Patent Publication Number: US-2023157782-A1

Title: Systems and methods for intraoperatively measuring anatomical orientation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/429,566, filed Feb. 10, 2017, and entitled “Systems And Methods For Intraoperatively Measuring Anatomical Orientation,” and now issued as U.S. Pat. No. 11,464,596. U.S. application Ser. No. 15/429,566 claims the benefit of U.S. Provisional Application No. 62/294,730, filed Feb. 12, 2016 and entitled “Systems And Methods For Intraoperatively Measuring Anatomical Orientation,” as well as U.S. Provisional Application No. 62/344,642, filed Jun. 2, 2016 and entitled “Systems And Methods For Intraoperatively Measuring Anatomical Orientation.” Each of these applications is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure is related to systems and methods for measuring anatomical position and/or orientation. In some embodiments, systems and methods quantitatively measure changes in the position and/or orientation of a portion of a patient&#39;s anatomy with respect to another portion of the patient&#39;s anatomy during surgery. 
     BACKGROUND 
     Many surgical procedures require a surgeon to intraoperatively assess changes in the position or orientation of one or more portions of a patient&#39;s anatomy. However, even in open surgeries, there can be obstructions that prevent a surgeon from viewing relevant anatomy at a surgical site, e.g., blood, adjacent soft tissue, etc. Traditional surgical procedures use imaging techniques, such as CT-scans, x-rays, etc., to pre-operatively plan for a desired anatomical correction and then to post-operatively assess whether the desired anatomical correction has been achieved. Viewing the anatomical changes intraoperatively using such imaging techniques can be difficult, however, as it may require interruption of the surgery. Also, many imaging techniques only provide snapshots illustrating progressive changes in a qualitative manner, but do not provide data of changes as they occur in real-time. A further limitation of such imaging techniques is that they may only provide qualitative data, thus requiring a surgeon to make a subjective assessment of when a desired anatomical orientation has been achieved. Such imaging techniques also expose the patient and the operating room staff to potentially-harmful radiation. 
     During a traditional pedicle subtraction osteotomy, surgeons remove bone from a vertebra of a patient suffering from a spinal deformity to correct spinal curvature. To intraoperatively determine when the appropriate amount of bone has been removed, the surgeon must be able to accurately assess the amount of correction that has been achieved at a given time. Traditionally, to make this assessment, the surgeon must step back from the surgical procedure while an imaging device is brought in and positioned to view the curvature of the spine. However, this provides only a subjective measure of angular correction and involves an interruption in the surgical procedure, adding time and inconvenience. Often times, this results in sub-optimal patient outcomes and repeat surgeries due to over- or under-correction of the deformity. 
     Thus, there is a need for improved systems and methods for intraoperatively measuring anatomical position and/or orientation. 
     SUMMARY 
     Systems and methods are disclosed in which changes in the position and/or orientation of an anatomical structure or of a surgical tool can be measured quantitatively during surgery. In some embodiments, a surgical electronic module can be configured to attach to a portion of a patient&#39;s anatomy and/or to a surgical device, to continually detect changes in a position and/or orientation of the patient&#39;s anatomy and/or the surgical device during surgery, and to communicate the changes to a user. Where the surgical device is attached to a portion of a patient&#39;s anatomy and/or is used to manipulate the patient&#39;s anatomy, the surgical electronic module can detect changes in the position and/or orientation of said anatomy. In embodiments where more than one module is used during surgery, the modules can continually detect changes in their positions and/or orientations relative to one another, which correspond to changes in relative positions and/or orientations of portions of the patient&#39;s anatomy and/or the surgical devices to which the modules are attached. 
     In one exemplary embodiment, a surgical electronic module is provided that includes a housing having one or more engagement features that are configured to removably attach the housing to a surgical device, a sensor, a processor, and a display. The sensor can be disposed in the housing and can be configured to detect a position or orientation of the module with respect to the earth. The processor can be coupled to the sensor and can be configured to calculate a change in position or orientation of the surgical device with respect to one or more reference points when the surgical device is attached to the module, based on the position or orientation detected by the sensor. The display can be configured to display the change calculated by the processor to thereby assist a user in assessing changes in position or orientation of anatomy coupled to or manipulated by the surgical device. In some embodiments, the display can be disposed on the housing. 
     In some embodiments, the surgical electronic module can include additional components. By way of non-limiting example, the surgical electronic module can further include a reset mechanism that, when actuated, sets an initial position or orientation of the module to be used in calculating the change in the position or orientation of the surgical device. Additionally or alternatively, the surgical electronic module can include a memory configured to store at least one of the position or orientation detected by the sensor and the change calculated by the processor. In still further embodiments, the surgical electronic module can include a communications interface configured to send the position or orientation detected by the sensor to an external device and to receive a position or orientation of the one or more reference points from the external device. The external device can be a second surgical electronic module. 
     In some embodiments, the one or more reference points can include a second surgical electronic module. In some embodiments, the sensor can be configured to detect the position or orientation at predetermined time intervals and/or the processor can be configured to calculate the change at the predetermined time intervals. The processor can further be configured to calculate first, second, and/or third derivatives of the position or orientation of the surgical device. In still further embodiments, the one or more engagement features can be configured to identify an aspect of the surgical device when the surgical device is attached to the module. 
     In another aspect, a surgical method is provided for measuring a change in anatomical position or orientation. The method can involve detecting an absolute angle of a first electronic module attached to a first surgical device by a sensor of the first electronic module, with the first surgical device being operatively coupled with a first portion of a patient&#39;s anatomy and detecting an absolute angle of a second electronic module attached to a second surgical device by a sensor of the second electronic module, with the second surgical device being operatively coupled with a second portion of the patient&#39;s anatomy. The method can also include calculating by a processor of at least one of the first and second electronic modules a change in an angle of the first electronic module with respect to the second electronic module multiple times during a surgery to determine a change in an angle of the first surgical device with respect to the second surgical device. The method can further include conveying to a user the change in the angle of the first surgical device with respect to the second surgical device to thereby assist the user in determining a change in an angle of the first portion of the patient&#39;s anatomy with respect to the second portion of the patient&#39;s anatomy. In some embodiments, the change in the angle of the first surgical device with respect to the second surgical device is conveyed to the user on a display of at least one of the first electronic module and the second electronic module. 
     In some embodiments, the method can further include actuating reset mechanisms of the first and second electronic modules to set an initial angle of the first module with respect to the second module. The initial angle can be used in calculating the change in the angle of the modules relative to one another. The calculating and the conveying steps can be repeated until a target position or orientation of the first surgical device with respect to the second surgical device has been reached. In such embodiments, the method can further include alerting the user when the target position or orientation has been reached. In still further embodiments, the method can include calculating a rate of the change in the angle of the first surgical device with respect to the second surgical device. 
     In some embodiments, the first and second portions of the patient&#39;s anatomy are first and second vertebra on opposite sides of an osteotomy site. When the first portion of the patient&#39;s anatomy is a first vertebra and the first surgical device is a first bone screw implanted in the first vertebra, the method can further include attaching the first electronic module to the first bone screw. Additionally, when the second portion of the patient&#39;s anatomy is a second vertebra disposed opposite an osteotomy site from the first vertebra and the second surgical device is a second bone screw implanted in the second vertebra, the method can further include attaching the second electronic module to the second bone screw. In such embodiments, the method can also include locking a spinal rod to the first and second bone screws after a target position or orientation of the first vertebra with respect to the second vertebra has been reached. 
     In yet another aspect, a surgical method is provided for guiding a surgical instrument. The method can include detecting an orientation of a first electronic module that is attached to the surgical instrument by a sensor of the first electronic module, detecting a position of the first electronic module via communications between the first electronic module and at least two electronic modules attached to at least two surgical devices, calculating by a processor of the first electronic module a change in the orientation of the surgical instrument and a change in the position of the surgical instrument over time, and conveying to a user the change in the orientation and the position of the surgical instrument to thereby assist the user in guiding the surgical instrument during surgery. In some embodiments, the change in the position and the orientation of the surgical instrument is conveyed to the user on a display of the first electronic module. In some embodiments, the at least two surgical devices do not move with respect to a patient&#39;s anatomy while the user is guiding the surgical instrument. 
     In another aspect, a surgical electronic module is provided that can include a housing, a sensor disposed in the housing and configured to detect a position or orientation of the module, an input disposed on an outer surface of the housing, and a processor coupled to the sensor and configured to record the position or orientation of the module in response to instruction from the input. The processor can be further configured to simultaneously calculate a change in position or orientation of the module in two orthogonal planes with respect to a reference point based on the position or orientation detected by the sensor. The module can also include a display configured to display the change calculated by the processor to thereby assist a user in assessing changes in position or orientation of anatomy coupled to the module. 
     Any of a variety of additional features or substitutions are possible. For example, in some embodiments the housing can include an engagement feature configured to removably attach the housing to a surgical instrument. In other embodiments, however, the module can be integrally formed in a surgical instrument. Any of a variety of surgical instruments can be utilized, including, for example, any of an osteotome, a chisel, a deformity correction instrument, a probe, an awl, a drill, a tap, and a gearshift. 
     The sensor included in the electronic module can be an inertial motion sensor that can include any of an accelerometer, a gyroscope, and a magnetometer. In some embodiments, for example, the sensor can be a 9-axis inertial motion sensor which can include an accelerometer, a gyroscope, and a magnetometer. In other embodiments, alternative sensors can be utilized that can include a subset of these components, e.g., an inertial motion sensor including solely a 3-axis accelerometer, a “10-axis” sensor including an altimeter, an “11-axis” sensor including a temperature sensor, etc. Moreover, in some embodiments a sensor can be configured to utilize only a subset of its available components, e.g., a 9-axis sensor having an accelerometer, a gyroscope, and a magnetometer that can be configured to make use of, for example, only the accelerometer and the gyroscope (thereby forming an effective “6-axis” sensor). In other embodiments various other sensors can be employed that can provide position and/or orientation information. 
     The input can have a variety of forms. In some embodiments, for example, the input can be a single button. In other embodiments, the input can include a plurality of buttons (e.g., 4 buttons, 5 buttons, etc.). In still other embodiments, button alternatives can be utilized, including switches, pressure sensitive sensors, etc. The buttons, switches, etc. can each be selectively lighted in some embodiments to provide feedback to a user and/or to prompt a user for input (e.g., a first button can light up to prompt recording of a first position or orientation, and a second button can light up to prompt recording of a second position or orientation). 
     As described in more detail below, in some embodiments the electronic module can be configured to calculate changes between two positions and project those changes into a previously-defined orthogonal reference planes. In some embodiments, the module can include a reference plane alignment feature for defining the two orthogonal planes. The reference plane alignment feature can be an extended ridge, protrusion, or other feature formed on the housing in some embodiments. 
     In one aspect, a surgical method is provided that can include positioning a surgical instrument in a first position or orientation relative to anatomy, receiving instruction from an input and recording the first position or orientation using a sensor coupled to the surgical instrument and configured to detect a position or orientation thereof, positioning the surgical instrument in a second position or orientation relative to anatomy, receiving instruction from the input and recording the second position or orientation using the sensor, calculating changes in position or orientation between the first and second positions or orientations in two orthogonal planes, and displaying the changes in position or orientation in the two orthogonal planes using a display coupled to the surgical instrument. 
     As with the module described above, a number of variations and additions are possible. For example, in some embodiments the method can further include coupling a surgical electronic module including the sensor and the display to the surgical instrument, whereas in other embodiments the sensor and the display can be integrally formed in the surgical instrument. Any of a variety of surgical instruments can be utilized and, in some embodiments, the surgical instrument can be any of an osteotome, a chisel, a deformity correction instrument, a probe, an awl, a drill, a tap, and a gearshift. 
     As noted above, in some embodiments the input can be a single button, while in other embodiments the input can be a plurality of buttons. In still other embodiments, any of a variety of button alternatives, such as switches, pressure-sensitive sensors, etc., can be utilized. In an embodiment in which a plurality of buttons is utilized, the method can further include resetting any of the recorded first and second positions or orientations in response to simultaneous depression of more than one button. 
     In some embodiments, the method can further include positioning the surgical instrument in a third position or orientation relative to anatomy, receiving instruction from the input and recording the third position or orientation using the sensor, positioning the surgical instrument in a fourth position or orientation relative to anatomy, and receiving instruction from the input and recording the fourth position or orientation using the sensor. The method can also include calculating changes in position or orientation between the third and fourth positions or orientations as projected onto a first plane defined between the first and second positions or orientations and a second plane orthogonal to the first plane, and displaying the changes in position or orientation between the third and fourth positions or orientations using the display. In this manner, the first and second positions of the surgical instrument recorded by the sensor can be used to define a first reference plane and a second reference plane orthogonal thereto, and position/orientation changes between third and fourth positions can be projected onto the two reference planes. 
     In another aspect, a method for performing bone or tissue correction or manipulation is provided that can include coupling first and second sensors to respective first and second portions of a patient&#39;s anatomy, and actuating an imaging device to capture an image of the first and second portions of the anatomy with the first and second sensors coupled thereto. The method can further include utilizing the image of the first and second portions of the anatomy in a simulated environment to identify a desired anatomical landmark with respect to each of the attached sensors, as well as, using a processor, determining a compensatory angle between each sensor and its respective anatomical landmark, and utilizing the compensatory angles and a measured angle between the first and second sensors to calculate an absolute anatomical angular orientation of the landmarks. 
     In some embodiments, coupling the first and second sensors can include implanting the first and second sensors in the patient. Further, in some embodiments the first and second sensors can be percutaneously implanted in the patient. As explained in more detail below, in some embodiments a mount can be employed to achieve percutaneous implantation of a sensor. 
     In some embodiments, the absolute angular orientation can be calculated in real-time or substantially in real-time, thereby providing a constant update to a surgeon or other user without the need to interrupt a procedure. In still other embodiments, the method can further include coupling a reference plane sensor to a patient&#39;s anatomy to define a reference plane in which angular orientation can be measured. In some embodiments, the reference plane sensor can be used to define a first reference plane and a second reference plane orthogonal to the first reference plane. 
