Spine graphic  260  may flip orientation depending on which side of the spine the user is digitizing (left or right). Additionally, spine graphic  262  may highlight the side of the patient the user is digitizing (left or right). The user may digitize the location of each implanted screw using, by way of example, the digitizer pointer  23  as described above. As each screw point  264  is digitized, its relative location with respect to the other acquired screw points  264  can be viewed in both sagittal and coronal views via the sagittal view panel  256  and the coronal view panel  258  as shown in  FIG. 19 . Optionally, the last screw point digitized may have a different graphic  266  than the previously-acquired screw points  264  (by way of example, the last screw point acquired  266  may be a halo and the previously-acquired screw points  264  may be circles). The screws locations may be digitized from a superior-to-inferior or inferior-to-superior direction and according to some embodiments, the system  10  can detect which direction the digitization is occurring in after the acquisition of two consecutive screw point locations. If during the digitization process, the user wishes to delete a digitized screw point, he/she may do so by pressing the “Clear Point” button  270 . If the user wishes to delete all digitized screw points, he/she may do so by pressing the “Clear All Points” button  268 . 
     Once the digitized screw points  264  are deemed acceptable, the user may press the “Calculate Rod” button  272  which initiates the curve calculation preferably using one of the algorithms discussed above. Once a rod solution has been calculated, a rod graphic  274  populates through the screw points  264 ,  266  and a confirmation graphic (e.g., a check) may appear on the “Acquire Screws” button  224  to indicate that the system  10  has generated a rod solution. Simultaneously, the “Calculate Rod” button  272  becomes the “Undo Rod” button  272 . If the user presses the “Undo Rod” button  272 , the rod solution  274  is cleared and the user may acquire more screw points or clear one or more screw points. After the “Undo Rod” button  272  is pressed, it then changes back to the “Calculate Rod” button  272 . Optionally, the system  10  may include a visual graphic for where along a rod the curve calculation is generating a severe bend (acute angle). The user may select “Undo Rod” button  272 , perform one or more surgical maneuvers (e.g. reduce the screw, backup the screw, adjust the screw head, etc.), redigitize the screw point, and generate a more feasible solution. If the rod solution is acceptable to the user, the Screw Acquisition step  194  is complete and the system  10  proceeds the Bend Instructions step  196  in the flowchart of  FIG. 15 . 
     The user then selects the “Bend Instructions” button  226  which navigates the display screen  200  to a Bend Instructions (left or right) screen shown by way of example in  FIG. 21 . The bend instructions within the bend instructions panel  276  allows the user to view the bend instructions corresponding to the resulting rod solution in the Acquire Screws screen ( FIG. 20 ). By way of example, the bend instructions panel  276  contains three fields containing various aspects of the bending instruction: upper message field  278 , bender instructions field  280 , and lower message field  282 . By way of example, the upper message field  278  may communicate the rod cut length, rod type, and/or rod loading instructions to the user (e.g. “Cut Rod: 175.00 mm Load Inserter End Into Bender”). The bender instructions field  280  displays rows  284  of bend maneuvers in location  286 , rotation  288 , and bend angle  290  to perform on the mechanical bender  18  as will be described in greater detail below. In the example shown in  FIG. 21 , there are five rows indicating five bend instructions. The lower message field  282  may communicate the direction of insertion or orientation of implanting the rod to the user. For example, the lower message field  282  shown in  FIG. 21  provides the following sample instruction: “Insert Rod head to foot.” In some implementations, the rod insertion direction into the patient is dependent on the sequence of screw digitization (superior-to-inferior or inferior-to superior). According to one or more preferred embodiments, the bend instruction algorithm takes into account the orientation of the inferior, superior, anterior, and posterior aspects of the rod and ensures that these aspects are known to the user. As the instructions for use direct the user to load the rod into the bender, the system  10  manages which bends are imparted on the rod first based on the severity of the bend angles. The section of the bend instructions with greater bend angles may be performed first then the straighter bend sections of the bend instructions are performed last. Further, the instructions may also direct the user to align a laser line or orientation line on the rod to an alignment arrow (not shown) on the mechanical rod bender  18 . This alignment controls the Anterior/Posterior orientation of the rod geometry and generates bend instructions accordingly. The user follows the bend instructions generated by the system  10  for location (location may be color-coded on the bender  18  and on the screen  200  as green triangle), rotation (rotation may be color-coded on the bender  18  and on the screen  200  as red circle), and bend angle (bend angle may be color-coded on the bender  18  and on the screen  200  as blue square), sequentially, starting at the first bend instruction and working sequentially until the final bend is completed. From here, the user may repeat steps  190 - 198  on the construct for the opposite side of the spine. 
