Abstract:
A measuring device  1  configured to receive a drill bit  3 , and determine the relative displacement of the drill bit  3 , is disclosed. The measuring device  1  is configured to be deployed proximate to the material to be drilled, reset, and subsequently determine the relative displacement of the drill bit  3 . The measuring device  1  can be used in orthopaedic surgery procedures for determining the depth of a bore hole in bone.

Description:
FIELD OF THE DISCLOSURE 
     The present invention relates to measuring devices, and in particular, a device for measuring the depth of a bore hole in bone. 
     BACKGROUND 
     In certain orthopaedic surgery procedures, a surgeon will have to drill holes in bone to subsequently accommodate a screw type. It is therefore essential to the procedure at hand that the depth of a bore hole in bone that has been drilled is accurately determined so that the appropriate screw type can be selected. 
     A common know procedure is to measure the depth of the bore hole using a standard mechanical/electronic depth gauge after drilling. However, this can lead to incorrect depths being obtained as the depth is only determined after the drilling has been completed. In addition, it can require several drilling steps to obtain the desired depth. Such a prior art device is known from US2010/0137874A1. 
     Another known option is to use drills with a scale and slider to indicate drill depth. However, obtaining an accurate reading from the scale can be a cumbersome process, especially when operating the drill. The problem is exaggerated when using small diameter drill bits. 
     SUMMARY 
     In a first aspect, there is provided a measuring device for a drill comprising a sensor configured to receive a drill bit and determine the relative displacement of the drill bit. This may allow the depth of the drill bit within a bore hole to be easily and accurately determined, either during or after the drilling process. 
     The sensor can have a conduit for receiving the drill bit. This may allow the relative displacement of the drill bit to be determined. The sensor may use an incremental encoder for accurately determining the displacement. The incremental encoder can be triggered by the drill bit interacting with either a mechanical, magnetic or electromagnetic sensor. A hall sensor or a sensor using an electromagnetic field generating component may be used for this purpose. 
     The sensor may advantageously be adapted to interact with a repetitive feature of the drill bit. This ensures accuracy in determining the relative displacement of drill bit. The repetitive feature may include grooves. The grooves may be filled with a plastics material, creating a flush outer surface for the drill bit, which aids sterilisation of the drill bit. A plastics material such as PEEK (Polyether ether ketone) may be appropriate. 
     Alternatively, the device sensor can be adapted to interact with a multi-pole magnetic sleeve for placement over a drill bit. This allows the use of standard drill bit. 
     Alternatively, the device sensor may be adapted to interact with a magnetic coating of drill bit. The magnetic coating may have a predetermined coded sequence of magnetic poles. This allows the use of standard drill bit. This coating may be applied to a standard drill bit. 
     The measuring device may comprise a housing that is integral with the sensor. The housing can display the relative displacement of the drill bit. This provides the user with a clear indication of the depth of the bore hole. Alternatively, the housing may be disposed remotely to the sensor and communicate wirelessly therewith, providing the user with an unobstructed view of the display. 
     The display may take the form of a touch screen for facilitating user interaction. The display may also advantageously indicate a screw/implant type required based on the relative displacement of the drill bit. The housing can also comprise a reset button for setting up the device prior to taking a measurement. The housing can also comprise a suitable power source and control circuitry. 
     The display may provide a menu based interface to facilitate user interaction, which can be advantageously navigated using at least one of a button, a trackball, or a thumb-joystick. 
     Embodiments in accordance with other aspects of the present invention comprise a drill bit or drill configured for use with the measuring device. 
     The present invention further provides a method of determining the relative displacement of a drill bit using a measuring device, comprising receiving a drill bit within a sensor, sensing motion of the drill bit through the sensor, and determining the relative displacement of the drill bit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in detail with reference to the accompanying drawings, in which: 
         FIG. 1  shows a perspective view of a measuring device in accordance with the present invention; 
         FIGS. 2 &amp; 2   a  show a drill bit in accordance with the present invention; 
         FIGS. 3 &amp; 4  show how a sensor in accordance with the present invention is implemented; 
         FIG. 5  shows a sleeve for use with a drill bit in accordance with the present invention; 
         FIG. 6  shows a drill bit with a magnetic coating in accordance with the present invention; 
         FIGS. 7 &amp; 8  show a drill in accordance with the present invention; 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, a measuring device  1  is configured to receive a drill bit  3 , and determine the relative displacement of the drill bit  3 . The measuring device comprises a conduit  5  for receiving and guiding the drill bit  3 . The length and diameter of the conduit  5  can be configured to accommodate drill bits having diameters that fall within a specific range. Alternatively, the conduit can be configured to accommodate a specific drill bit. 