     In another aspect, a method of measuring an absolute spinal angle is provided that can include coupling first and second sensors to respective first and second portions of a patient&#39;s spine, as well as capturing at least one image of the first and second sensors and the patient&#39;s spine using an imaging device. The method can further include identifying in the at least one captured image first and second vertebral endplates that define the absolute spinal angle to be measured, calculating, by a processor, a first compensation angle between the first endplate and the first sensor in the at least one captured image, and calculating, by a processor, a second compensation angle between the second endplate and the second sensor in the at least one captured image. The method can also include obtaining a relative angle measured between the first and second sensors, adjusting, by a processor, the relative angle based on the first and second compensation angles to calculate the absolute spinal angle, and communicating the absolute spinal angle to a user. 
     In some embodiments, the absolute spinal angle can be calculated and communicated to the user in real-time or substantially in real-time. Further, in some embodiments the absolute spinal angle can be at least one of lumbar lordosis and thoracic kyphosis. 
     In certain embodiments, communicating the absolute spinal angle can include displaying the angle on an electronic display. In other embodiments, different communication methods can be employed, such as audio communication through a speaker, or combinations of audio, visual, and tactile feedback. Further, in some embodiments, identifying the endplates can include interacting with a graphical user interface that displays the at least one captured image to select the endplates with an input device. 
     As with the method described above, in some embodiments coupling the first and second sensors can include implanting the first and second sensors in the patient. Moreover, in certain embodiments the first and second sensors can be percutaneously implanted in the patient. This can be accomplished in some embodiments using a percutaneous mount configured to receive a sensor and to extend into a patient. 
     In still another aspect, a surgical system is provided that can include a first sensor configured to couple to a first portion of a patient&#39;s anatomy, a second sensor configured to couple a second portion of the patient&#39;s anatomy, and an imaging device configured to capture an image of the patient&#39;s anatomy and the first and second sensors. The system can further include a processor configured to receive via an input identification of a desired anatomical landmark with respect to one of the first and second sensors, and determine a compensatory angle between each sensor and its respective anatomical landmark, wherein the processor is further configured to calculate an absolute anatomical angular orientation of the landmarks utilizing the compensatory angles and a measured angle between the first and second sensors. The system can further include a display configured to display the captured image and calculated absolute anatomical angular orientation of the landmarks. 
     The system can include a number of additions or variations. For example, in some embodiments the system can further include a reference plane sensor configured to be coupled to the patient&#39;s anatomy to define a reference plane in which angular orientation can be measured. In certain embodiments, the reference plane sensor can also define a second reference plane orthogonal to the reference plane. In some embodiments, the reference plane sensor can be direction-specific, e.g., a first end can be configured to point toward a user&#39;s head or other portion of anatomy while a second end can be configured to point toward a user&#39;s feet or other portion of anatomy. In such an embodiment, the reference plane sensor can include an orientation indicator to aid a user in aligning the reference plane sensor. An orientation indicator can be an arrow or other marking in some embodiments. In other embodiments, the reference plane sensor can be formed in the shape of a human patient such that a surgeon or other user can intuitively align the shape of the reference plane sensor with the shape of the patient during use. 
     In some embodiments, each of the first and second sensors can be an inertial motion sensor including any of an accelerometer, a gyroscope, and a magnetometer. For example, in some embodiments the sensor can be a 9-axis inertial motion sensor including an accelerometer, a gyroscope, and a magnetometer. In other embodiments, different inertial motion sensors can be utilized having a subset of these components. For example, in some embodiments a sensor having solely a 3-axis accelerometer can be employed, etc. 
     In some embodiments, each of the first and second sensors can be radiopaque to facilitate capture of the sensors by the imaging device. For example, if the imaging device is configured to capture X-ray images, radiopaque sensors can be clearly visible in the captured images. 
     As noted above, in some embodiments the first and second sensors can be implanted in a patient and, in certain embodiments, can be implanted percutaneously. In some embodiments, the system can further include first and second mounts configured to permit percutaneous coupling of the first and second sensors to the patient&#39;s anatomy. Each of the first and second mounts can include a needle-shaped distal portion for percutaneous insertion through a patient&#39;s skin and a proximal recess configured to receive one of the first and second sensors therewithin. 
     Any of the features or variations described above can be applied to any particular aspect or embodiment of the disclosure in a number of different combinations. The absence of explicit recitation of any particular combination is due solely to the avoidance of repetition in this summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1 A  is a schematic illustration of a spherical coordinate system; 
         FIG.  1 B  is a schematic illustration of relevant anatomical planes; 
         FIG.  2    is a schematic illustration of an exemplary surgical electronic module; 
         FIG.  3    is a perspective view of the surgical electronic module of  FIG.  2   ; 
         FIG.  4    is a perspective view of a spine of a patient with a spinal deformity; 
         FIG.  5    is a perspective view of one step of an exemplary method for correcting spinal deformity using surgical electronic modules; 
         FIG.  6    is a perspective view of another step of the method of  FIG.  5   ; 
         FIG.  7    is a perspective view of one step of another exemplary method for correcting spinal deformity using surgical electronic modules; 
         FIG.  8    is a perspective view of another step of the method of  FIG.  7   ; 
         FIG.  9 A  is a side view of one embodiment of a surgical instrument including an electronic module; 
         FIG.  9 B  is a top view of the surgical instrument of  FIG.  9 A ; 
         FIG.  10    is a schematic illustration of one embodiment of an electronic module with a single button input; 
         FIG.  11 A  is a schematic illustration of one embodiment of a surgical instrument with an electronic module measuring changes in position or orientation in a first plane; 
         FIG.  11 B  is a schematic illustration of the surgical instrument of  FIG.  11 A  measuring changes in position or orientation in a second plane orthogonal to the first plane; 
         FIG.  12    is a schematic illustration of one embodiment of a surgical instrument defining first and second orthogonal projection planes in which to measure changes in position or orientation; 
         FIG.  13 A  is a side view of one embodiment of an electronic module with a four-button input; 
         FIG.  13 B  is a top view of the electronic module of  FIG.  13 A ; 
         FIG.  14 A  is a schematic illustration of a first step in operating one embodiment of an electronic module with a four-button input; 
         FIG.  14 B  is a schematic illustration of a second step in operating the electronic module of  FIG.  14 A ; 
         FIG.  14 C  is a schematic illustration of a third step in operating the electronic module of  FIG.  14 A ; 
         FIG.  14 D  is a schematic illustration of a fourth step in operating the electronic module of  FIG.  14 A ; 
         FIG.  14 E  is a schematic illustration of a resetting step in operating the electronic module of  FIG.  14 A ; 
         FIG.  15 A  is a schematic illustration of one embodiment of a surgical instrument with an electronic module measuring changes in position or orientation in a first plane prior to surgical manipulation; 
         FIG.  15 B  is a schematic illustration of the surgical instrument of  FIG.  15 A  measuring changes in position or orientation in a second plane orthogonal to the first plane prior to surgical manipulation; 
         FIG.  15 C  is a schematic illustration of the surgical instrument of  FIG.  15 A  measuring changes in position or orientation in the first plane after surgical manipulation; 
         FIG.  15 D  is a schematic illustration of the surgical instrument of  FIG.  15 A  measuring changes in position or orientation in the second plane after surgical manipulation; 
         FIG.  16    is a schematic illustration of one embodiment of a surgical instrument defining first and second orthogonal projection planes in which to measure changes in position or orientation; 
         FIG.  17 A  is a schematic illustration of one embodiment of a surgical instrument defining a projection plane prior to surgical manipulation; 
         FIG.  17 B  is a schematic illustration of the surgical instrument of  FIG.  17 A  recording position or orientation information at a first point prior to surgical manipulation; 
         FIG.  17 C  is a schematic illustration of the surgical instrument of  FIG.  17 A  recording position or orientation information at a second point prior to surgical manipulation; 
         FIG.  17 D  is a schematic illustration of the surgical instrument of  FIG.  17 A  defining a projection plane after surgical manipulation; 
         FIG.  17 E  is a schematic illustration of the surgical instrument of  FIG.  17 A  recording position or orientation information at the first point after surgical manipulation; 
         FIG.  17 F  is a schematic illustration of the surgical instrument of  FIG.  17 A  recording position or orientation information at the second point after surgical manipulation; 
         FIG.  17 G  is a schematic illustration of the surgical instrument of  FIG.  17 A  displaying calculated position or orientation data after surgical manipulation; 
         FIG.  18 A  is a schematic illustration of one embodiment of a surgical instrument defining a projection plane prior to surgical manipulation; 
         FIG.  18 B  is a schematic illustration of the surgical instrument of  FIG.  18 A  recording position or orientation information at a first point prior to surgical manipulation; 
         FIG.  18 C  is a schematic illustration of the surgical instrument of  FIG.  18 A  recording position or orientation information at a second point prior to surgical manipulation; 
         FIG.  18 D  is a schematic illustration of the surgical instrument of  FIG.  18 A  displaying calculated position or orientation data prior to surgical manipulation; 
         FIG.  18 E  is a schematic illustration of the surgical instrument of  FIG.  18 A  saving calculated position or orientation data displayed in  FIG.  18 D ; 
         FIG.  18 F  is a schematic illustration of the surgical instrument of  FIG.  18 A  recalling calculated position or orientation data displayed in  FIG.  18 D ; 
         FIG.  18 G  is a schematic illustration of the surgical instrument of  FIG.  18 A  defining a projection plane after surgical manipulation; 
         FIG.  18 H  is a schematic illustration of the surgical instrument of  FIG.  18 A  recording position or orientation information at the first point after surgical manipulation; 
         FIG.  18 I  is a schematic illustration of the surgical instrument of  FIG.  18 A  recording position or orientation information at the second point after surgical manipulation; 
         FIG.  18 J  is a schematic illustration of the surgical instrument of  FIG.  18 A  displaying calculated position or orientation data; 
         FIG.  19 A  is a schematic illustration of one embodiment of a surgical instrument defining a projection plane; 
         FIG.  19 B  is a schematic illustration of the surgical instrument of  FIG.  19 A  recording position or orientation information at a first point; 
         FIG.  19 C  is a schematic illustration of the surgical instrument of  FIG.  19 A  recording position or orientation information at a second point; 
         FIG.  19 D  is a schematic illustration of the surgical instrument of  FIG.  19 A  displaying calculated position or orientation data; 
         FIG.  20 A  is a schematic illustration of one embodiment of an electronic module configured to selectively couple to a surgical instrument; 
         FIG.  20 B  is a schematic illustration of the electronic module and surgical instrument of  FIG.  20 A  measuring a first position or orientation relative to earth; 
         FIG.  20 C  is a schematic illustration of the electronic module and surgical instrument of  FIG.  20 A  being zeroed at the first position or orientation; 
         FIG.  20 D  is a schematic illustration of the electronic module and surgical instrument of  FIG.  20 A  at a second position or orientation measuring a relative change from the first position or orientation; 
         FIG.  21 A  is a schematic illustration of one embodiment of a surgical instrument recording a first pedicle screw trajectory; 
         FIG.  21 B  is a schematic illustration of the surgical instrument of  FIG.  21 A  aligned at a second pedicle screw trajectory that mirrors the first pedicle screw trajectory; 
         FIG.  22 A  is a schematic illustration of one embodiment of a surgical instrument for spinal rod bending in a first position; 
         FIG.  22 B  is a schematic illustration of the surgical instrument of  FIG.  22 A  in a second position and displaying position or orientation change from the first position; 
         FIG.  23    is a schematic illustration of one embodiment of a surgical system according to the teachings provided herein; 
         FIG.  24    is a schematic illustration of exemplary spinal regions and spinopelvic parameters; 
         FIG.  25 A  is a schematic illustration of various embodiments of a display according to the teachings provided herein; 
         FIG.  25 B  is a schematic illustration of various embodiments of surgical system components according to the teachings provided herein; 
         FIG.  25 C  is a schematic illustration of various embodiments of sensor mounts according to the teachings provided herein; 
         FIG.  25 D  is a schematic illustration of exemplary sensor placement on patient anatomy; 
         FIG.  25 E  is another schematic illustration of exemplary sensor placement on patient anatomy; 
         FIG.  25 F  is a schematic illustration of various embodiments of image capture according to the teachings provided herein; 
         FIG.  25 G  is a schematic illustration of one embodiment of a graphical user interface; 
         FIG.  25 H  is a schematic illustration of another embodiment of a graphical user interface; 
         FIG.  26 A  is a schematic illustration of one embodiment of sensors mounted percutaneously to patient anatomy; 
         FIG.  26 B  is a schematic illustration of one embodiment of sensors mounted to a spinal fixation element; 
         FIG.  26 C  is a schematic illustration of one embodiment of sensors mounted to derotation instruments; 
         FIG.  26 D  is a schematic illustration of one embodiment of sensors mounted to an osteotomy closure instrument; 
         FIG.  27 A  is a schematic illustration of one embodiment of a graphical user interface showing pre-operative measurements; 
         FIG.  27 B  is a schematic illustration of one embodiment of a graphical user interface showing user selection; 
         FIG.  27 C  is a schematic illustration of one embodiment of a graphical user interface showing pre-operative measurements and correction targets; 
         FIG.  27 D  is a schematic illustration of one embodiment of a graphical user interface menu; 
         FIG.  27 E  is a schematic illustration of one embodiment of a graphical user interface showing instructions for sensor activation; 
         FIG.  27 F  is a schematic illustration of one embodiment of a graphical user interface showing instructions for sensor placement; 
         FIG.  27 G  is a schematic illustration of one embodiment of a graphical user interface showing instructions for sensor placement and calibration; 
         FIG.  27 H  is a schematic illustration of one embodiment of a graphical user interface showing intraoperative measurements; 
         FIG.  27 I  is a schematic illustration of one embodiment of a graphical user interface showing instructions for relocating sensor to instrument; and 
         FIG.  27 J  is a schematic illustration of one embodiment of a graphical user interface showing intraoperative sensor measurements in two orthogonal planes. 