     Within a surgical procedure, a user may wish to toggle between left and right screens to view left and right digitized screw points, rod previews, and bend instructions for reference or comparison. Selecting the Left “L” toggle/status button  220  and right “R” toggle/status button  222  allows the user to do so. According to one more implementations, the GUI may additionally include a History feature. Selecting the History button (not shown) will allow the user to refer back to any previous rod bending solution. The user navigates to the Bend Instructions screen  226  based on choice of the L/R toggle buttons  220 , 222  and pressing Bend Instruction button  226 . If navigating to previous bend instructions, the Bend Instructions screen will display previous bend instructions. Once the user has selected the desired rod solution, the user then executes the bends using the mechanical bender  18 . 
     The embodiments described with respect to  FIGS. 15 and 18-21  above contemplate digitizing the implanted screw positions and outputting bend instructions for a rod shaped to custom-fit within those implanted screws. In one or more additional embodiments, the system  10  obtains position information of the implanted screws (steps  192  and  194 ), accepts correction inputs via one or more advanced options features (step  195 ), and generates for viewing bend instructions for a rod shaped to fit at locations apart from those implanted screw positions (step  196 ) as depicted in the flowchart of  FIG. 22 . Installing a rod shaped in this manner could correct a curvature or deformity in the patient&#39;s spine according to a user&#39;s prescribed surgical plan. Details of the surgical bending system  10  are discussed now discussed with examples for obtaining a rod bent according to one or more surgical plans. 
     As depicted in  FIG. 23 , selecting the “Advanced Options” button  292  expands an Advanced Options menu  294  from which the user may perform one or more corrections to the digitized screw points and the system  10  generates bend instructions that will achieve those desired corrections on the patient&#39;s spine once the rod is implanted and the screws are brought to the rod. 
     In some surgical procedures, a user may wish that the rod bend solution will consider a point that is not a digitized screw point in determining the bend instructions. According to some implementations, this point is an adjusted distance from the digitized screw point location. Selecting the “Adjust Points” button  296  from the Advanced Options menu  294  navigates the user to an Adjust Points screen as depicted in  FIG. 23 . Selecting a digitized screw location of interest (for example the screw point represented as dot  304  in  FIG. 24 ) highlights the screw point and brings up an adjust points control  306  in each of the sagittal and coronal views  256 ,  258 . The user adjusts point  304  to its desired location in the sagittal and coronal planes using arrows  308 ,  310 ,  312 , and  314 . In some implementations, as the point moves, dot  304  changes color based on the distance from the originally digitized screw location as shown in  FIG. 25 . Preferably, that color corresponds to color-coded offset distance indicator  322  which provides visual feedback to the user as to the distance the point has been adjusted. As depicted by way of example, dot  304  appears yellow in  FIG. 25  indicating that the point has moved 4 mm in each of the sagittal and coronal planes. In some implementations, the system  10  may have a maximum distance from the digitized point past which it will not allow the manipulated point to exceed (by way of example only, this distance may be 5 mm). The user may adjust as many points as desired in this fashion. The user may reset all adjusted points to their original configurations via “Reset” button  316  or may undo the last adjusted point via the “Undo Last” button  318 . Once satisfied with the adjusted points, the user may either proceed to one or more additional advanced options as set forth below or select “Calculate Rod”  272 . Once “Calculate Rod”  272  has been selected, the system  10  generates a rod in which the curve traverses the adjusted points, as in  FIG. 26 , thereby creating a correction-specific rod and providing the user with the ability to correct the curvature or deformity in the spine to his or her prescribed curve. 
     According to other implementations, a user may wish for a smoother rod bend. When the “Virtual Point” button  320  (shown by way of example in  FIG. 25 ) is selected, the system  10  allows the user to add an additional point anywhere in between the superior-most and inferior-most digitized screw locations. While there is no screw at this location, this point is taken into consideration during the curve calculation and may coerce the curve into a more natural shape yielding a smoother rod bend. Once satisfied with the virtual points, the user may either proceed to one or more additional advanced options as set forth below or select “Calculate Rod”  272  and as described above, the system  10  generates a correction-specific rod solution  274  that the user may use to correct the spine to the shape of the rod. 
