Abstract:
Use of measurement system and detection module with a burr instrument for surgical procedures. The measurement system may include sensors (e.g., displacement, acceleration, and/or force sensors) that may be utilized to detect potentially hazardous operation of the burr instrument. For example, when penetrating through the cortex of the bone the measurement system may be operated in a unicortical mode such that when the burr passes through the cortex of the bone, the instrument operating may be arrested to avoid damage to surrounding tissue. Moreover, the system may be utilized to detect a slip or sudden loss of contact of the burr instrument such that the instrument may be arrested to avoid damage to surrounding tissue. Furthermore, the burr instrument may have a predetermined ramp-up speed.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/252,193 filed Nov. 6, 2015, entitled “MEASUREMENT SYSTEM FOR USE WITH SURGICAL BURR INSTRUMENT,” which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Powered instruments are often used in surgical procedures, and especially in orthopedic procedures. For instance, such instruments are often used to perform operations on a bone of the patient. Examples of such operations including drilling, sawing, grinding, or the like. The objectives of such operations are many and may, for example, include for inserting an orthopedic appliance, shaping a bone, or other appropriate operations. As may be appreciated, powered tools may assist a surgeon in performing a surgical operation by speeding such operations. Improving the speed at which operations may occur may be beneficial to limit the duration of anesthesia for the patient and/or may assist ih avoiding other complications. 
         [0003]    Despite the assistance that powered tools may provide, control of such instruments may be more difficult than hand-powered tools. In turn, unintended contact of the instrument with surrounding tissue (e.g., soft tissue surrounding the bone on which the operation is to occur) may cause damage to such surrounding tissue. In turn, reducing the risk of such unintended contact of the instrument with tissue may assist in improving surgical outcomes. 
       SUMMARY 
       [0004]    In view the foregoing, a measurement system may be used with an instrument for use in connection with operation of an instrument. A detection module may be provided in operative communication with the measurement system to measure one or more parameters measured by the measurement system. The detection module may arrest the instrument to help reduce the possibility of damaging tissue surrounding a bone on which the instrument operates. For example, U.S. Pat. No. 6,665,948, U.S. Pat. Pub. No. 2015/0066035, U.S. Pat. Pub. No. 2015/0141999, and U.S. application Ser. No. 14/845,602, all of which are incorporated by reference herein in their entirety, disclose various systems and approaches to monitoring a powered surgical instrument. However, such disclosures have conventionally been directed to the measurement of the position of a working portion of a tool relative to a bone of a patient. In turn, these contexts involve measurement of the depth of a bore created by a drill bit or arresting a working tool portion upon engagement of the working tool with a specific portion of the bone. 
         [0005]    In the present disclosure, the instrument may comprise a powered burr tool for use in grinding a bone. The burr tool may be used to bore into a bone in a manner similar to a drill as described in the disclosures incorporated by reference above. Additionally or alternatively, a burr tool may be used on a surface of a bone to grind a portion thereof. That is, the burr tool may be advanced radially relative to an axis of rotation of the burr tool. However, when using a burr tool, it may be more likely that the bone substrate fails, resulting in release of the instrument from the bone. Furthermore, given that a burr tool may be used to operate on an exterior surface of a bone, the potential that the instrument may slips from the external surface may be increased. 
         [0006]    As such, the disclosure presented herein includes use of an instrument (e.g., a burr tool or other instrument) to operate on a bone. Furthermore, a measurement system may be utilized to measure one or more parameters related to the movement of the burr tool. In turn, a detection module may be provided to monitor the one or more parameters and control the instrument in response thereto. As such, the detection module may help reduce the potential of surrounding tissue damage when using a burr tool to perform an operation (e.g., a grinding operation) on an exterior portion of a bone. For instance, given that the measurement system may be operative to measure an applied force and/or displacement of the tool relative to a reference point, the measurement system may be operative to detect an instance in which the instrument slips from (i.e., suddenly loses contact with) the bone. Upon detection o such slippage or loss of contact, the instrument may be arrested to prevent and/or limit damage to&#39;surrounding tissue. Furthermore, the burr tool may be arrested upon completion of an operation relative to the bone (e.g., whether performed unicortically or bicortically). 
         [0007]    As such, the measurement system may include a displacement sensor, acceleration sensor, and/or a force sensor that may be utilized to monitor the instrument. A detection module may in monitor for an acceleration that exceeds a threshold or a force acting on the burr tool is reduced relative to a threshold (e.g., that may be indicative that the instrument has slipped). The instrument may be arrested upon completion of such an operation. In another context, the measurement system may be able to detect rapid accelerations of the instrument (e.g., using the displacement sensor). These rapid accelerations may result from the instrument slipping from the bone. As such, detection of such rapid acceleration may also cause the instrument to arrest to prevent damage surrounding tissue. 
         [0008]    Furthermore, the burr instrument may have a predetermined ramp-up speed in which the angular velocity of the burr instrument upon start up slowly accelerates along a ramp-up profile. This may assist in providing safety with the burr instrument to prevent unintentional contact of the burr instrument at high speed with soft tissue. 