     
    
    
     DETAILED DESCRIPTION 
     Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those of skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention. 
     In the present disclosure, like-numbered components of the embodiments generally have similar features and/or purposes. Further, to the extent that linear or circular dimensions are used in the description of the disclosed systems, devices, and methods, such dimensions are not intended to limit the types of shapes that can be used in conjunction with such systems, devices, and methods. A person skilled in the art will recognize that an equivalent to such linear and circular dimensions can easily be determined for any geometric shape. Sizes and shapes of the systems and devices, and the components thereof, can depend at least on the size and shape of the components with which the systems and devices are being used, the anatomy of the patient, and the methods and procedures in which the systems and devices will be used. The figures provided herein are not necessarily to scale. 
     Systems and methods are disclosed in which changes in a position and/or orientation of an anatomical structure or of a surgical tool can be measured quantitatively during surgery. In some embodiments, a surgical electronic module can be configured to attach to a surgical device, to continually detect changes in a position and/or orientation of the surgical device during surgery, and to communicate the changes to a user. In this way, where the surgical device is attached to a portion of a patient&#39;s anatomy and/or is used to manipulate the patient&#39;s anatomy, the surgical electronic module can detect changes in the position and/or orientation of said anatomy. In embodiments where more than one module is used during surgery, the modules can continually detect changes in their positions and/or orientations relative to one another, which correspond to changes in relative positions and/or orientations of the surgical devices to which the modules are attached. Additionally or alternatively, at least one of the modules can help to establish a reference 3D location in the operating room, particularly where the at least one of the modules is stationary. In some embodiments, the modules can include a resetting or “zeroing” function that allows a user to selectively set an initial relative position and/or orientation of the modules to zero. Subsequent changes in the relative positions and/or orientations of the modules can then be measured and displayed to the user so that the user knows when a desired change in position and/or orientation of the modules has been reached. In some embodiments, all of the components necessary for detecting, calculating, and/or communicating positional information (i.e., position and/or orientation) are contained within the module itself, thus eliminating the need for an external base station or other additional bulky equipment. By thus providing a means for quantitatively measuring changes in anatomical orientation in real-time during surgery, exemplary systems and methods provided herein can enhance the accuracy of the surgery and reduce or eliminate the need for intraoperative imaging, thereby reducing radiation exposure and increasing efficiency. 
     The positional information detected and/or calculated by the surgical electronic module can include one or more angles of the module with respect to the earth (referred to hereinafter as “absolute” angles), one or more angles of the module with respect to a some other reference point (referred to hereinafter as “relative” angles), distances between the module and one or more external reference points, changes in any of these values, a rate of changes in any of these values, and/or higher order derivatives of any of these values. The module can be configured to detect and/or calculate the positional information in a variety of units and coordinate systems. To provide relevant anatomical measurements during surgery, in some embodiments the module can be configured to translate positions and/or orientations detected in a spherical coordinate system, illustrated in  FIG.  1 A  and defined by an absolute azimuth angle θ, an absolute polar angle φ, and a radial distance r, into positions and/or orientations along the sagittal, axial, and coronal planes, illustrated in  FIG.  1 B . 
     The surgical electronic module can include one or more components for detecting, processing, communicating, and/or storing positional information of the module and the surgical device to which it is attached. As schematically illustrated in  FIG.  2   , an exemplary module  10  can include a processor  22 , a memory  24 , a communications interface  26 , and a sensor  28 —all of which can be in communication with each other. Any of these components can exist external to the module  10 , however, for example at a remote base station configured to communicate with the module  10  through the communications interface  26 . Further, although each of these components are referred to in the singular, it will be appreciated by a person skilled in the art that the various functions described as being carried out by one of the components can actually be carried out by multiple of those components, e.g., the functions described as being carried out by the processor  22  can be carried out by multiple processors. The electrical components can be powered by a battery contained within the module  10 , for example a lithium ion battery, or can be powered by an external power source coupled to the module  10  via an adaptor. 
     The sensor  28  can be or can include any type of sensor that is configured to detect positional information of the module  10 . By way of non-limiting example, the sensor  28  can include an accelerometer (e.g., a 9-axis accelerometer for measuring one or more angles of the module  10  with respect to a reference point such as the earth), a gyroscopic sensor, a geomagnetic sensor, and the like. Additionally or alternatively, where the module  10  is configured to detect a distance of the module from a reference point, the sensor  28  can include ultrasound, electromagnetic, and/or infrared transceivers for communicating with a positioning system. In an exemplary embodiment, the sensor  28  can be configured to detect an absolute position and/or orientation of the module in the spherical coordinate system. The sensor  28  can be configured to detect the positional information at intervals throughout a surgical procedure, for example every second, every millisecond, every microsecond, etc., such that the positional information is effectively detected continuously and in real-time. The positional information can be detected regularly, intermittently, or at non-regular intervals. The positional information can be conveyed to the surgeon, stored in the memory  24 , conveyed to the processor  22  for processing, and/or communicated to one or more external devices via the communications interface  26  for processing or storage. 
     Where the sensor  28  is configured to detect both an orientation and a position (e.g., a distance of the module  10  from some reference point), the module  10  can be configured to switch between an orientation detection mode in which the sensor  28  detects only the orientation and a full detection mode in which the sensor  28  detects both the orientation and the position. The module  10  can be configured to switch between the orientation detection mode and the full detection mode at the request of the surgeon, for example via actuation of an input device on the module  10 , and/or based on an identity of the surgical device to which the module  10  is attached. 
     The processor  22  can include a microcontroller, a microcomputer, a programmable logic controller (PLC), a field-programmable gate array (FPGA), an application specific integrated circuit (ASIC), integrated circuits generally referred to in the art as a computer, and other programmable circuits, and these terms are used interchangeably herein. The processor  22  can be configured to generate positional information and/or perform various calculations based on the positional information detected by the sensor  28 , stored in the memory  24 , and/or received from an external device via the communications interface  26 . By way of non-limiting example, the processor  22  can be configured to calculate a relative position and/or orientation of the module  10  with respect to an external reference point based on an absolute position and/or orientation of the module  10  that is detected by the sensor  28  and/or an absolute position and/or orientation of the external reference point that is received through the communications interface  26 . The processor  22  can be configured to calculate changes in the absolute and relative positions and/or orientations of the module  10  and/or a speed at which those changes occur, which will correspond to changes and/or a speed of the surgical device to which the module  10  is attached. 
     The processor  22  can be coupled to the memory  24 , which can include a random access memory (RAM), a read-only memory (ROM), a flash memory, a non-transitory computer readable storage medium, and so forth. The memory  24  can store instructions for execution by the processor  22  to implement the systems disclosed herein or to execute the methods disclosed herein. Additionally or alternatively, the memory  24  can store the positional information sensed by the sensor  28 , calculated by the processor  22 , and/or received from an external device through the communications interface  26 . 
     The communications interface  26  can be configured to receive and transmit information from any of the processor  22 , the memory  24 , and the sensor  28  with one or more external devices, e.g., another surgical electronic module, a base station, etc. The communications interface  26  be wireless (e.g., near-field communication (NFC), Wi-Fi, Bluetooth, Bluetooth LE, ZigBee, and the like) or wired (e.g., USB or Ethernet). In the case of NFC, for example, the module  10  can include a radio transceiver configured to communicate with a radio transceiver of another device, e.g., a second module, using one or more standards such as ISO/IEC 14443, FeliCa, ISO/IEC 18092, and those defined by the NFC Forum. The communication interface  26  can be selected to provide the desired communication range. In some embodiments, Bluetooth (e.g., class 2 Bluetooth having a range of 5-10 meters) can be used for the communication interface to allow the module  10  to remain somewhat distant from the device with which it is communicating, e.g., the second module and/or a base station, while at the same time limiting the communication range such that other mobile devices unlikely to be used in the surgery are not needlessly involved. 
     As shown in  FIG.  3   , the exemplary module  10  can include a proximal housing  20  and a distal shaft  30 . In general, the housing  20  and the shaft  30  can be any size and shape configured to be inserted at least partially into a patient&#39;s body during surgery while not substantially obstructing a surgeon&#39;s view of or access to a surgical site. Any or all of the above described components for detecting, processing, and/or communicating positional information can be housed within the housing  20 . Further, the housing  20  can have various external features for inputting and outputting the positional information, for example the housing  20  can include an electronic display  50  for communicating information detected and/or calculated by the module  10  and/or a zeroing button  60  to allow the user to indicate that the module  10  is in an initial position and/or orientation. The shaft  30  can extend distally from the housing  20  and can be configured to rigidly and mechanically attach the module  10  to the surgical device such that changes in the position and/or orientation of the surgical device result in corresponding changes in the position and/or orientation of the module  10 . 
     The display  50  can be configured to communicate the positional information detected and/or calculated by the module  10  to assist the surgeon in assessing anatomical changes effected by the surgical device to which the module  10  is attached. In the illustrated embodiment, the display  50  is formed on a proximal-facing surface of the housing  20 , although the display  50  can be located anywhere on the module  10 , e.g., such that it is visible to the surgeon during surgery, or it can be located remotely from the module. The display  50  can be any type of display screen, e.g., liquid crystal, light emitting diode, etc., and in some embodiments can be configured to withstand exposure to sterilization, liquids, and/or high levels of moisture. In an exemplary embodiment, the display  50  can display a change in the absolute or relative position and/or orientation of the module  10  during surgery, which corresponds to a change in the position and/or orientation of the surgical device to which the module  10  is attached. In some embodiments, the display  50  can additionally or alternatively provide positive and/or negative feedback to the surgeon about the position and/or orientation of the module  10 . By way of non-limiting example, when the module  10  detects that a desired position and/or orientation has been reached, the display  50  can provide positive feedback to the surgeon, e.g., a green light. When the module  10  is determined to be outside a desirable positional range, the display  50  can provide negative feedback to the surgeon, e.g., a red light, an error message, etc. Other means for communicating information to the surgeon can include, without limitation, a vibrator, a speaker or buzzer for providing audio feedback and an internal or external display in communication with the module  10  for providing visual feedback. The external display can be larger than the display  50  and, in some embodiments, can provide a real-time graphical illustration of the movement of the module  10  and optionally one or more other modules during surgery. 
     The positional information output by the module  10 , for example on the display  50 , can be reset to zero at any time by user actuation of a resetting or “zeroing” mechanism to thereby indicate that the module  10  is in an initial position and/or orientation. For example, a position and/or orientation of the module  10  displayed at a starting point of the surgery can to be set to zero upon actuation of the zeroing button  60  by the surgeon, although it will be appreciated by a person skilled in the art that the zeroing mechanism can be any feature on the module  10  or it can be remote to the module  10 . After the zeroing button  60  has been pressed, the display  50  can display a change in the position and/or orientation of the module  10  relative to a zero position and/or orientation, such that the surgeon can readily know the difference between the initial position and/or orientation of the module  10  and a current position and/or orientation of the module  10 . Thus, where the surgery requires changing a position and/or orientation of a patient&#39;s anatomy that is connected to the module  10  via the surgical device by a desired amount, the surgeon can know that the desired change has been effected when the desired change of the module  10  is displayed on the display  50 . In some embodiments, actuation of the button  60  can also initiate detection and/or calculation of the position and/or orientation of the module  10 . 
     The module  10  can be configured to attach directly to a patient&#39;s anatomy and/or to the surgical device via one or more engagement features  40  formed on a distal portion of the module  10 , for example on the distal end of the shaft  30 . The surgical device can be anything used in the operating room that facilitates the surgery, including, by way of non-limiting example, surgical implants, surgical instruments, fixtures in the operating room, e.g., an operating table, etc. The engagement features  40  can be specifically configured to mate the module  10  only to a single type of surgical device, or they can be adaptable or modular to allow for mating of the module  10  to any of a variety of surgical devices. Further, the engagement features  40  can be configured to mate the module  10  to more than one surgical device at a time. The engagement features  40  can provide for direct rigid mechanical attachment of the module  10  to the surgical device to thereby ensure that changes in a position and/or orientation of the surgical device result in corresponding changes in the position and/or orientation of the module  10 . In some embodiments, the engagement features  40  can be configured to rigidly attach to engagement features of another surgical electronic module to calibrate the module  10  with the other surgical electronic module, e.g., by synchronizing coordinate systems. Non-limiting examples of engagement features  40  include a snap mechanism, a lock-and-key mechanism, an electronic contact, a screw or other threaded feature, etc. 
     In some embodiments, the engagement features  40  can be configured to detect identification information about the surgical device to which the module  10  is attached. For example, the engagement features  40  can comprise one or more buttons, switches, pressure transducers, etc. that are configured to align with one or more protrusions on the surgical device. The number and arrangement of protrusions can serve to uniquely identify the surgical device. In this way, the number and arrangement of buttons or other components that are engaged by the one or more protrusions on the surgical device can convey identification information about the surgical device. In another embodiment, the engagement features  40  can include a radio frequency identification (RFID) transceiver or optical scanner that is configured to read a unique device identifier (UDI) contained in either an RFID tag or bar code, respectively, on the surgical device. The identification information can include a type of the surgical device, a serial number of the surgical device, an angle at which the surgical device is configured to attach to the module  10 , an age of the surgical device, an intended use of the surgical device, etc. 