     It may be advantageous for some patient anatomies for a user to use a pre-bent rod. Use of a pre-bent rod eliminates the need for making additional bends to a rod while assuring that a desirable rod curve is achieved. After all screw points have been digitized in the Acquire Screws step  194 , selecting the “View Pre-Bent Rod” button  298  from the Advanced Options menu  294  navigates the user to a “View Pre-Bent Rod” screen as depicted in  FIGS. 27-28 . Based on the digitized screw locations shown in  FIG. 27 , the system  10  calculates and outputs the best pre-bent rod geometry based on the selected manufacturer&#39;s rod system that was chosen during the setup step  190  (e.g. NuVasive® Precept®) and displays the best fit virtual pre-bent rod solution  324  available on top of the digitized screw points for viewing in the sagittal and coronal views  256 ,  258  (see  FIG. 28 ). Preferably, the system  10  only generates a pre-bent rod solution if the geometry of the pre-bent rod fits the digitized screw points within a predetermined curve fitting tolerance (e.g. 7 mm). According to one or more embodiments (as depicted in  FIG. 28 ), a color-coded offset distance indicator  322  may provide the user with an indication of the distance each screw position will be from the pre-bent rod construct. If the user is satisfied with the pre-bent rod suggestion, the system  10  proceeds to the Bend Instructions step  196  which displays the corresponding pre-bent rod specifications in the Bend Instructions Screen ( FIG. 29 ). The upper message field  278  instructs the user that, based on the digitized screw points, an 85.0 mm pre-bent rod is recommended. From here, the user may decide whether the patient&#39;s anatomical and surgical requirements would be better suited with a pre-bent option or a custom-bent option. Armed with the information from  FIGS. 27-29 , the user may then adjust the screw positions to fit the pre-bent rod if needed (e.g., adjust the screw head, adjust the screw depth, etc.). 
     In some instances, a user may want to align or correct the patient&#39;s spine in the sagittal plane (i.e., add or subtract lordosis or kyphosis). The surgical bending system  10  includes a sagittal correction feature in which the user is able to measure the amount of lordosis in the spine and adjust angles in the sagittal plane. The system  10  then incorporates these inputs into the bend algorithm such that the rod solution includes the desired alignment or correction. 
     Selecting the “View Vectors” button  300  from the Advanced Options menu  294  initiates the sagittal correction feature. The user may select at least two points of interest and the system then determines the appropriate vector in the sagittal view. According to the embodiment shown in  FIGS. 30-31 and 33 , the angles are measured and adjusted based on the screw trajectory screw axis position) using the digitized screw data acquired in the Acquire Screws step  194 . As shown in  FIG. 30 , the user selects at least two screw points of interest (e.g., screw points  338  and  342 ). The system  10  then measures the angle between the screw trajectories (shown here as 35 degrees). In some implementations, the system  10  may measure the total amount of lumbar lordosis by measuring the lumbar lordosis angle  334  in the superior lumbar spine (shown in  FIG. 30  as 15 degrees) and the lumbar lordosis angle  336  in the inferior lumbar spine (show in  FIG. 30  as 35 degrees). Using the angle adjustment buttons  328 ,  330  on the Angle Adjustment Menu  326 , the user may increase or decrease the desired angle correction of the spine in the sagittal plane (i.e., add or subtract lordosis or kyphosis superiorly or inferiorly). As the angle is adjusted, the angular position  336  between the two screw points  338 ,  342  is changed as well.  FIG. 31  illustrates an example in which the angular position  336  between points  338  and  342  is increased to 50 degrees). The system  10  may include a color-coded offset distance indicator  322  to provide the user with an indication of the distance each digitized screw position will be adjusted in the sagittal plane as described above. Once the desired amount of angular correction is achieved, the user may select the “Set” button  332 , and then the “Calculate Rod” button  270 . The system  10  then displays a rod solution  274  incorporating the user&#39;s clinical objective for correction of the spine in the sagittal plane as depicted in  FIG. 33 . 
     According to the embodiment of the sagittal correction feature shown in  FIG. 32 , the superior and inferior lumbar lordosis angles  334 ,  336  are measured, displayed, and adjusted referencing anatomy from an imported lateral radiographic image. By way of example, lateral radiographic image  358  may be inputted into the system  10 . The user may touch the screen  200  and move lines  360  over at least two points of interest (e.g. the superior endplate of V1 and the inferior endplate of V3) and the system  10  then measures the angle between the two lines  360 . The Using the angle adjustment buttons  328 ,  330  on the Superior Angle Adjustment Menu  346  or Inferior Angle Adjustment Menu  348 , the user may increase or decrease the desired angle correction of the spine in the sagittal plane (i.e., add or subtract lordosis or kyphosis superiorly or inferiorly). As either the superior or inferior lumbar lordosis angle is adjusted, the amount of adjustment is dynamically altered in its respective angle measurement box (i.e., either superior lumbar lordosis angle box  354  or inferior lumbar lordosis angle box  356 ). As depicted in  FIG. 32 , the user adjusts angle lines  360  as part of the inferior lumbar lordosis angle. The system  10  measures this angle as 20 degrees as depicted in angle measurement field  350 . The user then uses button  330  in superior angle adjustment menu  346  to increase the angle. This change is depicted in inferior lumbar lordosis angle box  356 . Once the desired amount of correction is achieved, in this example, it is achieved at 50 degrees. The user may then press the capture angle button  352  and this parameter may be correlated to the digitized screw positions corresponding to the vertebral levels that those angles were measured off of. The system  10  may include a color-coded offset distance indicator  322  to provide the user with an indication of the distance each digitized screw position will be adjusted in the sagittal plane as described above. Once the desired amount of angular correction is achieved, the user may select the “Set” button  332 , and then the “Calculate Rod” button  272 . The system  10  then displays a rod solution  274  incorporating the user&#39;s clinical objective for correction of the spine in the sagittal plane as depicted in  FIG. 33 . 