         [0009]    Accordingly, a first aspect relates to an instrument for use in a surgical operation. The instrument may include a drive system for rotationally driving a working tool engaged with the instrument about a working axis and a burr tool engaged with the instrument for rotation of the burr tool by the drive system about the working axis. The burr tool is contactably engageable with a medium. The instrument may also have a measurement system for monitoring the contactable engagement of the burr tool with the medium and a detection module in operative communication with the measurement system to detect a loss of the contactable engagement of the burr tool with the medium. 
         [0010]    A number of feature refinements and additional features are applicable to the first aspect. These feature refinements and additional features may be used individually or in any combination. As such, each of the following features that will be discussed may be, but are not required to be, used with any other feature or combination of features of the first aspect. 
         [0011]    For example, the measurement system may include an accelerometer for detecting an acceleration of the burr tool relative to the medium. The accelerometer may include a displacement sensor for outputting a displacement measure of the burr tool relative to the medium and an acceleration value may be determined based on a second derivative of the displacement measure. Additionally or alternatively, the accelerometer may measure an acceleration of the instrument directly. In either regard, the detection module may detect the loss of the contactable engagement of the burr tool with the medium in response to the acceleration of the burr tool relative to the medium exceeding an acceleration threshold value. 
         [0012]    In an embodiment, the acceleration threshold value may be a value that is selectable by a user of the instrument. The acceleration threshold value may include a maximum acceleration magnitude value. Alternatively, the acceleration threshold value may include a maximum rate of change of the acceleration of the burr tool. 
         [0013]    In an embodiment, the measurement system may include at least one force sensor. The at least one force sensor may be arranged relative to the burr tool to measure a force acing axially along the working axis. Also, the at least one force sensor may be arranged relative to the burr tool to measure a force acing radially relative to the working axis. 
         [0014]    In an embodiment, the detection module may detect the loss of the contactable engagement of the burr tool with the medium in response to a reduction in force acting on the burr tool exceeding a force threshold value. The force threshold value may include a maximum rate of change of force and a loss of the contactable engagement of the burr tool is detected upon the rate of change in the force acting on the burr tool exceeding the maximum rate of change. 
         [0015]    In an embodiment, the drive system may be operative to initiate rotation of the working tool such that a rotational speed of the working tool increases along a defined acceleration profile. The defined acceleration profile may be linear. Moreover, upon detection of the loss of the contactable engagement of the burr tool with the medium by the detection module, the drive system may be deactivated. Also, the instrument may include a brake in operative communication with the drive system. In turn, upon detection of the loss of the contactable engagement of the burr tool with the medium, the brake is activated to cease rotation of the working tool. 
         [0016]    A second aspect is directed to a method for use of an instrument. The method includes operating an instrument to perform an operation relative to a bone of the patient by contacting a working portion of the instrument with the bone of the patient and monitoring the working portion of the instrument. The method also includes detecting a loss of contact between the working portion and the bone and arresting the instrument upon detection of the loss of contact between the working portion and the bone. 
         [0017]    A number of feature refinements and additional features are applicable to the second aspect. These feature refinements and additional features may be used individually or in any combination. As such, each of the following features that will be discussed may be, but are not required to be, used with any other feature or combination of features of the second aspect. 
         [0018]    For instance, the monitoring may include measuring one or more parameter using a measurement system of the instrument. Specifically, the measurement system may include an acceleration sensor and the one or more parameters may be an acceleration of the working tool relative to the bone. The detecting may include measuring the acceleration of the working tool relative to a bone that exceeds an acceleration threshold value. 
         [0019]    Additionally or alternatively, the measurement system may include a force sensor and the one or more parameters may be a change in force acting on the working tool. In turn, the detecting may include measuring the change in force acting on the working tool that exceeds a force threshold value comprising a maximum rate of change in force. 
         [0020]    The arresting may include ceasing operation of a drive system of the instrument. Additionally or alternatively, the arresting may include applying a brake to a drive system of the instrument. 
         [0021]    A third aspect is directed to a method of use of a powered surgical burr tool. This method includes initiating rotation of the burr tool with the instrument. Specifically, the rotation of the burr tool accelerates according to an acceleration profile such that full speed of the burr tool is delayed by a known time after the initiating. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0022]      FIG. 1  depicts an embodiment of a working portion of a burr tool. 
           [0023]      FIG. 2A  depicts an embodiment of an instrument having a measurement system that may be used with a burr tool. 
           [0024]      FIG. 2B  depicts an embodiment of an instrument having a measurement system that may be used with a burr tool. 
           [0025]      FIGS. 3A-3C  depict various vies of an embodiment of an instrument having a measurement system that may be used with a burr tool. 
           [0026]      FIG. 4  depicts an embodiment of an instrument having a measurement system with a housing thereof not shown for clarity. 
           [0027]      FIG. 5  depicts an embodiment of a burr tool assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    The following description is not intended to limit the invention to the forms disclosed herein. Consequently, variations and modifications commensurate with the following teachings, skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular applications(s) or use(s) of the present invention. 