     The identification information can be conveyed to the surgeon, for example to ensure that the module  10  has been securely attached to the correct surgical device. Where the module  10  is determined not to have been attached to the correct surgical device, the module  10  can alert the surgeon to the error, for example by displaying an error message on the display  50 . In some embodiments, where the identification information includes an angular offset of a portion of the surgical device from the module  10  when the surgical device is attached to the module  10 , the identification information can be used to calculate an absolute position and/or orientation of that portion of the surgical device. Additionally or alternatively, the identification information, e.g., a type of the surgical device, can cause the module  10  to detect and/or calculate different types of positional information. By way of non-limiting example, the module  10  can be configured to switch into the full detection mode when the engagement features  40  detect that the module  10  is connected to a surgical instrument that is intended to change position and orientation during surgery, and into the orientation detection mode when the engagement features  40  detect that the module  10  is connected to a surgical device, e.g., an implant, that is only or primarily intended to change orientation during the surgery. In still further embodiments, where the module  10  is in communication with an external display that provides a graphical depiction of the surgery in real-time based on positional information transmitted from the module  10 , the external display can use the identification information to incorporate an illustration of the surgical device to which the module  10  is attached in the graphical depiction. The identification information can be stored along with positional information collected and/or calculated by the module  10  during surgery, e.g., to facilitate later reconstruction of the surgery. 
     The surgical electronic modules disclosed herein can generally be used to detect a position and/or orientation of a surgical device to which they are attached as well as changes in said position and/or orientation. Where the surgical device is also attached to a portion of a patient&#39;s anatomy, the surgical electronic module can be used to detect a position and/or orientation of that portion of the patient&#39;s anatomy as well as changes in said position and/or orientation. In an exemplary embodiment, two surgical electronic modules can be attached to two pedicle screws to detect an amount of correction in a patient&#39;s spinal curvature during a pedicle subtraction osteotomy. 
     The steps of an exemplary pedicle subtraction osteotomy utilizing the module  10  and a second module  10   a , which can be identical to the module  10 , are illustrated in  FIGS.  4 - 6   . However, it will be appreciated by a person skilled in the art that any surgical electronic module as described herein can be used, either the same or different from one another, and that the modules  10 ,  10   a  can be used in a variety of surgical procedures that effect changes in anatomical position and/or orientation. Further, it will be appreciated that the calculations said to be performed by the modules  10 ,  10   a  can either be performed by both of the processors  22 ,  22   a  or by only one of the processors  22 ,  22   a . Where the below-described calculations are performed by both of the processors  22 ,  22   a , the modules  10 ,  10   a  can communicate the results of the calculations with one another to check for accuracy and can display an error message to the user when there is a mismatch. The calculations can also be performed by a remote base station configured to receive positional information from the modules  10 ,  10   a.    
     As shown in  FIG.  4   , prior to the exemplary osteotomy procedure, the patient&#39;s lumbar spine includes a kyphotic deformity in which a first vertebra VI is positioned at an angle α in the sagittal plane relative to a second vertebra V2. A purpose of the osteotomy procedure can be to reduce the angle α to a desired value, e.g., by removing a corresponding amount of bone from a vertebra disposed between the vertebrae VI, V2. The amount of angular correction can be determined based on pre-operative imaging and calculations. In some embodiments, prior to surgery, the modules  10 ,  10   a  can be calibrated to one another to thereby synchronize their coordinate systems. For example, the modules  10 ,  10   a  can be mechanically connected to one another via, e.g., the engagement features  40 ,  40   a , and rotated through space as a pair until sufficient information has been gathered to synchronize their coordinate systems. An output signal can be provided to the user, for example a visual signal on the display  50 , a vibration, and/or an audio signal, to indicate when synchronization has been completed. Such synchronization can facilitate automated error correction, e.g., for axial rotations, and/or can facilitate quantification of coronal plane changes. 
     First and second pedicle screws  70 ,  70   a  can be implanted into first and second pedicles P1 and P2 of first and second vertebrae VI and V2, as shown in  FIG.  5   , according to customary surgical procedures. The first module  10  can be rigidly attached to the first pedicle screw  70  and the second module  10   a  can be rigidly attached to the second pedicle screw  70   a  via the engagement features  40 ,  40   a  of the first and second modules  10 ,  10   a . The modules  10 ,  10   a  can be attached to the pedicle screws  70 ,  70   a  either before or after the pedicle screws are implanted. Through this series of connections, changes in the positions and/or orientations of the first and second modules  10 ,  10   a  can correspond to changes in positions and/or orientations of the first and second pedicle screws  70 ,  70   a , respectively, and to changes in positions and/or orientations of the first and second pedicles PI, P2 to which the pedicle screws  70 ,  70   a  are attached. 
     Once the modules  10 ,  10   a  have been attached to the screws  70 ,  70   a  and the screws  70 ,  70   a  have been implanted in the pedicles PI, P2 in an initial position and/or orientation, the modules can be powered up and the zeroing buttons  60 ,  60   a  can be actuated to indicate to the modules  10 ,  10   a  that the modules  10 ,  10   a  are in the initial position and/or orientation. Thus, as shown in  FIG.  5   , the displays  50 ,  50   a  can each display “0” to indicate that the modules  10 ,  10   a  are oriented at an initial angle relative to one another. As the procedure is performed, the sensors  28 ,  28   a  of the modules  10 ,  10   a  can detect absolute azimuth and polar angles θ, φ of each of the modules  10 ,  10   a  with respect to the earth. The modules  10 ,  10   a  can communicate their absolute azimuth and polar angles θ, φ to one other via the communications interfaces  26 ,  26   a . Given this information, the processors  22 ,  22   a  can then calculate a relative angle ß of the first module  10  with respect to the second module  10   a  in the sagittal plane (e.g., by subtracting the absolute angles measured by the modules). The relative angle ß at the initial position and/or orientation of the modules  10 ,  10   a  can be stored in the memories  24 ,  24   a.    
     Angular correction of the spine along the sagittal plane can then be performed according to customary surgical procedures, which can include removal of bone between the first and second vertebrae VI, V2 at an osteotomy site O of a vertebra disposed between the vertebrae VI, V2. During the correction, the sensors  28 ,  28   a  can continually detect the absolute azimuth and polar angles θ, φ of the modules  10 ,  10   a  and the processors  22 ,  22   a  can continually calculate the relative angle ß based on the updated azimuth and polar angles θ, φ. As the relative angle ß changes during the surgery, the processors  22 ,  22   a  can further calculate a change Δß in the relative angle ß over a specified period of time. In the illustrated embodiment, where the modules  10 ,  10   a  were zeroed at the initial position and/or orientation, the change Δß in the relative angle ß between the initial angle and the current angle (and thus the amount of correction achieved) can be displayed on the displays  50 ,  50   a . In this way, the user can be provided with a real-time, quantitative measurement of angular correction throughout the surgery. When the desired angular correction has been achieved ( FIG.  6   ), as indicated for example by the value of Δß displayed on the displays  50 ,  50   a , the patient&#39;s spine can be stabilized in the corrected position and/or orientation. 
     In some embodiments, the processors  22 ,  22   a  can further calculate derivatives of values detected by the sensors  28 ,  28   a  and/or calculated by the processors  22 ,  22   a , such as ß, θ, and φ. By way of non-limiting example, the processors  22 ,  22   a  can calculate a first derivative of ß, i.e., a rate of change Δß/Δt in the relative angle ß over time, a second derivative of p, i.e., a relative acceleration Δß/Δt 2 , and/or a third derivative of ß, i.e., a relative jerk Δß/Δt 3  of the modules  10 ,  10   a  The rates of change Δß/Δt, Δθ/Δt and/or Δφ/Δt can be useful for error checking, for example to indicate whether the patient has been accidentally moved during the procedure. For example, in embodiments where the processors  22 ,  22   a  calculate a rate of change Δθ/Δt for each of the modules  10 ,  10   a , it can be assumed that the patient is moving when the rate of change Δθ/Δt of the first module  10  is equal to the rate of change Δθ/Δt of the second module  10   a , since it is unlikely that the first and second modules  10 ,  10   a  would be moved at precisely the same rate as part of the surgical procedure. Thus, when the rate of change Δθ/Δt of the first module  10  is equal, or at least substantially equal, to the rate of change Δθ/Δt of the second module  10   a , either or both modules  10 ,  10   a  can alert the surgeon to the patient&#39;s movement, for example by displaying an error message on the displays  50 ,  50   a . Additionally or alternatively, to provide clinical feedback, the rates of change Δß/Δt, Δθ/Δt and/or Δφ/Δt can be displayed, e.g., on the displays  50 ,  50   a , and/or stored, e.g., in the memories  24 ,  24   a . Information about the rates of change Δß/Δt, Δθ/Δt and/or Δφ/Δt can be useful for clinicians because they provide a measure of how quickly an anatomical adjustment is made, which may correlate to patient outcomes. 
     In some embodiments, spinal fixation or stabilization hardware (e.g., screws and rods) can be coupled to a first side of the patient&#39;s spine before correction is performed without locking down the fixation hardware. The modules can be coupled to screws implanted in a second, contralateral side of the patient&#39;s spine. After the desired amount of correction is achieved, the fixation hardware in the first side of the patient&#39;s spine can be locked down to maintain the corrected angle. The modules can then be removed and a spinal fixation element  80  can be attached to the pedicle screws  70 ,  70   a  implanted in the second, contralateral side to complete the fixation. In other embodiments, spinal fixation or stabilization hardware can be coupled only to a single side of the patient&#39;s spine, e.g., a side on which the modules  10 ,  10   a  are attached. It will be appreciated that the modules  10 ,  10   a  can be removed from the pedicle screws  70 ,  70   a  either before or after a spinal fixation element or rod  80  is coupled to the pedicle screws. 
     The above-described method involves a single level osteotomy and first and second modules  10 ,  10   a  configured to measure a local correction, however it will be appreciated that more complex deformity correction can also be performed. For example, rotational deformities or angular deformities in any of the sagittal, axial, and/or coronal planes can be corrected and the degree of correction monitored using the modules disclosed herein. By way of further example, several modules (e.g., three, four, five, six, seven, eight, or more) can be coupled to corresponding vertebrae to provide correction measurements for a spinal segment (e.g., a lumbar region, a thoracic region, a cervical region, etc.) or for an entire spine (e.g., from skull to tailbone). Measurement data associated with such procedures can be communicated to an external display to give the surgeon a graphical depiction of overall spinal correction. 
     Although not shown, additional information can be displayed on the displays  50 ,  50   a  and/or on an external display in communication with the modules  10 ,  10   a . The information displayed on the display  50  can be selected by a user before the procedure, can be impacted by the surgical device to which the module  10  is attached, and/or can be preconfigured as part of the factory settings of the module  10 . By way of non-limiting example, the modules  10 ,  10   a  can convey positive and/or negative feedback to the surgeon during surgery. For example, the displays  50 ,  50   a  can convey an error message to the user when the change Δß in the relative angle ß exceeds the desired angular correction, when the engagement features  40 ,  40   a  detect that they are not attached to the correct surgical device, and/or when engagement between the engagement features  40 ,  40   a  and the pedicle screws  70 ,  70   a  has been lost or weakened. In some embodiments, where the processors  22 ,  22   a  are configured to calculate the rate of change Δß/Δt in the relative angle ß, the displays  50 ,  50   a  can convey an error message to the user when the rate exceeds a predetermined speed limit. In still further embodiments, should the patient be rotated in the axial plane during the surgery, for example due to a table rotation or rolling over of the patient, one or both of the modules  10 ,  10   a  can detect the change and can be configured to alert the surgeon via an error message on the displays  50 ,  50   a , which may include an instruction to recalibrate. In case of a need to recalibrate, the modules  10 ,  10   a  can be detached from the screws  70 ,  70   a  and can be attached to one another to repeat the calibration procedure described above. 
     Information detected and/or calculated by the modules  10 ,  10   a  during the procedure can be collected and stored for later use. The information can be stored locally in the memories  24 ,  24   a  and/or can be transmitted via the communications interfaces  26 ,  26   a  to one or more external base stations. The stored information can be used at a later time for various purposes, for example to create a reproduction of the surgery, for clinical improvement, research, and/or ethnography. 
     Another exemplary pedicle subtraction osteotomy using one or more surgical electronic modules as described herein is illustrated in  FIGS.  7  and  8   . The procedure according to this method involves the use of four surgical electronic modules  10   b ,  10   c ,  10   d ,  10   e , which can detect, calculate, store, and/or transmit information in a similar manner to the modules  10 ,  10   a  described above during the exemplary pedicle subtraction osteotomy of  FIGS.  4 - 6   . It will be appreciated by a person skilled in the art that any surgical electronic module as described herein can be used, either the same or different from one another, and that the modules  10   b ,  10   c ,  10   d ,  10   e  can be used in a variety of surgical procedures that effect changes in anatomical position and/or orientation. 
     Similarly to the procedure described with reference to  FIGS.  4 - 6   , a desired angular correction of the spine can be determined prior to the surgery. The modules  10   b ,  10   c ,  10   d ,  10   e  can be calibrated by freely rotating mated pairs of the modules  10   b ,  10   c ,  10   d ,  10   e  until enough positional information has been acquired to synchronize their coordinate systems. For example, the module  10   b  can be attached to each of the other modules  10   c ,  10   d ,  10   e , and synchronized with each of the other modules  10   c ,  10   d ,  10   e  to ensure that all of the modules  10   b ,  10   c ,  10   d ,  10   e  are synchronized to each other. 
     The first three modules  10   b ,  10   c ,  10  can be rigidly attached to three pedicle screws  70   b ,  70   c , and  70   d , while the fourth module  10   d  can be rigidly attached to a surgical cutting instrument such as a rongeur  90 . The engagement features  40   b ,  40   c ,  40   d ,  40   e  can detect an identity of the device to which the modules  10   b ,  10   c ,  10   d ,  10   e  are attached, such that the first three modules  10   b ,  10   c ,  10   d  can detect that they are each attached to a pedicle screw and the fourth module  10   e  can detect that it is attached to a rongeur. Based on this information, the first three modules  10   b ,  10   c ,  10   d  can switch into the orientation detection mode in which only orientation information is displayed to the user, and the fourth module  10   e  can switch into the full detection mode in which orientation and position information is displayed. Further, as explained in detail below, the processors  22   b ,  22   c ,  22   d  of the first three modules  10   b ,  10   c ,  10   d  can be configured to calculate different positional information from the processor  22   e  of the fourth module  10   e . It will be appreciated by a person skilled in the art, however, that the procedure can be performed utilizing only three modules, two of which are attached to two pedicle screws and the third of which is attached to a surgical cutting instrument. 