     It is to be appreciated that, because patient position (e.g., pelvic tilt) may have an effect on the lumbar lordosis measurements, the sagittal correction feature of the system will be able to account for any patient positioning-related deviations. It will also be appreciated that in addition to lordotic corrections, the sagittal angle assessment tool may be useful for other types of surgical maneuvers, including but not limited to pedicle subtraction osteotomy (PSO) procedures and anterior column reconstruction (ACR) procedures. 
     In some instances, a user may want to align or correct the patient&#39;s spine in the coronal plane (i.e., correct scoliosis). The system  10  includes a coronal correction feature in which the user is able persuade one or more screw locations towards a particular coronal alignment profile by manually or automatically biasing which direction the rod bend curve is adjusted. The system  10  then incorporates these inputs into the bend algorithm such that the rod solution includes the desired alignment or correction. 
     Selecting the “Coronal Correction” button  302  from the Advanced Options menu  294  initiates the coronal correction feature. The user may select at least two points of interest and the system then generates a best fit reference line through all points including and lying between the at least two points of interest. In some instances, the ideal correction of the spine in the coronal plane is a straight vertical line extending between the superior-most and inferior-most screw locations of interest. However, depending on a patient&#39;s individual anatomy, achieving a straight vertical line may not be feasible. The user may wish to achieve a certain amount of correction relative to the ideal correction. From the display screen, the user may select a percentage of relative correction between the screw points as digitized (0% correction) and the best fit reference line (100%). Furthermore, the system then calculates a rod solution and shows an off-center indicator  322  to provide a user with an indication of the distance each screw is from the coronally-adjusted rod construct as wet forth above. 
     According to the embodiment shown in  FIGS. 34-37 , the user may straighten all points within the construct (global coronal correction). From the display screen  200 , the superior and inferior screw points  362 ,  364  are selected and the system  10  generates a best fit global reference line  366  through all points  362 ,  364 ,  368 . Using the Coronal Correction Menu  370 , the user manipulates the + and − buttons  372 ,  374  to adjust the percentage of correction desired. In the example shown in  FIG. 36 , the amount of desired correction is shown as 100% on the percentage correction indicator  376 , meaning the rod solution  274  will be a straight line in the coronal plane and all screw locations will be adjusted to fit the rod/line. As depicted in  FIG. 36 , the system  10  may include a color-coded offset distance indicator  322  to provide the user with an indication of the distance each digitized screw position will be adjusted in the coronal plane as set forth above. If the user deems this an acceptable rod solution, the user selects the “Calculate Rod” button  272  to view the rod solution  274  ( FIG. 37 ) and receive bend instructions or proceeds to another advanced feature as will be described in greater detail below. 
     According to the embodiment shown in  FIGS. 38-40 , the user may straighten a subset of the screw points within the construct (segmental coronal correction). Based on the sequence those points are inputted into the system, a best-fit segmental reference line is generated through the points in the direction of the last chosen point. If an inferior point  364  is selected first and then a superior point  362  is selected second, the system  10  will draw the best-fit segmental reference line  378  superiorly as shown in  FIG. 38 . Conversely, if a superior point  362  is selected first and then an inferior point  364  is selected second, the system  10  will draw the best-fit segmental reference line  378  inferiorly. Using the Coronal Correction Menu  370 , the user manipulates the + and − buttons  372 ,  374  to adjust the percentage of correction desired. In the example shown in  FIG. 39 , the amount of desired correction is shown as 100% on the percentage correction indicator  376 , meaning the rod solution  274  will be a straight line in the coronal plane and all selected screw locations will be adjusted to fit the rod/line. As shown in  FIG. 40 , however, unselected screw locations  380  will not be adjusted to fit the rod/line and their relative locations will be inputted into the system  10  and taken into consideration when the rod calculation is made. As depicted in  FIG. 39 , the system  10  may include a color-coded offset distance indicator  322  to provide the user with an indication of the distance each digitized screw position will be adjusted in the coronal plane as set forth above. If the user deems this an acceptable rod solution, the user selects the “Calculate Rod” button  272  to view the rod solution  274  ( FIG. 40 ) and receive bend instructions or proceeds to another advanced feature as will be described in greater detail below. 