         [0029]    As described above, the present disclosure generally relates to an instrument  50  such as that shown in the figures. The instrument  50  may be a powered surgical instrument such as a drill, saw, reciprocating tool, or the like. In one embodiment, the instrument  50  may be engaged with a burr tool  16  like the one shown in  FIG. 1 . The burr tool  16  may include a working portion  12  that may have fluted or ridged portions. The working portion  12  may allow for abrading or grinding a bone or other calcified or hardened substrate. As such, it may be appreciated the burr tool  16  may be used in a context similar to a drill bit, where the burr tool  16  is advanced axially along a working axis  120  about which the burr tool  16  is rotated. Additionally or alternatively, the burr tool  16  may be used to grind by advancing the burr tool  16  in a direction perpendicular to the working axis  120 . Moreover, a combination of radial and axial movement may also be used and monitored as described in greater detail below. 
         [0030]    In this regard, as has been described in U.S. Pat. No. 6,665,948, U.S. Pat. Pub. No. 2015/0066035, U.S. Pat. Pub. No. 2015/0141999, and U.S. application Ser. No. 14/845,602, all of which are incorporated by reference above, various methodology and approaches to monitoring a tool as it is advanced bicortically or unicortically through a bone of a patient. In this regard, in the event the burr tool  16  is utilized in a manner similar to a drill bit where the burr tool  16  is advanced to create a bore in the bone, any of the methodology or structure utilized in the references incorporated therein may be used in conjunction with the burr tool  16 . That is, the burr tool  16  may stand in the place of any tool, instrument, or drill bit described in any of the incorporated disclosure. As such, as shown in  FIG. 2A, 2B, 3A, 3B, 3C, 4, and 5 , the burr tool  16  may be used with any type of measurement system described in the incorporated references. As such, the burr tool  16  may be used to create a bore. Upon the burr tool  16  passing through a bone ora portion of a bone, the instrument driving the burr tool  16  may be arrested. This may prevent unintended damage to tissue surrounding the bone when using the burr tool  16 . 
         [0031]    Referring to  FIGS. 2A and 2B , an instrument  50  may include a measurement system  100  that comprises a displacement sensor  102 , a force sensor  104 , and a controller assembly  106 . The displacement sensor  102  may be connected to the instrument housing  26 . The connection can be made by a variety of well known mounting methods such as a mount that clamps to the displacement sensor  102  and is attached to the instrument housing  26  by one or more threaded fasteners. Alternative methods such as welding or adhesive bonding could also be used. 
         [0032]    The displacement sensor  102  may include a transducer  108  that outputs a displacement signal that is representative of a displacement, with respect to the reference point, of the leading edge  16   a  of the burr tool  16 . The displacement sensor  102  may have an extension  110  that is displaceable along a longitudinal axis. The extension  110  has a distal end  110   a  that can be placed in registry with the reference point when the leading edge  16   a  of the burr tool  16  is positioned relative to a bone (e.g., placed in contact therewith). The distal end  110   a  may be maintained in registry with the reference point throughout the process conducted using the burr tool  16 . The reference point can be any anatomical structure proximal to the location at which the burr tool  16  is used. The extension  110  has a proximal end  110   b  that is attached to the first sensor  102 . Preferably the transducer  102  is a linear variable differential displacement transducer (“LVDT”). 
         [0033]    Furthermore, it may be appreciated that the spacing of the extension  110  of the displacement sensor  102  from the burr tool  16  may introduce the potential for errors or other disadvantages in determining the displacement of the burr tool  16  relative to the reference point. For instance, as the extension  110  may contact a structure that is offset from the contact point between the leading edge  16   a  of the burr tool  16  and the medium to be subjected to an operation using the burr tool  16 . Accordingly, any movement between the structure contacted by the extension  110  and the medium on which the burr tool  16  is used may be falsely registered as relative movement of the burr tool  16  with respect to the reference point. Furthermore, there may not be a rigid structure to contact adjacent to the medium to be contacted, leading to displacement of the structure contacted by the extension  110  (e.g., such as in the case where the extension  110  may contact soft tissue adjacent to the medium to be contacted given the offset from the location to be contacted). Furthermore, the offset nature of the extension  110  relative to the contact between the burr tool  16  and the medium to be contacted may lead to other complications such as having to expose a greater surface of the medium to be contacted, which may adversely affect patient outcomes. 
         [0034]    In this regard, and with further reference to  FIG. 2B , an embodiment of an instrument  50  may be provided that includes an acceleration sensor  112 . An acceleration sensor  112  may be provided as an alternative to or in addition to the displacement sensor  102  described above. The acceleration sensor  112  may be a MEMS accelerometer, an optical accelerometer, a piezoelectric accelerometer, or any other appropriate accelerometer. In turn, the acceleration sensor  112  may be operative to determine an acceleration of the instrument  50 , which may be rigidly interconnected with the burr tool  16 . As such, an acceleration of the burr tool  16  may be sensed by the acceleration sensor  112 . 
         [0035]    Further still, in either of the embodiments of an instrument  50  depicted in  FIGS. 2A or 2B  may include a force sensor  104 . Specifically, the working portion load measurement assembly  104  may include one or more force sensors. The one or more force sensors may be arranged to measure a force applied to the burr tool  16  when in use. A force sensor element may be arranged to measure an axial load acting on the burr tool  16  that results in advancement of the burr tool  16  as the burr tool  16  is advanced axially relative to the working axis  120 . In addition, one or more force sensor elements may be provided that are arranged to measure a force acting in a radial direction relative to the working axis  120 . In this regard, in a grinding operati 9 n where the burr tool  16  is advanced in a direction radial to the working axis  120 , a force acting on the burr tool  16  may be monitored. In turn, monitoring of the force acting on the burr tool  16  (e.g., either axially or radially) may assist in determining unintended slippage of the burr tool  16  as will be discussed in greater detail below. 