     The pedicle screws  70   b ,  70   c ,  70   d  can be implanted into pedicles PI, P2, and P3 on vertebrae VI, V2, and V3, either before or after the modules  10   b ,  10   c ,  10   d  are attached thereto. At least one of the pedicle screws  70   b ,  70   c ,  70   d  can be implanted on an opposite side of an intended osteotomy site O from at least one of the other pedicle screws  70   b ,  70   c ,  70   d . Similarly to the modules  10 ,  10   a  used in the exemplary procedure of  FIGS.  4 - 6   , an initial position and/or orientation of the modules  10   b ,  10   c ,  10   d  with respect to one another can be set by actuating the zeroing buttons  60   b ,  60   c ,  60   d . Thus, as shown in  FIG.  7   , the displays  50   b ,  50   c ,  50   d  can each display “0” to indicate that the modules  10   b ,  10   c ,  10   d  are oriented at an initial angle relative to one another. Angular correction of the spine along the sagittal plane can then be performed according to customary surgical procedures, which can include removal of bone from a vertebra disposed between the second and third vertebrae V2, V3 by the rongeur  90 . During the procedure, the sensors  22   b ,  22   c ,  22   d  can continually detect absolute azimuth and polar angles θ, φ of each of the modules  10   b ,  10   c ,  10   d . The modules  10   b ,  10   c ,  10   d  can communicate their absolute azimuth and polar angles θ, φ with each other (e.g., via Bluetooth or other wired or wireless communication) to thereby allow for the processors  22   b ,  22   c ,  22   d  to calculate a relative angle ß 1  of the first module  10   b  with respect to the second module  10   c , a relative angle ß 2  of the second module  10   c  with respect to the third module  10   d , and a relative angle ß 3  of the first module  10   b  with respect to the third module  10   d . Further, the modules  10   b ,  10   c ,  10   d  can calculate changes Δß 1 , Δß 2 , Δß 3  in the relative angles ß 1 , ß 2 , ß 3  throughout the procedure, rates of changes Δß 1 /Δt, Δß 2 /Δt, Δß 3 /Δt and/or rates of changes Δθ/Δt, Δφ/Δt in the azimuth and polar angles θ, φ of each of the modules  10   b ,  10   c ,  10   d.    
     Because the relative angle ß 1  of the modules  10   b ,  10   c  with respect to one another does not change throughout the procedure since the modules  10   b ,  10   c  are on the same side of the osteotomy site O, the modules  10   b ,  10   c ,  10   d  can be configured to display only Δß 2 . The displays  50   b ,  50   c ,  50   d  can be configured not to display the change Δß 1  since it will remain substantially equal to zero throughout the procedure, and not to display the change Δß 3  because Δß 2  and Δß 3  will remain substantially equal to one another throughout the procedure. Of course, it will be appreciated by a person of skill in the art that the modules  10   b ,  10   c ,  10   d  could display either Δß 2  and Δß 3 , since they are substantially equal to one another, and Δß 2  has been chosen solely for purposes of illustration. Also, if at any point during the surgery, Δß 1  ceases to be substantially equal to zero and/or Δß 2  and Δß 3  cease to be substantially equal to one another, all three relative angular changes Δß 1 , Δß 2 , Δß 3  can be displayed on the displays  50   b ,  50   c ,  50   d . Any of these values can be displayed on an external display alternatively or in addition. 
     The module  10   e  can be attached to the rongeur  90  via engagement features  40   e  on the module  10   e  at any point during the surgery to help the surgeon remove a desired amount of bone from a desired location. Like the modules  10   b ,  10   c ,  10   d , the module  10   e  can be “zeroed” by user actuation of the zeroing button  60   e  when the module  10   e  is placed in an initial position and/or orientation, e.g., when the rongeur  90  to which the module  10   e  is coupled is inserted at a desired cutting angle and at a maximum desired cutting depth into the patient&#39;s body. Thus, as shown in  FIG.  7   , the module  10   e  can display two zeros, one indicating an initial angle and one indicating an initial distance. Preferably, the module  10   e  is zeroed before any angular correction in the patient&#39;s spine has been achieved and/or at the same time that the other modules  10   b ,  10   c ,  10   d  are zeroed. 
     Because the module  10   e  is able to detect that it is attached to a surgical instrument, e.g., the rongeur  90 , as opposed to a surgical implant, e.g., the pedicle screws  70   b ,  70   c ,  70   d , it can be configured to calculate and/or display different positional information than the modules  10   b ,  10   c ,  10   d . This information can supplement the information displayed by the modules  10   b ,  10   c ,  10   d  to confirm that a desired angular correction has been achieved. In particular, whereas the modules  10   b ,  10   c ,  10   d  are configured to calculate and/or display changes in positional information with respect to one another, the module  10   e  can be configured to calculate and/or display changes in its own positional information throughout the surgery. Further, whereas the modules  10   b ,  10   c ,  10   d  are configured to calculate and/or display changes in their relative orientations, the module  10   e  can be configured to calculate and/or display changes in both orientation and position. 
     To perform these calculations, the module  10   e  can continually detect absolute azimuth and polar angles θ, φ of the module  10   e  with the sensor  28   e , calculate an absolute angle ß 4  of the module  10   e  in the sagittal plane with the processor  22   e , and store the absolute angle ß 4  for any given time in the memory  24   e . Similarly, the sensor  28   e  can continually detect an absolute position (e.g., including a distance d 4  of the module  10   e  relative to a starting position) via triangulation, time-of-flight, or other positioning algorithms using ultrasonic, electromagnetic, and/or infrared location signals sent by each of the modules  10   b ,  10   c ,  10   d ,  10   e  and communicated therebetween. It will be appreciated by a person skilled in the art that, where at least three modules are used, unique position information can be created through location signals sent out by each of the modules and communication among all of the modules of the information received from the signals while the modules  10   b ,  10   c ,  10   d  are stationary, e.g., before they are moved together as part of reduction procedure. It will further be appreciated by a person skilled in the art that the position of the rongeur  90  can be determined through communication between the module  10   e  and other surgical electronic modules positioned in the operating room. As the position and/or orientation of the rongeur  90  changes during surgery, the processor  22   e  can calculate and the display  50   e  can display a change Δß 4  in the angle ß 4  and/or a change Δd 4  in the distance d 4  of the module  10   e —and therefore of the rongeur  90 —in the sagittal plane. For example, as shown in  FIG.  8   , the display  50   e  can indicate that the rongeur  90  has moved by a distance of 49 mm and by an angle of 27 degrees from the initial position and orientation. In this way, the surgeon can know when the rongeur  90  has completed a desired motion to thereby remove a desired amount of bone. In some embodiments, the module  10   e  can be configured to alert the surgeon when the rongeur  90  is moved outside of or beyond a surgical plane, e.g., beyond a desired angle and/or distance, for example by displaying an error message on the display  50  and/or providing an audio signal or vibration. For example, the surgeon can be warned when the distal end of the rongeur is approaching or has exceeded a predetermined maximum insertion depth in the anterior direction (e.g., when the axial displacement of the rongeur relative to the starting position approaches zero or becomes negative). 
     Similarly to the exemplary pedicle subtraction osteotomy of  FIGS.  4 - 6   , when the desired angular correction of the spine in the sagittal plane has been achieved, the change Δß 2  in the relative angle ß 2  displayed on the displays  50   b ,  50   c ,  50   d  will be equal to the desired angular correction. The patient&#39;s spine can then be stabilized in the corrected position via a spinal fixation element  80  that can be attached to the implanted pedicle screws  70   b ,  70   c ,  70   d . The modules  10   b ,  10   c ,  10   d  can be removed from the pedicle screws  70   b ,  70   c ,  70   d  and the module  10   e  can be removed from the rongeur  90  either before or after fixation with the spinal fixation element  80 . 
     It will be appreciated by a person skilled in the art that a greater number of modules can enhance the accuracy of the procedure by providing for a greater amount of positional information. For example, using more modules can provide positional information to a greater degree of precision and/or specificity, e.g., with more significant digits, which can be displayed to the surgeon. As each module is added in the procedure, the number of significant digits displayed to the surgeon can increase, thereby providing a measure of the increase in accuracy added by each additional module to the surgeon. Additionally or alternatively, using a greater number of modules can enable the modules to detect and/or calculate their positions and/or orientations in more dimensions. The positions, orientations and/or changes in the positions and/or orientations of the modules can be displayed to the user for each plane in which information is acquired. However, it will also be appreciated by a person of skill in the art that a position and/or an orientation of the module in certain planes need not be calculated since it can be assumed that the patient will not move in certain planes. 
     It will further be appreciated by a person skilled in the art that the devices and methods described herein can be particularly useful for robotic assisted surgery. For example, one or more surgical electronic modules as described herein can transmit positional information to a robotic manipulator, which can manipulate the one or more modules until they have reached a desired final position that has been input to the manipulator. 
     Further embodiments of devices and methods for intraoperatively measuring position or angular orientation are also provided. In one embodiment, measuring simultaneous angles in multiple orthogonal planes can be accomplished using a single device that includes an integrated display. Such a device can be permanently assembled to medical tools and instruments in some embodiments, while in other embodiments it can be modular to allow for use with a number of medical tools and instruments during surgery. A 9-axis inertial motion unit (IMU) or sensor consisting of a 3-axis accelerometer, a 3-axis gyroscope, and a magnetometer can be housed inside the device to allow angular measurements. In another embodiment, a 3-axis accelerometer alone can be used to allow angular measurement between orthogonal planes. In other embodiments, any of a variety of other sensors can be employed that can provide information related to the position and/or orientation of the sensor. 
       FIGS.  9 A and  9 B  illustrate one embodiment of such a device  100  that is integrally formed with an electronic module  102  for detecting position or orientation information during use. The module  102  can include a housing  104  containing a sensor (not shown), a processor (not shown), and a display  106  for communicating information to a user. The device  100  can also include an input for receiving instruction from a user. In the illustrated embodiment, the input can be a single button  108 , though in other embodiments different inputs, such as toggles, switches, a plurality of buttons, etc. can be utilized. For example, and as shown in  FIGS.  9 A- 12   , the button  108  can allow a user to zero and successively complete angular measurement. By way of further example, a single press of the button  108  can calibrate the sensor to initialize or zero the angular measurement at a desired starting location (see  FIG.  10    left side, referred to as “State A”). This can be represented by a thin box  110  around the 0° displays, as shown in  FIG.  10   . A second press of the button  108  at a new location can define a desired plane between the starting and the new location. The instrument  100  can provide angular measurement between these locations in two planes, i.e., the newly defined plane  112  and the plane  114  that is orthogonal to the defined plane but still parallel to the gravitational field (see  FIG.  10    right side, referred to as “State B”). 
     In another embodiment, a device  120  can provide for defining a projection plane, parallel to a gravitational field, between two locations by using, for example, two depressions of a button or other input. For example, a user can press a button  122  once at a starting location (labeled State A and passing through point A in  FIGS.  11 A and  11 B ) and press the button a second time at a second, new location (labeled as State B and passing through point B in  FIGS.  11 A and  11 B ). This can allow for defining a desired projection plane (Plane X in  FIGS.  11 A- 12   ) for angular measurement. Depression of the button  122 , for example third and fourth times, can allow angular measurements between any other two locations. Measurements between these locations can be displayed as projected angles in the newly defined projection plane (Plane X in  FIGS.  11 A- 12   ) and a plane orthogonal to the defined projection plane (Plane Y in  FIGS.  11 A- 12   ). 
     By way of further example,  FIG.  11 A  illustrates the measurement of an angle θ in labeled Plane X between a first position of the instrument  120  at labeled State A and a second position of the instrument at labeled State B.  FIG.  11 B  illustrates an orthogonal section view and the measurement of an angle φ in labeled Plane Y (which is orthogonal to Plane X in  FIG.  11 A ) between the first position of the instrument  120  at labeled State A and the second position of the instrument at labeled State B.  FIG.  12    illustrates the labeled planes X and Y from a top view, showing their orthogonal relationship to one another. 
       FIG.  12    also illustrates a top view the instrument  122  showing a reference plane alignment feature  124  to aid a user in defining orthogonal projection planes relative to a patient  126 . The reference plane alignment feature  124  can be, for example, a ridge, depression, line, or other marking that a user can reference to align the instrument  122  with a desired reference or projection plane. In the illustrated embodiment, for example, the instrument is positioned such that the reference plane alignment feature  122  is aligned with the projection plane labeled Plane X. The projection plane labeled Plane Y is established based on Plane X without need for a dedicated reference feature on the instrument  122 . In some embodiments, however, a secondary marking or feature could be included to explicitly denote the orientation of the orthogonal projection or reference plane. 
     The devices and methods described herein, including the sensors used to detect changes in position or orientation, can be utilized in connection with a number of surgical instruments. In one embodiment, for example, sensors can be attached to an osteotome or chisel for accurate bone cutting to create a desired wedge during a pedicle subtraction osteotomy to achieve planned sagittal balance. During cutting these osteotomies, surgeons often want to know both the angle for the wedge and also the angle of attack in the orthogonal plane. In some cases, surgeons cut osteotomies in a medial to lateral direction at a certain angle to avoid any unintended damage to the critical anatomy in case of slips and overcuts. 
     The devices and methods described herein can also allow surgeons to create asymmetric wedges at each level for complex deformity correction procedures that require correction of sagittal and coronal balance. Prior techniques often include in-vivo use of many cobbled together methods, including a combination of “eyeballing,” experience, estimation, multiple fluoro images or the use of templates, wedges, and rasps to estimate bone removal work. Utilizing the devices and methods described herein can provide a number of advantages over these prior techniques. Furthermore, the devices and methods can be applied to other osteotomies performed in trauma, joints, and CMF (craniomaxillofacial) procedures. 