     In some spinal procedures (e.g., anterior column deformity correction procedures), restoring a patient&#39;s spine to a balanced position may be a desired surgical outcome. The surgical bending system  10  may include a Global Spinal Balance feature configured to receive preoperative and/or theoretical spinal parameter inputs, use these spinal parameter inputs to determine a target rod shape that will restore or improve spinal balance, display the balanced rod curvature and how that rod would traverse the screws in the deformed spine, and output a target rod that may be used to correct the spine to the rod and achieve a desired balanced alignment. Depending on user preference, these spinal parameters may comprise Pelvic Incidence (PI), Pelvic Tilt (PT), Sacral Slope (SS), Lumbar Lordosis (LL), Superior Lumbar Lordosis (↑ LL), Inferior Lumbar Lordosis (↓LL), C7 Plumb Line offset (C7PL), and Thoracic Kyphosis (TK) measurements. The surgical bending system  10  may be further configured to assess spinal parameter inputs intraoperatively to determine how the surgical correction is progressing. 
       FIG. 41  depicts a flowchart indicating the steps of the Global Spinal Balance feature according to one embodiment. At step  390 , the system  10  inputs a patient&#39;s preoperative spinal parameter measurements. Next, the system generates theoretical target spinal parameter measurements (step  392 ). One or more target spinal parameter measurements may be optionally adjusted the user in accordance with a surgical plan a step  394 . At step  396 , a target spinal rod may be scaled to match the patient&#39;s anatomy using the theoretical or adjusted target spinal parameter measurements from step  392  or  394 . This scaled target rod may then be displayed  398  to the user. Optionally, the system  10  may generate one or more measurements (step  400 ) during the surgical procedure. At step  402 , the target spinal parameter data may then be adjusted based on the intraoperative measurements from step  400 . Finally, the system  10  may generate bend instructions for balanced spine correction. 
     The user may input a patient&#39;s preoperative measurements into the system  10  as depicted, by way of example in  FIG. 42 . Selecting the Pre-Op measurement button  404  allows the user to input measurements into PI, LL, Superior LL, Inferior LL, C7PL, and TK input fields  408 ,  410 ,  412 ,  414 ,  416 ,  418 , and  420  respectively. These pre-operative anatomical measurements may be used to understand the imbalance in the patient&#39;s deformed spine as well as help determine an operative plan to implant devices that would adjust or form the spine to a more natural balance (e.g., rods, screws, a hyperlordotic intervertebral implant, etc.). 
     As depicted in  FIG. 43 , the global spinal balance feature allows the user to adjust the patient&#39;s anatomical measurement values to the user&#39;s preferred target spinal parameters for a balanced and/or aligned spine. According to one implementation, selecting the target measurement button  406  populates measurements into input fields  410 ,  412 ,  414 ,  416 ,  418 ,  420  that represent an ideal or properly balanced spine. If the user accepts these target spinal parameters, the system  10  would output a theoretical rod solution comprising rod shapes and curves representing an ideal or properly balanced spine scaled and overlaid onto the digitized screw points as shown in  FIG. 44 . The system  10  may also include a color-coded offset distance indicator  322  to provide the user with an indication of the distance each digitized screw position is from the rod solution in the sagittal and coronal planes as set forth above. Alternatively, if the user seeks to achieve a different alignment, he or she may use buttons  422 ,  424 ,  426  to adjust these target spinal parameters. The user could then refer to the correction indicator  428  for an indication of how much correction (relative to the pre-operative and theoretical spinal parameters) would be achieved based on those adjusted input correction values. The user&#39;s input correction values would then drive the rod bending algorithm (based on the digitized screw locations) to a rod shape customized to the user&#39;s plan for that particular placement. The final rod could be positioned within the patient and the screws and spine would be adjusted to the rod at the desired alignment. 
     In accordance with the Global Spinal Balance feature, spinal parameter inputs may be assessed intraoperatively. For example, the user may wish to intraoperatively measure the amount of lumbar lordosis that has been achieved (for example, after placement of an intervertebral implant). As depicted in  FIGS. 45-46 , the system  10  may include be configured to obtain or import one or more lateral images, generate one or more lines between two or more landmarks on the patient&#39;s anatomy, determine a relationship between those landmarks, and adjust one or more spinal parameters to be used in generating the rod solution. As shown by way of example in  FIG. 45 , the user first selects the intraoperative measurement button  408 . Next, lateral radiographic image  358  may be inputted into the system  10 . The user may touch the screen  200  and move lines  360  over at least two points of interest (e.g. the superior endplate of V1 and the superior endplate of V2) and the system  10  then measures the angle between the two lines  360 . As shown in  FIG. 45 , the system  10  measures this angle as 15 degrees as indicated in the angle measurement field  350 . Optionally, the system may compare the intraoperative measurement to the preoperative and/or target spinal parameter value and provide an indication to the user of how much correction has been achieved relative to the pre-operative and theoretical spinal parameters. Using the angle measurement buttons  328 ,  330 , the user may increase the desired angle of correction of the spine in the sagittal plane (i.e., add or subtract lordosis or kyphosis). As the angle is adjusted, the amount of adjustment may dynamically displayed within angle measurement field  350 . The system  10  may include a color-coded offset distance indicator (not shown) to provide the user with an indication of the distance each digitized screw position will be adjusted in the sagittal plane as described above. Once the desired amount of angular correction is updated, the user may press the “Set” button  332  and then the “Calculate Rod” button (not shown in this view). The system then displays a rod solution  274  incorporating the user&#39;s intraoperative objective for correction of the spine in the sagittal plane. 