         [0036]    Another embodiment of an instrument with a displacement sensor including a displacement sensing arm that extends from the instrument may be provided. For example, such a displacement sensing arm may be provided that may coordinate with a bushing member of a burr tool assembly that may be used with the instrument. In this regard, the bushing may move along the burr tool in a direction corresponding to the axis of rotation of the burr tool. Upon engagement of the bushing and the displacement sensing arm, the bushing and displacement sensing arm may undergo corresponding movement. As such, the bushing may be disposed in contact with the medium to be contacted when the leading edge of the burr tool is in contact with the medium. As such, a reference point may be established when the bushing and leading edge of the burr tool are both in contact with the medium to be contacted by the burr tool. As the bushing is located adjacent to (e.g., partially or fully surrounding the burr tool), the bushing may facilitate contact with the medium at or very near the location to be contacted prior to initiation of an operation using the burr tool. In this regard, the reference point may be more accurately maintained as the bushing may contact at least a portion of a periphery of the bone adjacent to where the burr tool is used to remove tissue. That is, the bushing may remain in intimate contact with the medium to be contacted adjacent to the burr tool. This may prevent false displacement readings attributable to the foregoing problems associated with an offset extension  110 . Furthermore, the amount of contact of the bushing may be localized at the location to be contacted by the burr tool, thus allowing for potentially less intrusion when performing operations using the burr tool. 
         [0037]    For example, with additional reference to  FIGS. 3A, 3B, 3C, and 4 , an embodiment of an instrument  50  comprising an embodiment of a measurement system  400  is shown. The instrument  50  may be adapted for use with a burr tool assembly  60  (shown in  FIG. 5 ) that may include a bushing  452 . The instrument  50  may integrally comprise at least some components of the measurement system  400  to facilitate operation of the measurement system  400  in connection with the instrument  50  (e.g., which may be according to the description above regarding measurement system  100 ). For example, at least a portion of a displacement sensor  410  may be integrated into a housing  26  of the instrument  50 . In this regard, the displacement sensor  410  may include a depth sensing arm  412  that is specifically adapted for engagement with a bushing  452  of a burr tool assembly  60  that may be engaged by the chuck  420  of the instrument  50 . 
         [0038]    In this regard, the depth sensing arm  412  maybe used to establish a reference point from which displacement of the burr tool  16  may be measured as described above. In this regard, as follows herein, a general description of the features and operation of the instrument  50  used in conjunction with the burr tool assembly  60  is provided. 
         [0039]    The displacement sensor  410  may include a depth sensing arm  412  that may extend from the instrument housing  26 . For example, the depth sensing arm  412  may extend distally (e.g., from a distal face  30  of the instrument  26 ) in a direction corresponding with the direction in which the burr tool  16  extends from a chuck  420  of the instrument  50 . As such, the chuck  420  may engage the burr tool  16 . At least a portion of the displacement sensing arm  412  may extend from the instrument housing  26  parallel to an axis of rotation  120  of the instrument  50 . The depth sensing arm  412  may also include a distal portion  414  that is adapted to engage a bushing  452  provided with the burr tool assembly  60  shown in  FIG. 4 . As used herein, distal may correspond to a direction from the instrument  50  toward the leading edge  16   a  of the burr tool  16  and proximal may correspond to a direction from the leading edge  16   a  of the burr tool  16  toward the rear portion of the instrument  50 . In this regard, at least a portion of the depth sensing arm  412  (e.g., the distal portion  414 ) may be adapted to engage the bushing  452  of the burr tool assembly  60  as will be described in more detail below. In any regard, at least a portion of the depth sensing arm  412  may extend into the housing  26 . The housing  26  may contain a coil  416 . As such, a proximal end  418  of the displacement sensing arm  412  may interface with the coil  416  of the displacement sensor  410  that may be disposed within the instrument housing  26 . 
         [0040]    Specifically, in  FIG. 4 , the depth sensing arm  412  is shown in a retracted position relative to the burr tool  16 . As such, this retracted position shown in  FIG. 11  may occur when the burr tool  16  is advanced relative to the bushing  452  during an operation. In this regard, the proximal end  418  of the displacement sensing arm  412  is disposed within the coil  416  of the displacement sensor  410 . Accordingly, the displacement sensor  410  may comprise an LVDT sensor as described above that is adapted to sense the position of a core  422  relative to a coil  416 . The displacement sensing arm  412  may incorporate a core  422  at the proximal end  418  thereof. Accordingly, as the proximal end  418  of the displacement sensing arm  412  is moved relative to the coil  416 , the location of the core  422  may be determined to provide an output corresponding to the position of the core  422 , and in turn the displacement sensing arm  412  relative to the instrument housing  26 . That is, the depth sensing arm  412  may be displaceable relative to the coil  416  such that the displacement sensor  410  may be operable to sense a change in position of the depth sensing arm  412  relative to the instrument housing  26  and output a measure of the displacement of the burr tool relative to a reference point. In an embodiment, the total measurable travel of the core  422  relative to the coil  416  may be at least about 2.5 in (6.4 cm). Furthermore, the resolution of the output of the displacement sensor  410  may be about 0.1% (e.g., about 0.002 inches (0.06 mm) for a sensor having a total measureable travel of 2.5 inches). 