     For example, there is currently no quick way to measure sagittal balance and regional curves intraoperatively during deformity and degenerative surgeries. Prior techniques often involve taking fluoro-images, sending them to a PACs (Picture Archiving and Communication) system, and using the images to measure correction with a manual protractor. Such a technique can require a surgeon to leave a sterile field, which is not desirable and increases surgery time. In one embodiment, a device according to the teachings provided herein can be coupled to any medical device, tool, or instrument that can be aligned with two endplates at the apex of a desired curve by means of fluoro-images to measure desired lordosis or kyphosis plus scoliosis angles simultaneously. 
     In another embodiment, a sensor can be coupled to a deformity correction instrument, such as quick-sticks, towers, frames, or flex-clips, during derotation and axial correction of the spine. Surgeons can attach sensors to these instruments before starting the correction or maneuvers in some embodiments. During this time, a surgeon can decide to zero the sensors and, after final correction, the sensors can provide more information on the angular correction achieved in the axial and sagittal planes. 
     In still another embodiment, a surgeon can attach one or more sensors to Lenke probes, gearshifts, awls, drills, or taps that can assist surgeons to achieved planned pedicle screw trajectory or pedicle preparation trajectory. The devices and methods described herein can also be used to train surgeons to place mPACT (medialized Posterior Approach Cortical Trajectory) cortical screws, iliac screws and SAI (Sacral-Alar-Iliac) screws using an optimized trajectory. 
     Furthermore, a surgeon can use the devices described herein to measure and register an angle used to place a pedicle screw on one side of the vertebrae, and then use this registered information to mirror a pedicle screw trajectory on other side. This can allow surgeons to have pedicle screws inserted symmetrically across the spine, which can further allow similar bent rods to be placed on each side. Rod bending is a highly cumbersome and skilled art, using similar bent rods on each side of a patient&#39;s spine can save significant time. 
     The embodiments described above and illustrated in  9 A- 12  include a single sensor and a one-button interface. In other embodiments, a device can include, for example, a different number of buttons for an input or interface. By way of further example, a single sensor device can include a four-button interface, as described below and illustrated in  FIGS.  13 A- 16   . The number of buttons can be varied, however, based on desired workflow and/or usability. For example, short and long presses of one or more buttons can be utilized to provide functionality equivalent to other embodiments in which a larger number of buttons are included. 
     The embodiment described below can be utilized for measuring anatomical, instrumental, or device based change in position or angular orientation. In this embodiment, systems quantitatively measure changes in the position or angular orientation of a portion of a patient&#39;s anatomy or a medical device with respect to another portion of the patient&#39;s anatomy or a medical device during surgery. The devices and methods described herein can be utilized to achieve the same functionality described above and in U.S. application Ser. No. 14/471,120, entitled “SYSTEMS AND METHODS FOR INTRAOPERATIVELY MEASURING ANATOMICAL ORIENTATION,” filed Aug. 28, 2014, and now issued as U.S. Pat. No. 9,993,177, the entire contents of which are hereby incorporated by reference. One advantage of the embodiments described herein, however, is that this functionality can be achieved with a single sensor housed in a medical device. 
     As shown in  FIGS.  13 A and  13 B , a device  130  can include an electronic module  132  permanently assembled to a medical tool and/or instrument  134  in one embodiment. In another embodiment, the module  132  can be modular to allow for use with number of medical tools and instruments during a surgery. As noted above, a 9-axis inertial motion unit (IMU) or sensor that can include a 3-axis accelerometer, a 3-axis gyroscope, and a magnetometer, can be housed inside the device to provide angular measurements. In another embodiment, a 3-axis accelerometer alone can be used to provide angular measurement between orthogonal planes. 
     Operation of devices, such as the device  130 , illustrated in  13 A- 16  is described below. The device  130  can include an input  136  that, in the illustrated embodiment, includes four buttons  136   a ,  136   b ,  136   c ,  136   d  that can allow a user to provide instruction regarding measurement of a change in position or angle. Further, a display  138  can communicate measured or calculated position or orientation data back to a user during use. 
     As shown in  FIG.  14 A  (and referred to as “State A”), a first button  136   a  can be initially lit to indicate to a user that this button should be pressed for a next action. Pressing the first button  136   a  can allow a user to zero and calibrate the sensor in a first starting position. The display can display one or more zeros  140  to denote positioning of the instrument at a starting position/orientation. For example, in  FIG.  15 A  the instrument  130  is attached to the screw A in the starting position. 
       FIG.  14 B  (referred to as “State B”) illustrates a second button  136   b  that lights to indicate to a user to press this button for a next action. Prior to pressing the second button  136   b , the instrument  130  can be moved to a new end point location. For example, in  FIG.  15 A  the instrument is attached to the screw B in the new end point location. Pressing the second button  136   b  can allow the user to measure an angle between a start point location and this new end point location in two orthogonal planes (e.g., angles θ 1  and φ 1  in planes X and Y, respectively, as shown in  FIGS.  15 A and  15 B ). The top-left of the display  138  can be updated with the measured angle  142  at the end point location (e.g., when the instrument  130  is attached to the screw B in  FIG.  15 A ). 
       FIG.  14 C  (referred to as “State C”) illustrates a third button  136   c  that can be illuminated to indicate to a user to press this button for a next action. When a user has gone through an osteotomy closure during deformity correction, for example, they can take the sensor to the previous start point location (e.g., the position where the device  130  is in contact with the screw A shown in  FIG.  15 C ). The third button can be pressed to zero the sensor at this location, as reflected by the display of 0° (ref. no.  144 ) in the upper right corner of the display  138 . 
     The user can then move the sensor to the end point location (e.g., the position where the device  130  is in contact with screw B shown in  FIG.  15 C ). A fourth button  136   d  can be lit indicating that the surgeon or other user can press it for a next action. Upon pressing the button  136   d , as shown in  FIG.  14 D  (referred to as “State D”), the top-right corner of the display  138  can be updated with a new measured angle  146  between the new start and end points in two orthogonal planes (e.g., angles θ 2  and φ 2  in planes X and Y, respectively, as shown in  FIGS.  15 C and  15 D ). Furthermore, a first delta (e.g., θ 1 -θ 2 )  148  in the defined plane (Plane X) and a second delta (φ 1 -φ 2 )  150  in the orthogonal plane (Plane Y) can be updated indicating changes in the angular orientation of the original location (an initial location of a patient&#39;s anatomy, medical device, or implant before correction) with respect to the new locations (a final location of a patient&#39;s anatomy, medical device, or implant after correction). 
     In some embodiments, pressing any two buttons (e.g.,  136   c ,  136   d , etc.) together can instruct the system to reset all values and ready the device  130  for a new differential angular measurement, as shown in  FIG.  14 E  (referred to as “State E”). 
     In another embodiment, the device can allow for defining a projection plane, parallel to a gravitational field, between two locations with two depressions of a fifth button positioned at the center of the four buttons. Differential Measurements between any new locations can be displayed as projected angles in this newly defined projection plane (Plane X in  FIGS.  15 A- 16   ) and a plane orthogonal to this defined projection plane (Plane Yin  FIGS.  15 A- 16   ). For angular measurements, a similar workflow as outlined above for States A through E above can be used. 
     As noted above,  FIGS.  15 A- 15 D  illustrate one example of how a device  130  with a four-button input or interface can be utilized to measure changes in position or orientation simultaneously in two orthogonal planes. As shown in the orthogonal section views of  FIGS.  15 A and  15 B , the device  130  can be zeroed when in a first position where it is in contact with screw A by pressing the first button  136   a . The device can then be moved to a second position where it is in contact with screw B and the second button  136   b  can be depressed. This can record the angle θ 1  in the Plane X between the first and second positions, as well as the angle φ 1  between the first and second positions in the Plane Y (that is orthogonal to the Plane X). 
     Following surgical manipulation of the patient&#39;s anatomy  152 , a second set of measurements can be made using the device  130 . As shown in  FIGS.  15 C and  15 D , the device  130  can be zeroed when in the first position where it is in contact with screw A by pressing the third button  136   c . The device can then be moved to the second position where it is in contact with screw B and the fourth button  136   d  can be depressed. This can record the angle θ 2  in the Plane X between the first and second positions, as well as the angle φ 2  between the first and second positions in the Plane Y (that is orthogonal to the Plane X). 
     The display  138  of the device  130  can communicate measured and calculated position or orientation data to a user. As shown in  FIG.  15 C , for example, the display  138  can include the angles θ 1  and φ 1  in an upper left corner  154 , the angles θ 2  and φ 2  in an upper right corner  156 , the delta θ 1 -θ 2  in the center  158 , and the delta φ 1 -φ 2  in the lower right corner  160 . The positioning of the information on the display  138  can be different in alternative embodiments, and any of a variety of types of information in different display formats can be communicated to a user. 
       FIG.  16    illustrates the instrument  130  from a top view and highlights that the instrument can include a reference plane alignment feature  162  to aid a user in defining the reference plane (Plane X in the figure) and the orthogonal plane (Plane Y in the figure). In the illustrated embodiment, a flat surface on a side of the instrument  130  can serve as the reference plane alignment feature that can be oriented parallel to the reference plane (Plane X). In other embodiments, a marking, recess, protrusion, or other feature can be utilized, as described above. Further, in some embodiments an additional marking or feature can be included to denote the orientation of the orthogonal plane (Plane Y in the figure). 
     The above-described embodiments can have a number of advantages over prior devices and techniques. For example, the devices and methods described herein can provide an instrument for intraoperatively measuring anatomical orientation that can be easy to use and can be independent of a wirelessly (e.g., Wi-Fi®, Bluetooth®, etc.) or otherwise remotely-connected tablet or display. Moreover, there is currently no quick way to measure sagittal balance and regional curves intraoperatively during deformity and degenerative surgeries. Many prior techniques involve taking fluoro-images, sending them to a PACs (Picture Archiving and Communication) system, and using the images to measure correction using a manual protractor. This type of technique can require a surgeon to leave a sterile field, which is not desirable and can increase surgery time. In one embodiment, a device according to the teachings provided herein can be coupled to any medical device, tool, or instrument that can be aligned with two endplates at the apex of the desired curves by means of fluoro-images to measure starting lordosis or kyphosis plus scoliosis angles simultaneously as soon as patient is placed on the operating table. This can be advantageous because placing a patient prone on a table can change their regional curve. Surgeons are often interested in knowing this angle and registering it as reference starting point. All further corrections and improvements in sagittal balance, as well as subsequent changes in regional angular values, can be measured relative to this reference point using, for example, the third and fourth buttons  136   c ,  136   d  of the device described above. For example, the steps shown in  FIGS.  14 C,  14 D,  15 C, and  15 D  can be repeated as a surgeon or other user manipulates a patient&#39;s anatomy and measurements can be taken relative to the original starting positions recorded as shown in  FIGS.  14 A,  14 B,  15 A, and  15 B . 
     The above described devices and methods can be utilized for a number of different procedures, including, for example, as a tool for measuring angular orientation of a portion of a patient&#39;s anatomy, an osteotome with angular orientation sensing capability, a tool for pedicle targeting, and a tool for rod bending, among others. These devices can provide surgeons with an ability to intraoperatively measure spinal correction achieved at each regional curve, including lordosis, kyphosis, and scoliosis. Further, devices and methods described herein can provide an association between correction achieved in a standing and a prone position, as well as an ability to provide coronal correction measurement. Such methods and devices can aid surgeons in achieving coronal alignment, shoulder and pelvic leveling during complex deformity correction procedures, and can provide degeneration and minimally invasive procedure surgeons with tools to intraoperatively monitor spinal alignment. 
     The devices and methods described herein can provide such functionality while minimizing sensor size and reducing visual obstruction to a surgeon. This can provide surgeons with real-time regional curve and osteotomy closure angle measurement without compromising workflow or visualization. Furthermore, the devices described herein can make use of wired or wireless components (e.g., wired components can in some cases be made smaller than wireless components, further reducing the size of a device). The methods described herein can provide quick, simple to use, low profile, and easy to connect components that can be used with a surgeon&#39;s existing tools. This can allow surgeons to quickly validate angles utilizing their existing instrumentation to achieve consistent surgical outcomes. Further, the devices described herein can include a variety of user interfaces, including a digital display, a single button, four buttons, five buttons, or another configuration. The devices described herein can be configured to couple to bone cutting tools, taps, Lenke probes, osteotomy closure clamps, harmonic tools, or rod benders. The devices and methods described herein could also be configured for use with lordotic cages to confirm achievement of proper angular correction at a particular level. 
     Moreover, the devices described herein can be incorporated into a modular handle that can be moved between various surgical instruments. This can allow the devices and methods described herein to be utilized in connection with other deformity, degenerative, and minimally invasive surgery (MIS) applications. 
     Exemplary features of the devices and methods described herein can include a disposable integrated handle with an angle sensor, a built-in display, and a low profile easy to connect feature for mating with existing surgical instruments. Such a device can allow a surgeon to intraoperatively measure actual regional angles after aligning a tool with respective end plates, or measure a change in angles projected on to any two orthogonal planes. A free-hand tool according to the teachings provided herein can be used to validate osteotomy wedge angles, enable accurate bone removal (as in osteotomies), target trajectory for pedicle screws, mirror trajectory for pedicle screws, and bend spinal rods. 