     The user may also wish intraoperatively measure the patient&#39;s pelvic incidence angle. As shown in  FIG. 47 , selecting the intra-op measurement button  408  optionally brings up a PI assessment tool. The system  10  obtains a fluoroscopic image  452  of the patient&#39;s pelvis. The user first selects the femoral head button  432  and uses arrows  450  on the PI Adjustment Menu  448  to locate the center point of the femoral head  434 . Next, the user selects the posterior sacrum button  436  and uses arrows  450  to identify the posterior aspect of the sacral endplate  438 . Then, the user selects the anterior sacrum button  440  and uses arrows  450  to identify the anterior aspect of the sacral end plate  442 . With all PI inputs selected, the user may press the “Draw PI” button  446  after which the system  10  automatically draws and measures the pelvic incidence angle  446  for the user. 
     In some circumstances, the user may want to assess the amount/severity of coronal plane decompensation and/or intraoperatively ascertain the amount of correction achieved with a given rod bend configuration. The system may include a Coronal Offset Assessment feature configured to obtain or import one or more Anterior-Posterior images, acquire digital position information regarding landmarks on the patient&#39;s anatomy, generate one or more lines between those landmarks, and determine a relationship between those landmarks. 
     According to some implementations, the system  10  first obtains a fluoroscopic image  454  of the iliac sacral region ( FIG. 48 ). The user digitizes two points  456  and selects the Iliac Line: Set button  460  to establish a horizontal iliac line  458 . Next, the user digitizes the midpoint  462  of the sacrum and selects the CSVL Line: Set button  466  and the system  10  automatically generates an orthogonal line (CSVL line  464 ) from the sacral midpoint  462  to the iliac line  458 . The system  10  then obtains a fluoroscopic image  468  of the C7 vertebra as depicted in  FIG. 49 . The user digitizes the midpoint  470  of the C7 vertebra and selects the “C7PL: Set” button  474  and the system  10  automatically generates an orthogonal line (C7PL line  476 ) from the midpoint  470  of C7 to the iliac line  458 . Finally, the system  10  calculates the coronal offset distance (in box  476 ) using the offset distance between the CSVL line  464  and the C7PL line  476  line. As such, the user is given an intraoperative assessment of the amount of coronal offset corrected or left to be corrected which affords the opportunity to decide if a surgical planning goal has been achieved or if one or more spinal parameter inputs need to be updated with respect to coronal alignment. 
     Once the user has selected the desired rod solution, the user then executes the bends using a mechanical rod bender  18 . It is contemplated that the mechanical rod bender  18  may be any bender that takes into account six degrees of freedom information as it effects bends onto a spinal rod. By way of example, according to one implementation, the mechanical rod bender  18  may be the bender described in commonly-owned U.S. Pat. No. 7,957,831 entitled “System and Device for Designing and Forming a Surgical Implant” patented Jun. 7, 2011, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein. According to a second implementation, the mechanical rod bender  18  may be the bender shown in  FIG. 50 . First and second levers  106 ,  110  are shown as is lever handle  108  designed for grabbing the lever  106  manually and a base  112  for holding lever  110  in a static position. Second lever  110  has a rod pass through  114  so that an infinitely long rod can be used as well as steady the rod during the bending process with the rod bending device  18 . The user grabs handle  108  and opens it to bend a particular rod by picking an angle on the angle gauge  132  and closing the handle  108  such that levers  106 ,  110  are brought closer together. The mechanical rod bender  18  in other embodiments could be produced to bend the rod during the handle opening movement as well. The rod moves through mandrel  118  and in between moving die  120  and fixed die  122 . The rod is bent between the two dies  120 ,  122 . Gauges on the bender  18  allow the user to manipulate the rod in order to determine bend position, bend angle, and bend rotation. The rod is held in place by collet  126 . By sliding slide block  128  along base  112 , the rod can be moved proximally and distally within the mechanical rod bender  18 . Position may be measured by click stops  130  at regular intervals along base  112 . Each click stop  130  is a measured distance along the base  112  and thus moving a specific number of click stops  130  gives one a precise location for the location of a rod bend. 
     The bend angle is measured by using angle gauge  132 . Angle gauge  132  has ratchet teeth  116  spaced at regular intervals. Each ratchet stop represents five degrees of bend angle with the particular bend angle gauge  132  as the handle  106  is opened and closed. It is to be appreciated that each ratchet step may represent any suitable degree increment (e.g., between 0.25 degrees to 10 degrees). The bend rotation is controlled by collet knob  134 . By rotating collet knob  134  either clockwise or counterclockwise, the user can set a particular rotation angle. The collet knob  134  is marked with regular interval notches  136  but this particular embodiment is continuously turnable and thus has infinite settings. Once a user turns knob  134 , the user can set the knob  134  at a particular marking or in between or the like to determine a particular angle rotation to a high degree of accuracy. Additionally, base  112  may have a ruler  138  along its length to aid the user in measuring a rod intraoperatively. 