         [0041]    While a LVDT displacement sensor is shown and described in relation to the instrument  50  shown in the accompanying figures, it may be appreciated that other types of displacement sensors may be provided. For instance, the sensor may provide for the absolute or relative measurement of the position of the distal end  418  of the displacement sensing arm  412  to provide a displacement measure. For instance, in another embodiment, an optical displacement sensor may be provided. Other types of displacement sensors are also contemplated such as, for example, a capacitive displacement sensor, ultrasonic sensors, Hall effect sensors, or any other sensors known in the art capable of outputting an absolute or relative position measure. 
         [0042]    In an embodiment, the coil  416  may define a passage  424  extending at least partially through the housing  26 . Specifically, the passage  424  may extend from a proximal face  32  of the housing  26  to the distal face  30  of the housing  26 . That is, the passage  424  may extend entirely though the housing  26 . An end cap  34  may be provided that is operable to close the proximal end of the passage  424  at the proximal face  32  of the instrument housing  26 . Furthermore, a biasing member  426  (e.g., a coil spring) may be provided in the passageway  424  at a proximal end thereof. The biasing member  426  may be provided between the end cap  34  and the proximal end  418  of the displacement sensing arm  412 . In this regard, the biasing member  426  may act on the proximal end  418  of the displacement sensing arm  412  to bias the displacement sensing arm  412  distally relative to the passage  424  and instrument housing  26 . 
         [0043]    As such, the displacement sensing arm  412  may include features that selectively prevent ejection of the displacement sensing arm  412  from the distal end of the passage  424 . For example, the displacement sensing arm  412  may include at least one flat  428  that extends along a portion of the arm  412 . At the proximal and distal extents of the flat  428 , the displacement sensing arm  412  may include shoulders  436  that project from the flats  428 . As such, at the proximal opening of the passage  424 , a selectively displaceable stop  438  (best seen in  FIG. 23 ) may be disposed relative to the flat  428  such that the flat  428  may move relative to the stop  438 , but interfere with the shoulder  436  defined in the displacement sensing arm  412  to prevent passage of the shoulder  436  beyond the stop  438 . In this regard, the length of the displacement sensing arm  412  along which the flat  428  extends may be moveable relative to the stop  438 , and the stop  438  may limit proximal and distal movement of the displacement sensing arm  412  beyond the stop  438 . 
         [0044]    However, the stop  438  may be displaceable upon depressing a button  440  provided on an exterior of the housing  26 . Other types of actuators may be used to control the stop  438  and a button  440  is provided as merely one example. Thus, upon depressing the button  440 , the stop  438  may be displaced away from the displacement sensing arm  412  to allow the shoulder  436  to pass distally from the distal end of the passage  424  such that the displacement sensing arm  412  may be removed entirely from the passage  424 . The distal end of the flats  438  may include a detent  442  that may be engageable with the stop  438  so as to maintain the displacement sensing arm  412  in a proximally disposed, retracted position relative to the housing (e.g., as shown in  FIG. 4 ). Once the button  440  is depressed and released, the detent  442  at the proximal end of the flat  428  of the displacement sensing arm  412  may be released by the stop  438  and the displacement sensing arm  412  may move proximally (e.g., under influence of the biasing member  426 ). The displacement sensing arm  412  may move proximally until the shoulder  436  at the distal end of the flat  428  are engaged to prevent further distal movement of the displacement sensing arm  412 . Accordingly, the displacement sensing arm  412  may be retained in a retracted position (e.g., for improved visibility of the distal end of the burr tool  16 ), released to be moveable relative to and biased proximally with respect to the housing  26 , and removable altogether from the housing  26 . 
         [0045]    In the latter regard, removal of the displacement sensing arm  412  and biasing member  426  from the passage  424  may allow for separate cleaning (e.g., in an autoclave) of those members. Additionally, removal of the end cap  34  may allow for a cleaning apparatus (e.g., a brush or the like) to be passed through the full length of the passage  424  to facilitate cleaning thereof. 
         [0046]    As referenced above, the distal portion  414  of the displacement sensing arm  412  may be adapted to engage a burr tool assembly  60  (e.g., a bushing  452  thereof) that is correspondingly adapted for use with the instrument  50 . For instance, the displacement sensing arm  412  may generally be linear along the proximal portion  418  of the displacement sensing arm  412 . In this regard, the proximal portion  418  may be adapted to be collinear with the passage  424  and moveable within the passage  424 . Furthermore, the distal portion  414  of the displacement sensing arm  412  (e.g., the portion distal to the linear portion of the displacement sensing arm  412 ) may extend from the linear portion of the displacement sensing arm  412  toward the burr tool assembly  60  that may be engaged by the chuck  420  of the instrument  50 . In this regard, the linear portion of the displacement sensing arm  412  may be substantially parallel to and offset from the axis of rotation  120 . The distal portion  414  may extend from the linear portion in a direction corresponding with the offset such that the distal portion  414  extends toward the burr tool assembly  60 . This may facilitate engagement between the displacement sensing arm  412  and the bushing  454  of the burr tool assembly  60 . As shown, in  FIGS. 3A-4 , the distal portion  414  may be an at least partially arcuate member extending along a radius of curvature toward the burr tool assembly  60 . However, the distal portion  414  may be shaped differently (e.g., the distal portion  414  may be a linear portion extending at an angle or perpendicularly from the proximal  418  toward the burr tool assembly  60 ). 