     Further illustration of embodiments of the devices and methods described herein is shown in  FIGS.  17 A- 22 B .  FIGS.  17 A- 17 G , for example, illustrate one embodiment of a method of operation of an instrument for intraoperatively measuring anatomical orientation having a three-button input interface.  FIG.  17 A , for example, illustrates coupling of an electronic module  170  to an instrument  172  and positioning of the instrument  170  such that it is in contact with a pedicle screw or other rigid bony attachment  173  that can be implanted in, for example, a patient&#39;s spine  174 . The module  170  can include a display  176 , an input  178  including three buttons  178   a ,  178   b ,  178   c , and a reference plane alignment feature  180 . In the illustrated step of the method, the instrument  172  with electronic module  170  coupled thereto can be positioned at a first point in contact with the pedicle screw  173  or a portion of the patient&#39;s anatomy and the reference plane alignment feature  180  can be aligned with the desired reference plane (e.g., a patient&#39;s sagittal plane) into which measured angles can be projected. The first button  178   a , which can be illuminated to prompt a user in connection with, for example, a prompt on the display  176 , can be depressed to record the current position and/or orientation and define the reference plane. 
       FIG.  17 B  illustrates another step in which the instrument is moved to a first position and the second button  178   b  (which can be illuminated to prompt a user after the first button  178   a  is depressed to define the reference plane) can be depressed to record the current position and/or orientation at the first position. In another step illustrated in  FIG.  17 C , the instrument  172  can be moved to a second position and the third button  178   c  (which can be illuminated to prompt a user after the second button  178   b  is depressed to define the first position) can be depressed to record the current position and/or orientation at the second position. A processor included in the electronic module  170  can then calculate angular differences between the position or orientation of the instrument  172  in the first and second positions, as projected onto the reference plane and a plane orthogonal thereto. This measurement can serve as a baseline or pre-manipulation reference. 
     Following surgical manipulation of the patient&#39;s anatomy (e.g., spine  174 ), the measurement process can be repeated as shown in  FIGS.  17 D- 17 F . In particular, the reference plane can be redefined using the first button  178   a , as shown in  FIG.  17 D . In addition, position and/or orientation information can be captured at the first position using the second button  178   b , as shown in  FIG.  17 E , and position and/or orientation information can be captured at the second position using the third button  176   c , as shown in  FIG.  17 F . The processor of the module  170  can then communicate measured and calculated information to a user via the display  176 , including a first angle  184  measured between the first and second positions before surgical manipulation and a second angle  186  measured between the first and second positions after surgical manipulation, as well as a delta  188  between those angles, as shown in  FIG.  17 G . These angles can be projected into the defined reference plane  182  and a further angle  190  can be included representing change in a plane orthogonal to the reference plane  182 . 
       FIGS.  18 A- 18 J  illustrate another embodiment of a method of operation of an instrument for intraoperatively measuring anatomical orientation having a three-button interface. The method is similar to the method described above and shown in  FIGS.  17 A- 17 G , but illustrates additional features that can be provided in various embodiments. For example,  FIGS.  18 A- 18 C  illustrate a similar process for defining a reference plane (and a plane orthogonal thereto) and recording position and/or orientation information at first and second reference positions prior to surgical manipulation of a patient&#39;s anatomy.  FIG.  18 D  illustrates that, in some embodiments, initial measured or calculated position and/or orientation data can be immediately displayed to a user. Such information can include, for example, the angle  200  between the first and second reference points as projected into the reference plane, as well as the angle  202  between the first and second reference points as projected into the plane orthogonal to the reference plane. 
     In  FIG.  18 E , a step of simultaneously pressing the second button  178   c  and the third button  178   c  to save the captured information is illustrated. Alternatively, a user could re-take the measurements of  FIGS.  18 A-C  if, for example, the instrument was improperly positioned, etc. As shown in  FIG.  18 F , at subsequent times the saves data can be recalled to the display  176  by simultaneously pressing the second and third buttons  178   b ,  178   c  again. 
       FIGS.  18 G- 18 J  illustrate method steps similar to those described above and illustrated in  FIGS.  17 D-G  that can be performed following surgical manipulation of a patient&#39;s anatomy  174 . These can include redefining the reference plane ( FIG.  18 G ), capturing data from the first and second positions ( FIGS.  18 H and  181   ), and displaying measured and calculated position and/or orientation data to a user via the display  176  ( FIG.  18 J ). 
       FIGS.  19 A- 19 D  illustrate still another embodiment of a method of operation of an instrument for intraoperatively measuring anatomical orientation having a three-button interface. In this embodiment, the electronic module  170  and instrument  172  can be utilized without pre- and post-manipulation measurement comparisons to measure position and/or orientation changes between two reference points at any time. The method illustrated in  FIGS.  19 A- 19 D  mirrors that described above and illustrated in  FIGS.  18 A- 18 D . For example, the method can include defining a reference plane into which angles can be projected ( FIG.  19 A ), capturing position and/or orientation data at a first position ( FIG.  19 B ) and a second position ( FIG.  19 C ) and displaying measured or calculated data to a user ( FIG.  19 D ). The displayed data can include, for example, a first angle  200  between the first and second reference positions measured in the reference plane and a second angle  202  between the first and second reference positions measured in a plane orthogonal to the reference plane. 
       FIG.  20 A- 20 D  illustrate one embodiment of a method of operating an osteotome  210  with a disposable sensor pod or module  212  that can be selectively coupled thereto. In particular, a housing  214  of the module  212  can include one or more engagement features  216  configured to interface with complementary features on the osteotome  210  (or other surgical instrument) to removably attach the housing thereto. Examples of engagement features  216  can include a protrusion, recess, latch, or other feature. 
     The module  212  can include a single button input  218  that can be used to operate the module in a manner similar to the method described above and illustrated in  FIGS.  9 A- 12   . In the illustrated embodiment, the module  212  includes a different type of display  220  that conveys first angle information  222  related to a reference plane and second angle information  224  related to a plane orthogonal to the reference plane using two illuminated indicators oriented orthogonally to one another. 
       FIGS.  20 B- 20 D  illustrate one example of a method of operating the module  212 . Similar to the methods described above, the illustrated method can include positioning the osteotome in a first orientation, zeroing a display of orientation relative to gravity, moving the osteotome to a second orientation, and displaying a change in orientation relative to the first orientation. More particularly, as shown in  FIG.  20 B  the osteotome  210  can be placed in a first reference position and the module  212  can indicate an initial angular orientation relative to gravity or the earth. A user can press the input button  218 , as shown in  FIG.  20 C , to zero the module  212  when in the first reference position. The user can then move the osteotome  210  to a second reference position, as shown in  FIG.  20 D , and read the angular indications from the display  220  that are now shown relative to the first reference position. 
       FIGS.  21 A- 21 B  illustrate one embodiment of a tool  230  that can be used in conjunction with the module  212  to determine pedicle screw trajectory. As shown in  FIG.  21 A , the tool  230  can be aligned with an axis of a first pedicle screw A p1  being inserted into a patient&#39;s vertebra  232  and can record its position and/or orientation, e.g., by depressing the button  218 . Angles measured relative to a reference plane and a plane orthogonal thereto can be displayed on the display  220  using the illuminated indicators  222 ,  224 . The tool can then be utilized to determine a mirroring pedicle axis A p2  orientation on an opposite side of a patient&#39;s vertebra  232 , as shown in  FIG.  21 B . For example, a user can adjust a position of the tool  230  until the display  220  has a matching angular displacement readout, or the module  212  can be configured to flash the indicators  222 ,  224  or otherwise communicate feedback to a user when the position and/or orientation mirrors the orientation recorded as shown in  FIG.  21 B . The tool  230  and module  212  can thereby provide for symmetric insertion of pedicle screws on opposite sides of the spine. This can create an additional benefit of permitting similarly formed spinal fixation elements, such as spinal fixation rods, to be utilized on both sides of the patient&#39;s spine. Eliminating time spent customizing rod shape on each side of the spine can greatly increase the efficiency of a surgeon or other user by reducing complexity and time for a spinal correction procedure. 
       FIGS.  22 A- 22 B  illustrate another embodiment in which sensors  242 ,  244  are coupled to a tool  240  used to bend a spinal fixation rod  246 . The sensors  242 ,  244  can detect an angle of bend imparted to the rod  246  by the tool  240 , and this angle can be displayed to a user on a display  248  coupled to the tool. In other embodiments described in more detail below, the sensors  242 ,  244  can be coupled to the rod  246  directly to detect a degree of bend imparted thereto in one or more planes. 
       FIG.  23    is a schematic illustration of an exemplary system  300  for measuring anatomical position and/or orientation. As shown, the system can include one or more sensors  302  of the type described herein, a computer system  304  with an electronic display (e.g., a tablet computer, a laptop computer, a mobile device, or the like), and a software application executed by the computer system. The software application can receive user inputs via, e.g., a keyboard, touch-screen display, etc. and can display various information to a user using a graphical user interface  306 , such as data received from the sensors or information calculated using such data. The system can be configured to measure and/or calculate various anatomical parameters, including the various spinopelvic parameters shown in  FIG.  24   , such as thoracic kyphosis (TK), lumbar lordotic angle (LLA), sagittal vertical axis (SVA), sacral slope (SS), pelvic tilt (PT), and pelvic incidence (PI). 
     An exemplary method of using the system  300  is shown schematically in  FIGS.  25 A- 25 H .  FIG.  25 A  illustrates initial setup of the system  300  for embodiments that make use of a reusable tablet  310  and a disposable display  312 . One distinction between the two versions can be that a reusable tablet  310  may not be sterile and, as a result, can communicate wirelessly with one or more sensors  314  that are sterilized and provided in sterile packaging. In the disposable display embodiment  312 , all components can be sterilized and wired connections between the display and sensors can be employed because all components can be brought into the sterile field. 
       FIG.  25 B  illustrates removal of the various system components from a sterile packaging  319 . As shown, the system can include a reference plane sensor  320  and one or more angle sensors  322 . The system can also include a power and communication pod  324  which can be coupled to a reusable tablet or other computer system, or to a disposable computer system. The sensors  322  can be wired or wireless (e.g., Bluetooth, Wi-Fi, etc.). In the case of wired sensors, the wire size can be less than 5 mm. The sensors  322  can be approximately dime-sized, e.g., less than about 18 mm in diameter or width. Low profile sensors  322  can be used, for example sensors having a width less than 7 mm and a length less than 10 mm. In some embodiments, the sensors  322  do not include a housing, display, buttons, etc. It will be appreciated that sensors  322  and/or wires having any size can be used. The sensors  322  can be inertial measurement sensors, e.g., a 9-axis inertial measurement sensor with a gyroscope, magnetometer, and accelerometer. Other sensor types can also be used, including optical, infrared, ultrasound, RF, video tracker, video surface, fiber optic, surface RPT, laser scan, electromagnetic, combinations of sensors, and any sensors described herein. 
       FIG.  25 C  illustrates various mounts for attaching the angle sensors  322  to the patient. It will be appreciated that any of the mounts disclosed above can be used instead or in addition. The illustrated mounts include a clamp  326  for attaching to the spinous process or other anatomical structure, a Steinman pin or pedicle screw  328 , and an alligator clip  330 . The mounts can be less than 1 inch in height in some embodiments. 
       FIG.  25 D  illustrates placement of the angle sensors  322  with respect to the patient. The sensors  322  can be placed at any of a variety of locations on the patient. In an exemplary embodiment for measuring global spinal angles, such as lumbar lordosis and/or thoracic kyphosis, the sensors can be mounted in different regions of the patient&#39;s spine. For example, a sensor can be mounted at each of SI, LI, and T4 as shown by sensors  322   a ,  322   b , and  322   c . While not shown, sensors can also be mounted to the cervical spine to measure position or orientation of that region. The same or a similar arrangement of sensors can be used to measure coronal and derotation spinal angles. 
       FIG.  25 E  illustrates establishment of a reference plane  332  (e.g., the sagittal plane of the patient) using the reference plane sensor  320 . The reference plane sensor  320  can be the same as the angle sensors  322 , and can include any of the same features as the angle sensors. The reference plane sensor  320  can include reference markings or indicia to guide a user in aligning the sensor with the patient. For example, a housing of the sensor can be human-shaped, or can include head and feet markings to indicate proper orientation. The housing can include a flat surface to allow the housing to be laid against the patient or the operating table. The housing can include a longitudinal line indicator configured to be aligned by the user with the patient&#39;s sagittal plane. In use, the user can position the reference plane sensor  320  in alignment with the patient&#39;s sagittal plane and push a button on the sensor to establish the current sensor position as the reference plane. The sagittal plane of the patient can move during the course of a surgery, especially in scoliosis and other cases where significant correction is performed. Accordingly, at any time the user desires, the reference plane  332  can be re-established by picking up the sensor  320 , aligning it, and pushing the button to capture the current position as the reference plane. 
       FIG.  25 F  illustrates capture of patient and sensor images. The images can be captured from a lateral perspective as shown. The image can be captured using fluoroscopy, MM, CT, or other imaging techniques. The captured image can be communicated to the software application executing on the computer system  304  (e.g., tablet  310 ). For example, an image capture device  334  used to capture the image can be integrated with the computer system  304  or can be coupled thereto (e.g., via a wireless or USB connection). The image can also be communicated using a camera of the computer system  304  to capture an image of a display screen on which the patient image is shown. For example, the integrated camera on the tablet computer  310  can be used to take a picture of the display screen of a fluoroscope to load the patient image into the tablet computer. The angle sensors  322  can be radiopaque, magnetic, metallic, or can have other properties that allow them to be visualized in the patient image. It will be appreciated that, in the case of fluoroscopy, only a small number of fluoro-shots are needed to capture an image of the sensors and of the patient anatomy (e.g., less than 3 shots, less than 2 shots, and/or a single shot in certain embodiments). 