     According to another implementation, the rod bender  18  may be a pneumatic or motor-driven device which automatically adjusts the location, rotation and bend angle of the rod. By way of example, three motors may be utilized for each movement. A linear translator motor would move the rod in and out of the mandrel  118  and moving die  120 . One rotational motor would rotate the rod and moving die  120 . The bend calculations could be converted into an interface program that would run to power and control the motors. The automated bender would lessen the possibility of user error in following the manual bend instructions. It would also increase the resolution or number of bends that can be imparted in the rod making for a smoother looking rod. 
     Throughout this disclosure, discussion has centered around the ability to custom bend with specificity a spinal rod during a spinal fixation procedure utilizing a rod-based spinal fixation construct. An example method for the minimally invasive implantation of such a rod-based spinal fixation construct  510  will now be described with specific reference to  FIG. 51 . First, a spinal fixation anchor  512  is anchored through the pedicle of each vertebra to be fixated (e.g. three vertebra as shown in  FIG. 51 ). For the purpose of this disclosure, spinal fixation anchor  512  may be any type of bone anchor capable of receiving and securing a spinal fixation rod thereto (e.g., a bone screw). By way of example only the spinal fixation anchor  512  includes a rod housing  514  for receiving the spinal rod  516  therein and a shank (not shown) for anchoring the housing  514  to the bone. As shown in  FIG. 51 , each spinal fixation anchor  512  has an extension guide  518  mated thereto for the purpose of guiding the spinal rod  516  into the rod housing  514 . Although not expressly depicted in  FIG. 51 , it is to be understood that the extension guides  518  provide passage through minimally invasive surgical openings in the patient&#39;s skin from the outside to the surgical target site. 
     With the fixation anchors  512  in position, a rod  516  appropriately sized to span the distance between the superior and inferior spinal fixation anchors is selected. The rod  516  may then be prebent or custom-bent according to any of the implementations of the disclosure described above. In a traditional “open” surgical procedure, a long incision is made in the patient and the spinal rod is advanced to each screw via the attached guides at essentially the same time through the long incision. Thus, the incision must be at least as long as the rod. During some insertion techniques the rod  516  is inserted into the guide channel of the first fixation anchor  512  generally parallel to the extension guide while the insertion instrument is angled back towards the remainder of the extension guides  518 . As the inserter is rocked towards the insertion end, the rod  516  advances through each guide. 
     In a minimally invasive procedure, in contrast, the extension guides  518  are placed in a minimally invasive fashion (i.e. through individual openings without a long incision). As such, an additional opening is needed either cephalad or caudal to one terminus of the construct. The rod  516  is inserted (via rod inserter  536 ) through this additional opening and then sequentially guided (below the skin) through each successive extension guide  518 . Once the rod  516  has been fully positioned within the guides relative to each of the rod housings  514 , a rod pusher  520  is advanced along the screw guide and employed to seat the rod  516  within the rod housing  514 . The rod inserter  536  is then detached from the proximal end  522  of the rod  516  and removed from the additional opening. 
     Various examples of rod inserters and rod pushers that may be used (with or without further modification) with the present invention, as well as a more detailed discussion of the surgical technique involved are disclosed in commonly owned and co-pending U.S. patent application Ser. No. 13/456,210, filed on Apr. 25, 2012 and entitled “Minimally Invasive Spinal Fixation System &amp; Method,” the entire contents of which is hereby incorporated by reference into this disclosure as if set forth fully herein. 
       FIG. 52  illustrates one example of a rod pusher  520  that may be adapted for use with the present invention. The rod pusher  520  is used to fully seat (“reduce”) the spinal rod  516  into the rod housing  514  and thereafter insert a lock screw (not shown) to secure the rod  516  to the anchor  512 . The rod pusher  520  may be configured for use with any of the rod guide assemblies, for example the extension guides  518  shown herein. Generally, the rod pusher  520  has a proximal end  526 , a distal end  528 , an elongated central shaft  529  extending longitudinally therebetween. The proximal end  526  may further include a recess  527  configured to mate with an IR tracking array  22  described above. Distal end  528  further includes distal tip  530  for assisting with seating the rod  516  within anchor  512 . Alternatively, the rod pusher  520  may be provided with an integrated IR tracking array  22  located at the proximal end  526 . The rod pusher  520  further includes a handle  532  and a connector  534 . The handle  532  is located near the proximal end  526  and is configured to allow manual operation of the rod pusher  520  by the user. The connector  534  releasably couples the rod pusher  520  to a guide assembly (for example the extension guides  518  described herein). 