         [0047]    With further reference to  FIG. 5 , an embodiment of a burr tool assembly  60  that may be used in conjunction with the instrument  50  is depicted. The burr tool assembly  60  may include a shank  454  that is disposed adjacent to a proximal end of the assembly  60 . There may be sharped fluted ridges disposed in a working portion  453  adjacent to the leading edge  16   a  of the burr tool assembly  16 . These ridges may include cutting edges that, when rotated serves to abrade or grind the medium into which the burr tool  16  is advanced. In certain applications, the working portion  453  may be generally spherical. In other applications, the working portion  453  may be conical, ovoid, or any other appropriate shape. Moreover, the working portion  453  may be of any appropriate size for application in various different contexts in which the burr tool may be utilized. A cylindrical member  458  may extend between the shank  454  and the working portion  453 . The working portion  453 , cylindrical body  458 , and shank  454  may collectively define the burr tool  16 . 
         [0048]    In addition to the burr tool  16 , the burr tool assembly  60  may also comprise a bushing  452  as referenced above. The bushing  452  may engage the cylindrical member  458  to facilitate relative movement of the bushing  452  relative to the cylindrical member  458  along a direction corresponding to the axis of rotation  120 . For example, the bushing  452  may include an aperture  460  through which at least a portion of the cylindrical member  458  may be disposed. The aperture  460  may form a cylindrical opening that extends at least in a direction corresponding to the axis of rotation  120  of the burr tool  16 . The cylindrical opening may be sized to receive the cylindrical member  458  therein such relative movement between the cylindrical opening and the cylindrical member  458  is provided. Moreover, the bushing  452  may be sized so as to be operable to pass over the working portion  453 . In this regard, the bushing  452  may surround the working portion  453  when the bushing is in a distal position such that the leading edge  16   a  of the working portion  453  is aligned axially with the distal edge of the bushing  453 . As such, the burr tool  16  may be free to rotate within the aperture  460  (e.g., even when the bushing  452  is disposed about the working portion  453 ), and the bushing  452  may slideably engage the cylindrical member  458  for relative movement therebetween that is constrained along the direction corresponding to the axis of rotation  120 . Further still, as the bushing member  452  may surround the working portion  453  when in a biased, distal position, the working portion  453  may be at least partially shielded to prevent unintended contact with surrounding tissue. Moreover, as the working portion  453  is advanced relative to the bone to be the target to an operation, the bushing  452  may be moved proximally relative to the working portion  453  so as to allow measurement of the displacement of the working portion  453 . This may be particularly helpful in relation to monitoring the working portion  453  to detected unintended slippage that may otherwise lead to damage to the surrounding tissue. 
         [0049]    The bushing  452  may include an engagement member  456  that is disposed on the bushing  452  and adapted for engagement with a displacement sensing arm  412  of an instrument  50  to which the burr tool assembly  60  is engaged. For instance, as depicted in  FIG. 5 , the engagement member  456  may comprise a post  462  extending from the bushing  452 . The post  462  may extend away from the axis of rotation  120  of the burr tool assembly  60 . In an embodiment, the post  462  may extend perpendicularly to the axis of rotation  120 . Accordingly, the post  462  may engage a hole  464  provided on the distal portion  414  of the displacement sensing arm  412 . In this regard, the post  462  may extend into the hole  464 . Movement of the bushing  452  relative to the burr tool  16  in a direction corresponding to the axis of rotation  120  may result in the post  462  acting on the hole  464  such that the displacement sensing arm  412  undergoes corresponding movement upon movement of the bushing  452  relative to the burr tool  16 . In turn, as described above, the core  422  at the proximal portion  418  the displacement sensing arm  412  may also undergo corresponding movement relative to the coil  416 , which may be detected by the displacement sensor  410  and output as a displacement measure. 
         [0050]    It may be appreciated that other arrangements for engaging the bushing  452  with the displacement sensing arm  412  may be provided so that the bushing  452  and displacement sending arm  412  undergo corresponding movement. For example, other structures such as clasps, fasteners, or other mechanisms may be utilized to engage the bushing  452  to the displacement sensing arm  412 . Furthermore, the bushing  452  may, in some embodiments, be integrally defined on the distal portion  414  of the displacement sensing arm  412 . In this regard, a standard burr tool  16  may be engaged with a chuck  420  of the instrument  50  and the bushing  452  may be disposed relative to the burr tool  16 . In any regard, the bushing  452  may be pivotal relative to the displacement sensing arm  412  (e.g., in a direction perpendicular to the axis of rotation  120 ) to facilitate ease of engagement of the bushing  452  with the displacement sensing arm  412  or the bushing  452  with the burr tool  16  when engaging the burr tool  16  with the chuck  420  of the instrument  50 . 