     In some cases, a user might position the angle sensors  322  such that their sensor axis is slightly offset from the endplate plane of the patient&#39;s vertebrae. The system can be configured to apply a correction such that angles or other measurements made by the angle sensors  322  can be calculated relative to the endplates.  FIG.  25 G  illustrates an endplate definition step. As shown, an electronic display  336  of the computer system  304  (e.g., tablet  310 ) can display the captured patient image  338 , showing the angle sensors  322 . The computer system  304  can execute an image processing routine to identify the angle sensors  322  within the image  338  and to display a visual indicator  340  of the sensor axis to the user (a dotted line in the illustrated example). The user can then interact with the computer system  304 , e.g., using a touch screen, mouse, or other input device, to draw a line parallel to the vertebral endplate  342  (a solid line with circular endpoints in the illustrated example). The computer system  304  can then calculate an angular offset between the sensor axis  340  and the user-input endplate plane  342  to determine a correction factor  344  (15 degrees in the illustrated example). This process can be performed for each of the angle sensors  322 . Once complete, these compensatory angles can be used along with the angles measured by the angle sensors  322  to calculate actual absolute endplate angles, including regional angles such as lumbar lordosis, thoracic kyphosis, etc. The system can also track changes in such angles. The same or a similar approach can be used to determine coronal and/or axial angles, e.g., to obtain six degree-of-freedom information of the three-dimensional spine. 
       FIG.  25 H  illustrates the system software displaying on an electronic display  336  one or more measurements captured using the system. For example, the system can display the absolute lumbar lordosis angle  346  and the absolute thoracic kyphosis angle  348  as shown. The system can also display other spinopelvic parameters, such as pelvic incidence, pelvic tilt, sagittal vertical axis, PI-LL, and so forth. The system can also display changes in any of these parameters. The system can display these measurements in real-time as the surgery is performed and as any corrections are applied to the patient&#39;s spine. 
     In some embodiments, measurement of certain spinopelvic parameters can allow for calculation of others. For example, the following formulas can be used to calculate pelvic tilt and sagittal vertical axis: 
       PT=1.14+0.71×(PI)−0.52×(maximal lumbar lordosis)−0.19×(maximal thoracic kyphosis)
 
       SVA=−52.87+5.90×(PI)−5.13×(maximal lumbar lordosis)−4.45×(PT)−2.09×(maximal thoracic kyphosis)+0.513×(patient age)
 
     The system  300  can be used in any type of surgery, including open surgery and percutaneous or minimally-invasive surgery (MIS). As shown in  FIG.  26 A , the angle sensors  322  can be attached to the patient using percutaneous mounts  350  in some embodiments. Each mount  350  can include an elongate needle portion  352  that can be docked into vertebral bone of the patient to support the mount and attach the mount to the patient anatomy. A proximal end of the mount can include a recess  354  that can receive a sensor  322  and a set screw  356 . The mount  350  can be selectively mated to a driver  358  to facilitate application of a rotational torque to the set screw  356  and/or to the mount  350  itself. The mount  350  can thus serve as a modular instrument for degenerative and MIS procedures, allowing for global alignment and measurement via a percutaneous approach. 
     While use of the system is described above in the context of measuring various absolute spinal angles, it will be appreciated that the system can be used in any of a variety of other applications. For example, as shown in  FIG.  26 B , the system  300  can be used to measure a bend angle of a spinal rod  360  or other implant by coupling the angle sensors  322  thereto using clamps or other attachment mechanisms  362 . As shown in  FIG.  26 C , the system  300  can also be used to measure a correction or change in position associated with a derotation maneuver, for example by coupling the angle sensors  322  to bone anchors, extension tabs, quick sticks, or other implants or instrumentation  364  attached to the vertebrae  366  being corrected. As shown in  FIG.  26 D , the system  300  can measure an osteotomy angle by attaching the angle sensors  322  to bone anchors, osteotomy frames, osteotomy clamps, or other implants or instrumentation  368  attached on either side of an osteotomy  370 . 
       FIGS.  27 A- 27 J  illustrate exemplary software functionality of the system, e.g., as implemented using a software application executing on a computer system  304  comprising a processor and a memory. As shown in  FIG.  27 A , the system can include a patient information module  380  for receiving user information relating to the patient and for displaying information related to the patient. The patient information module can facilitate entry and/or display of various attributes of the patient, including name, age, sex, pre-operative anatomical measurements, notes, patient images, and so forth. The patient input module can display a graphical representation  382  or an actual image of the patient&#39;s spine. Entered information can be saved to a storage unit or database of the computer system. 
     As shown in  FIG.  27 B , the system can support use with multiple patients, and can include a patient selection screen  384  for selecting the current patient. 
     As shown in  FIG.  27 C , the system can include a planning module  386 . The planning module  386  can permit entry of pre-operative planned corrections  388  and can calculate and display a predictive post-operative estimate  390 . For example, a correction can be entered by specifying one or more locations (e.g., T10 and L3 as shown at ref. no.  388 ) and a correction angle to be performed at each level, for example via osteotomy. Based on the entered correction information, the system can calculate and display a predictive post-operative estimate  390 . In the illustrated embodiment, a 10-degree correction at T10 and a 12 degree correction at L3 are estimated to result in a −30-degree post-operative kyphosis angle as compared to a −40-degree pre-operative kyphosis angle, a 42-degree post-operative lordosis angle as compared to a 30-degree pre-operative lordosis angle, a 28-degree post-operative pelvic tilt as compared to a 36-degree pre-operative pelvic tilt, and a 57 mm post-operative sagittal vertical axis (SVA) as compared to a 103 mm pre-op SVA. The user can continue entering or editing proposed corrections until the desired post-operative geometry is displayed, thereby establishing a surgical plan. 
     As shown in  FIG.  27 D , the system can include a main menu screen  392  from which a user can begin a new procedure, recall a saved procedure, perform a quick measurement, change or view administrative settings, or exit the software application. 
     As shown in  FIG.  27 E , the system can include a sensor connection module  394 . The sensor connection module  394  can display the connection status of the various sensors  320 ,  322  of the system to the software application and other information such as the MAC address or other identifier associated with the sensor. This information can be utilized by a surgeon or other user to ensure proper set-up of the system prior to use. 
     As shown in  FIG.  27 F , the system can include a sensor positioning module  396 . The sensor positioning module can help guide a user in proper sensor placement. For example, the sensor positioning module  396  can control LEDs or other indicators on the sensors  322  to give the user an indication as to the use or positioning of any particular sensor. In the illustrated example, the sensor positioning module  396  can cause angle sensors for measuring a local angle to illuminate a blue (or other color) LED, and can display a color-coded graphical depiction  398  of the desired placement of the sensors relative to the patient. Similarly, the sensor positioning module  396  can cause angle sensors for measuring a global angle to illuminate a green (or other color) LED, and can display a color-coded graphical depiction  400  of the desired placement of the sensors relative to the patient. 
     As shown in  FIG.  27 G , the system can include a reference plane calibration module  402 . The reference plane calibration module can instruct the user to position the reference plane sensor  320  as shown to establish the sagittal plane of the patient (e.g., by correlating the sensor coordinate system to the patient&#39;s sagittal plane). In some embodiments, the calibration module  402  can perform a one-step calibration, in which the user simply aligns the reference plane sensor with the patient&#39;s sagittal plane and pushes a button on the sensor to establish the reference plane. 
     As shown in  FIG.  27 H , the system can include an intuitive measurement screen  404  for various corrections, such as multi-level osteotomies as shown. The measurement screen  404  can include any of the various measured or calculated values described above, including comparisons of planned to actual values for various spinopelvic parameters. 
     As shown in  FIG.  27 I , the system can include an option to use one or more of the angle sensors  322  for tracking an instrument during the surgery. For example, one of the local angle sensors can be attached to an osteotome to allow a position and/or orientation of the osteotome to be measured before and during bone cutting. In such an embodiment, an instrument tracking module  406  can instruct a user regarding which sensor  322  should be detached from the patient&#39;s anatomy and coupled to the instrument to be tracked. The module  406  can further provide for designating the type of instrument being tracked such that any necessary calibration can be performed and/or data regarding the instrument loaded from a data store. 
     As shown in  FIG.  27 J , the system can display angles measured in various planes, including, for example, sagittal plane  408  and axial plane  410  angle measurements. Accordingly, and as noted above, the system can be capable of measuring and calculating position and/or orientation data in both a reference plane and plane orthogonal to the reference plane. 
     The above system can be used to intraoperatively validate global spinal alignment. The system can allow continuous intraoperative monitoring, assessment, and validation of global alignment from the time a patient is placed on the operating table to the completion of the spinal deformity correction. The system can facilitate accurate and continuous intraoperative alignment validation during spinal reconstruction, which can be used to assist in measurement of the orientation of the anatomical regions of the spine prior to closing the wound when actions can still be taken to correct a less than optimal correction. The system can allow continuous validation of global spinal alignment, active real absolute value measurement of different regions of the spinal curve including thoracic kyphosis, lumbar lordosis, scoliosis and axial derotation, maintain association between standing and prone global alignment, allow spinopelvic parameter estimation, reduce wound exposure time, reduce number of steps, reduce setup errors, reduce frustration, improve speed, and enable ease of use. The system can allow segmental and global validation, measure sagittal, axial, and coronal angles, minimize setup time, enable simple calibration, provide an intuitive user interface workflow, provide accurate, repeatable and reproducible results, be adaptive to multi-level correction, use sensors having a size comparable to other instruments, and use modular sensor clips. 
     The system can allow surgeons to estimate spinal alignment correction achieved due to patient positioning, allow surgeons to estimate sagittal balance in concurrence with vertebral derotation, allow degenerative and MIS surgeons to maintain spinal alignment during surgery, assist surgeons in spinal rod bending, derotation maneuvers, and osteotomy closures, intraoperatively measure spinal correction achieved at each regional curve (e.g., lordosis, kyphosis, and scoliosis), maintain association between standing and prone patient position, provide actual regional angular value measurements and relative monitoring of the change in values during the correction, and/or allow monitoring of sagittal balance during degenerative and MIS cases. 
     The system can allow for optimization of interbody cage placement in some embodiments. For example, the system can be used to measure how much an interbody cage expanded disc space or corrected an angle in real time. In the case of expandable or adjustable cages, the system can be used to inform expansion or contraction of the cage based on real-time measurement of the achieved correction. This can increase the efficiency of operations to implant such cages. 
     Entire setup of the system can be completed before an actual osteotomy and correction is performed. As soon as setup is complete, a surgeon or other user can measure and associate changes in sagittal balance due to patient positioning. The surgeon can re-plan execution due to any correction achieved from patient positioning. The system can track changes in actual absolute regional angles due to operative execution. The system can allow continuous validation without requiring additional fluoro-shots, which is not possible with existing systems that require new fluoro-images be captured each time sagittal balance is to be validated. 
     Restoration of spinal global alignment can be important in complex spinal deformity surgery. Spinal osteotomies are established surgical techniques to correct spinopelvic malalignment. Reports have demonstrated that improved sagittal spinal alignment following spinal osteotomies correlate with improved health related quality of life (HRQOL) scores. Spine osteotomies can be broadly divided into four main types: Smith-Petersen, Ponte, Pedicle Subtraction Osteotomy (PSO), and Vertebral Column Resection (VCR). The type of osteotomy used depends on both the location of the spinal deformity and on the amount of correction that is required. A spinal fusion with instrumentation may also be performed along with spine osteotomy to stabilize the spine and prevent further curvature. It is commonly reported that many times outcome of these surgeries can result in negative impact of sagittal balance resulting in disability, pain, deficient forward gaze and poor health related quality of life (HRQOL). An intraoperative monitoring of the sagittal balance can help achieve optimum outcomes when treating spinal disorders. Even when addressing problems in the coronal and axial plane, an awareness of sagittal balance can avoid future complications. Global spinal malalignment is often difficult to assess and measure during the surgery when the patient is in the prone position. However, surgery is the crucial period because it is during surgery that alignment can be corrected. Currently there is no quick way to measure sagittal balance and regional curves intraoperatively during deformity and degenerative surgeries. Current techniques include in-vivo use of many cobbled together methods including a combination of “eyeballing,” experience, estimation, multiple fluoro-images, or the use of templates, wedges, or rasps to estimate spinal angular correction to facilitate intra-operative validation of the global alignment to spinal reconstruction. In some cases, the technique is to take fluoro-images, send them to a PACs system, and use the images to measure correction using a manual protractor. This technique requires the surgeon to leave the sterile field, which is not desirable and results in increased surgery time. In addition, placing that patient prone on the operating table can change their global alignment and regional curves from what has been assessed in standing pre-operative film. This change in correction due to the patient positioning can warrant a change in the original plan for the osteotomy execution. 
     The instruments disclosed herein can be designed to be disposed of after a single use, or they can be designed to be used multiple times. In either case, however, the instrument can be reconditioned for reuse after at least one use. Reconditioning can include any combination of the steps of disassembly of the instrument, followed by cleaning or replacement of particular pieces, and subsequent reassembly. In particular, the instrument can be disassembled, and any number of the particular pieces or parts of the instrument can be selectively replaced or removed in any combination. Upon cleaning and/or replacement of particular parts, the instrument can be reassembled for subsequent use either at a reconditioning facility, or by a surgical team immediately prior to a surgical procedure. Those skilled in the art will appreciate that reconditioning of an instrument can utilize a variety of techniques for disassembly, cleaning/replacement, and reassembly. Use of such techniques, and the resulting reconditioned instrument, are all within the scope of the present disclosure. 
     The instruments described herein can be processed before use in a surgical procedure. First, a new or used instrument can be obtained and, if necessary, cleaned. The instrument can then be sterilized. In one sterilization technique, the instrument can be placed in a closed and sealed container, such as a plastic or TYVEK bag. The container and its contents can then be placed in a field of radiation that can penetrate the container, such as gamma radiation, x-rays, or high-energy electrons. The radiation can kill bacteria on the instrument and in the container. The sterilized instrument can then be stored in the sterile container. The sealed container can keep the instrument sterile until it is opened in the medical facility. Other forms of sterilization known in the art are also possible. This can include beta or other forms of radiation, ethylene oxide, steam, or a liquid bath (e.g., cold soak). Certain forms of sterilization may be better suited to use with different portions of the instrument due to the materials utilized, the presence of electrical components, etc. 
     One skilled in the art will appreciate further features and advantages of the disclosure based on the above-described embodiments. Accordingly, the disclosure is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.