       FIG. 53  illustrates one example of a rod inserter  536  that may be adapted for use with the present invention. By way of example only, the rod inserter  536  is an adjustable-angle rod inserter that introduces the spinal rod  516  through the operative corridor at one angle (relative to the inserter), and then is capable of pivoting the rod at the surgical target site. However, fixed angle rod inserters may also be used without departing from the scope of the present invention. 
     By way of example only, the rod inserter  536  includes an outer sleeve  538 , an inner shaft  540 , a handle  542 , and a rod holder  544 . The outer sleeve  538  is an elongated cylindrical member having an inner lumen extending throughout. The inner shaft  540  is an elongated rod member having a proximal end  546  and a distal end (not shown). The proximal end  546  includes a knob  548  that is enables a user to employ the various functions of the rod inserter, which are not pertinent to the present invention. Further discussion of the rod inserter  536  is available in the above-referenced &#39;210 application (incorporated by reference). The proximal end  546  may further include a recess  550  in the top surface of the proximal end configured to mate with a IR tracking array  22  described above. Alternatively, the rod inserter  536  may be provided with an integrated IR tracking array  22  located at the proximal end  546 . The rod holder  544  is configured to releasably hold the proximal end  522  of a spinal rod  516 . 
     Once the fixation anchors  512  have been implanted and the spinal rod  516  has been contoured, in some instances the user may want to use the spatial tracking system  12  of the present disclosure to assist with the insertion and positioning of the spinal rod  516  in the patient. Such use may make minimally invasive placement of the spinal rod  516  of a long construct much easier than currently possible by giving the user a real time digitized image of the location of the rod tip  524  relative to the rod pusher  520  (and implanted anchors  512 /guides  518 ). To facilitate this, the Advance Options menu  294  discussed above ( FIG. 23 ) may include an additional option to initiate the rod tracking program, for example a “Rod Tracking” option (not shown). 
     In the instant case, the rod inserter  536  and rod pusher  520  are each provided with an attached (or integrated) IR tracking array, such as the IR tracking array  22  shown and described above with reference to  FIGS. 2-5 . Operation of the IR tracking array  22  with regard to the rod inserter and/or rod pusher is essentially the same as the operation of the IR tracking array  22  with the digitizer pointer  30  described above. 
     The user selects the “Rod Tracking” button from the Advance Options menu  294  that navigates the user to a Rod Tracking screen as depicted in  FIGS. 54 and 55 . The proximal end of the contoured rod  516  is attached to the rod holder  544  of the rod inserter  536  (with IR tracking array). To calibrate the system  110 , the user inserts the distal tip  524  of the spinal rod  516  into a calibration aperture  550  located on the distal end  528  of the rod pusher  520  and then presses the “Calibrate Rod” button  556  on the GUI. This calibration step calculates the distance and orientation of the rod inserter handle  542  to the rod pusher  520 . 
     The user then inserts the rod pusher  520  (with attached or integrated IR tracking array) into the first targeted extension guide  518  (i.e. the extension guide that this closest to the additional skin opening) and digitizes the screw via the process described above. As shown in  FIG. 54 , at this point the GUI then shows a graphic representation of the targeted screw head  552  (e.g. depicted as a static sphere, detailed tulip, and the like) in both the sagittal and coronal views. The user then inserts the rod  516  though the additional opening and advances it toward the first targeted extension guide  518 . As the distal tip  524  of the rod  516  approaches the target extension guide  518 , the GUI shows a dynamic graphical representation of the distal tip  554  in both distance and direction/orientation relative to the target extension guide  518 . Using this information, the user can then manipulate the rod inserter  536  to accurately guide the distal tip  524  of the rod  516  to the targeted screw, by guiding the graphical representation of the rod tip  554  to the graphical representation of the screw head  552  on the GUI, as shown in  FIG. 55 . Once the distal tip  524  of the rod  526  is successfully positioned within the first extension guide  518 , the rod pusher  520  is removed (or a second rod pusher  520  with IR tracking is used) and inserted into the extension guide  518  for the second targeted screw. This second location is then digitized using the process described above. The user then continues to advance the distal tip  524  of the rod  516  toward and through the second targeted extension guide  518 . This process continues until the entire rod  516  is properly positioned within the extension guides  518 . 
     Once the rod  516  is properly positioned within the extension guides  518 , the rod holder  544  of the rod inserter  536  is detached from the proximal end  522  of the rod  516  and the rod inserter  536  removed from the additional opening, which can then be closed. One or more rod pushers  520  are employed to seat and secure the rod  516  within the rod housing  514  of the screw head, and the pusher  520  and extension guides  518  are removed from the operative corridor(s) before closing the operative wounds. 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown, by way of example only, in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed. On the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined herein.