         [0051]    In any regard, the shank  454  of the burr tool assembly  60  may be engaged with the chuck  420  of the instrument  50 . In this regard, the burr tool  16  may be fixed relative to the instrument  50  in the direction along the axis of rotation  120 . In turn, the bushing  452  may be displaceable relative to the burr tool  16  along the axis of rotation  120 . In this regard, when the burr tool  16  is advanced relative to a medium during an operation, the bushing  452  may remain stationary at a reference point established prior to the operation and the displacement sensor  410  may be operable to detect the relative motion between the burr tool  16  and the bushing  452  retained in a stationary position relative to the reference point, thus providing a measure of the relative movement of the burr tool  16  relative to the reference point. 
         [0052]    As may be appreciated, when using the instrument  50 , a force sensor for measurement of force acting on the leading edge  16   a  of the burr tool  16  may also be provided. In this regard, a force sensor  118   a  (e.g., a force sensor such as piezoelectric crystal) may be disposed proximally to the instrument drive assembly  430  (which may include the chuck  420 , gear drive  434 , and motor  432 ). In turn, force acting on the leading edge  16   a  of the burr tool  16  as it is advanced axially may be transferred to the force sensor  118   a  via the drive system  430 . That is, the axial force acting on the leading edge  16   a  of the burr tool  16  may be transferred through the shank  454  of the burr tool  16  to the chuck  420 , and the drive system  430 . In turn, the drive  430  may act upon the force sensor  118   a  to produce an output corresponding to the force acting on the leading edge  16   a.  In this regard, it may be appreciated that the rigid assembly of the drive  430 , chuck  420 , and burr tool  16  may transmit the force acting on the leading edge  16   a  of the burr tool  16  to the force sensor  118   a.  It may further be appreciated that the drive assembly  430  may be fixed rotationally relative to the instrument housing  26  so as to impart rotation to the chuck  420 . However, the drive  430  may be preferably free to move in a direction along the axis of rotation  120  such that the at least a majority of the force acting on the leading edge  16   a  of the burr tool  16  may be transferred to the force sensor  118 . 
         [0053]    In addition to the force sensor  118   a  that is capable of measuring an axial force acting on the burr tool  16 , a number of additional force sensors  118   b  may also be provided. These force sensors  118   b  may be arranged radially about the working axis  120 . In turn, a force acting on the burr tool  16  in a direction radial relative to the working axis  120  may be measured by the force sensors  118   b.  As such, if the burr tool  16  is advanced in a radial direction (e.g., to perform a grinding operation or the like), the radial force acting on the burr tool  16  may also be measured. 
         [0054]    In an embodiment, the force sensors  118   a  and  118   b  may have a range of measureable force from about 0 lbf (0 N) to about 100 lbf (445 N). In an embodiment, the force sensors  118   a  and  118   b  may have a range of measurable force from about 0 lbf (0 N) to about 25 lbf (111 N). The force sensors  118   a  and  118   b  may have a precision of at least about 1% of the maximum measureable force. Accordingly, in an embodiment, the force sensors  118   a  and  118   b  may have a precision of at least about 0.25 lbf (1.1 N). In an embodiment, the force sensors  118   a  and  118   b  may have a precision of 0.5% (e.g., about 0.125 lbf (0.56 N)). 
         [0055]    As such, a displacement sensor (e.g.,  102  or  40 ), an acceleration sensor  112 , and/or one or more force sensors (e.g.,  104  or  118   a  or  118   b ) may be provided with a measurement system used with a burr tool  16 . As such, the various sensors may comprise a measurement system that may be used to monitor the burr tool  16  during operation. While the displacement and/or acceleration measured by the measurement system  40  may be used by the instrument  50  in conjunction with use of a burr tool  16  for creation of bore (e.g., to arrest the instrument upon passing through all or a portion of the bone), the measurement system  40  may also monitor the burr tool  16  during a grinding or boring operation. For instance, burr tools  16  may be used to shave a bone in the spine of a patient which may be conducted near blood vessels or nerves that may be damaged upon unintentional contact with the burr tool  16 . 
         [0056]    In this regard, the burr tool  16  may be used in a manner such that a working portion of the burr tool  16  is contacted with a medium (e.g., a bone of a patient). The contacting engagement may comprise advancing the burr tool  16  axially along the working axis  120 , radially relative to the working axis  120 , and/or a combination thereof. In turn, use of the burr tool may include risk that the burr tool  16  may slip from the bone or other hard structure being ground using the burr tool  16 . Such loss of contact, which may be sudden) with the medium with which the burr tool  16  is in contact (which is also referred to herein as a slip) may increase the risk that surrounding tissue is inadvertently contacted by the working portion of the burr tool  16 . This contact may cause damage to surrounding tissue, and is therefor preferably minimized. 
         [0057]    In turn, a detection module may be in operative communication with the measurement system. The detection module may monitor for any such loss of contact or slippage and may arrest the instrument  50  upon detecting any such slippage. The instrument  50  may cease operation of a drive of the instrument and/or apply a brake to cease rotation of the instrument. 
         [0058]    One or more of a number of parameters may be monitored by the detection module. In turn, slippage or sudden loss of contact of the burr tool  16  with a medium with which the burr tool  16  is in contact may be detected based on one or more of the parameters monitored by the detection module. Specifically, when the burr tool  16  slips, the burr tool  16  may experience an unexpected and relatively large acceleration. As the burr tool  16  may be monitored for displacement relative to a reference point and/or the acceleration of the instrument may be monitored by an acceleration sensor  112 , the measurement system  40  may be operative to detect an acceleration of the burr tool  16 . For instance, an acceleration may be detected by a dedicated acceleration sensor or may be derived from a measured displacement of the burr tool  16  relative to a reference point. In the latter case, a signal representative of displacement may be output by a displacement sensor. A derivative may be determined reflecting a rate of change of the displacement (e.g., a velocity). A second derivative of the displacement sensor may also be calculated that is indicative of an acceleration of the burr tool  16 . In this regard, a displacement sensor along with a measurement system capable of processing the signal to arrive at an acceleration value may also be used as an acceleration sensor to determine an acceleration of a burr tool  16  for use in monitoring the burr tool  16  in a manner described herein. 
         [0059]    Some minor accelerations may be associated with normal operations. As such, an acceleration threshold value may be established such that if the observed acceleration exceeds the acceleration threshold value, the instrument may be arrested. The acceleration threshold value may comprise a predetermined acceleration value. In other embodiments, the acceleration threshold value may be selectable by a user. Such an acceleration threshold value may comprise a magnitude of acceleration of the burr tool  16  and/or a value related to a maximum rate of change of the acceleration of the burr tool  16 . As such, a measured acceleration magnitude that exceeds the acceleration threshold value (e.g., due to slippage of the instrument  50 ) may result in the instrument being arrested. This may prevent and/or limit damage to surrounding issue in the case of slippage. This monitoring may also occur during use of the burr tool  16  in an operation to create a bore. For instance, in the event of a bone failure or other fracture that result in slippage or other sudden acceleration, even in the case of creation of a bore, the instrument may be operative to arrest upon detection of such an acceleration to prevent and/or limit damage to surrounding tissue. 
         [0060]    Furthermore, a slip of the instrument may also be accompanied by a decrease (e.g. a rapid decrease) in the force acting either axially or radially on the burr tool  16 . In turn, a rapid deceleration (e.g., indicated by a rate of decrease in force above a given magnitude in force reduction or a rate of change in force) may indicate the burr tool  16  has slipped from the bone. That is, a force threshold value may be established. The force threshold value may comprise a detection of a decrease in force greater than a predetermined magnitude or a predetermined rate in change of force. In response, the instrument  50  may be stopped (e.g., by deactivating the drive and/or applying a brake) upon the measured force exceeding the force threshold value. However, a decrease in force that occurs below the force threshold value may not interrupt operation. Such a condition may be present when a user is still contacting a desired medium, but with a lesser force than previously applied. The force threshold value may be controllable by a user to assist in preventing false detection of a loss of contact or slip. 
         [0061]    As such, a detection module may be provided in connection with a controller of the instrument  50 . The detection module may monitor one or more signal provided from corresponding ones of one or more sensors (e.g., a displacement, acceleration, and/or force sensor). In response to one or more of the conditions described above, the detection module may be in operative communication with a drive control of the instrument to cease operation of the instrument and/or engage a brake to cease rotation of the instrument. The detection module may allow the predetermined thresholds for acceleration, rate of change in force, or other variables associated with monitoring of the instrument  50  to be adjusted. 
         [0062]    The detection module may comprise any appropriate hardware, software, or combinations thereof. For instance, the module may be executed by a processor in operative communication with a memory that stores instructions for configuration of the processor to perform the monitoring and detection functions described above. In this regard, the processor may be in operative communication with a measurement system of an instrument. As such, the processor that executes the detection module may receive signals associated with displacement, acceleration, and/or force as described above. The processor may then apply rules in relation to a determination of whether the parameters monitored by the processor exceed a condition associated with a loss of contact of the burr tool  16 . For instance, the rules may relate to one or more of the acceleration threshold value and/or force threshold value described above. For instance, the processor of the detection module may apply a rule wherein if a monitored parameter exceeds the threshold value established for that parameter, a loss of contact is detected and the detection module may cease operation of the instrument and/or apply a brake. In some embodiments, a rule may be established that requires simultaneous occurrence of exceeding the threshold value for acceleration and force for detection of a loss of contact of the burr tool  16 . In other embodiments, structures other than a processor and memory may be provided to execute the detection module such as application specific integrated circuits (ASICs), field programmable gate arrays, or other appropriate hardware and/or software. Further still, the detection module may be integrated into an instrument controller or provided as an independent module for execution in connection with the instrument. 
         [0063]    As a further safety feature of the instrument (e.g., that may be especially helpful for operations involving a burr tool  16 ), upon initiation of rotation of the instrument  16 , the acceleration of the rotational speed of the instrument may be ramped such that top rotational speed of the instrument is reached relatively gradually. This value may be preset or adjustable by the user. In this regard, upon initiation of rotation, the instrument may begin to rotate relatively slowly with gradual (e.g., a set or variable increase over time) ramping up of the speed of the instrument. This may be a linear increase, an exponential increase, or stepped increase in speed without limitation. 
         [0064]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character. For example, certain embodiments described hereinabove may be combinable with other described embodiments and/or arranged in other ways (e.g., process elements may be performed in other sequences). Accordingly, it should be understood that only the preferred embodiment and variants thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.