     The measuring device  1  further comprises an incremental encoder  7  for measuring the relative displacement of the drill bit  3 . The incremental encoder can be triggered by the drill bit interacting with either a mechanical, magnetic or electromagnetic sensor. In the illustrated embodiment, a hall sensor  11  (see  FIG. 3 ) detects the change in magnetic field as the drill bit  3  moves relative to the hall sensor  11 . The hall sensor also incorporates a magnet  9 . A cyclic change in magnetic field can be used to establish the relative displacement of the drill bit  3  as it moves through the conduit  5 . In an alternate embodiment a differential transformer sensor is used instead of the hall sensor  11 . An example of such a sensor is a “Sensor ID1101G” produced by POSIC, which is currently designed to be used for gear wheel tooth detection. 
     In accordance with the present invention, the cyclic change in magnetic field that is indicative of relative displacement can be generated by a repetitive feature of the drill bit  3 . The repetitive feature can take the form of circumferential grooves  13 ,  13 ′,  13 ″ disposed along the length of the drill bit  3 . The grooves may be truncated as shown in  FIG. 2A .  FIG. 2A  also indicates the dimensions in mm, and pitch angle of the truncated grooves in a preferred embodiment of the present invention. Although shown as regularly spaced grooves  13 ,  13 ′,  13 ″, the grooves  13 ,  13 ′,  13 ″ may be disposed irregularly. The spacing of the grooves  13 ,  13 ′,  13 ″ can be set depending on the tolerance required for the procedure at hand. Additionally, the grooves may be filled with a plastics material, which is preferably non-magnetic. An example plastics material is PEEK (Polyether ether ketone). This allows the drill bit  3  to maintain a flush outer surface, which aids sterilization. 
     The underlying principle of detecting a cyclic change in magnetic field that is indicative of relative displacement can be applied by other means. Other means in the form of magnetic sensors includes a magneto-transistor, a magnetoresistance sensor, an AMR magnetometer, a MEMS sensor, a MEMS compass, a fluxgate magnetometer or a search coil magnetic field sensor. Other means include the use of an electromagnetic field generating component with a hall sensor that is triggered by a repetitive feature of the drill bit  3 . Alternatively, a mechanical sensor can be adapted to be triggered by the grooves  13 ,  13 ′,  13 ″. Alternatively, as shown in  FIG. 5 , a sleeve  10  for receiving a drill bit  3  may be used to implement the incremental encoder  7 . Magnetic poles  12 ,  12 ′  12 ″ on the sleeve can trigger a sensor (not depicted), such as a reed switch, thereby providing a cyclic indication used to establish the relative displacement of the drill bit  3  as it moves through the conduit  5 . The magnetic poles  12 ,  12 ′  12 ″ may be disposed either regularly or irregularly depending on the measuring device setup. The spacing of the magnetic poles  12 ,  12 ′  12 ″can also be set depending on the tolerance required for the procedure at hand. 
     As an alternative to using a sleeve  10 , a drill bit  3  shown in  FIG. 6  may be provided with a magnetic coating  14  having predefined magnetic poles. 
     The measuring device further comprises a housing  15 . Although the housing  15  is depicted as integral with the conduit  5 , it may be remotely located and communicate wirelessly with the incremental encoder  7 . The housing contains a display  17  for indicating the relative displacement of the drill bit  3 . The display  17  may take the form of an LCD or LED display, such as a seven segment display. 
     A reset button  19  allows the measurement to be reset to zero. The display will receive an output from the incremental encoder  7  and indicate the relative movement of the drill bit  3  from the position it was at when the reset button  19  was pressed. This allows the measuring device  1  to adopt a wide range of positions relative to the drill bit prior to commencing drilling and/or measurement. This can also be particularly useful if a drilling process is used in which bores of different diameters are drilled sequentially. 
     The display  17  may optionally comprise touch screen functionality integrated with a menu based user interface (not depicted). In addition, this user interface can be navigated using at least one of a button, a trackball, or thumb joystick (not depicted). The display can further indicate to the user a screw/implant type required based on the measured relative displacement of the drill bit (and hence hole drilled). The user can optionally configure the device to apply an offset to the measured relative displacement when indicating a screw/implant type required. The user can also indicate the type of drill bit  3  that is to be used for drilling, so that the incremental encoder can be calibrated accordingly. 
     The housing  15  also contains control circuitry and a power source. The power source may be controlled by the reset button  19 . 
     An exemplary method of using the device will now be described. The measuring device is placed with an end  21  of the conduit flush against the material (e.g. bone cortex) to be drilled. A drill bit  3  is received within the conduit. The reset button  19  of the device is pressed to ‘zero’ the device. The drilling operation commences. The movement of the drill bit  3  with circumferential grooves  13 ,  13 ′,  13 ″ (disposed substantially parallel to one another and substantially perpendicular to the length of the drill bit) past the hall sensor  9  (which is placed next to the drill bit at a distance of approximately 0.6 mm) creates a cyclic change in magnetic field. The number of recurrences of the cyclic change is indicative of the relative displacement of the drill bit  3 , and is displayed on the display  17 . Thereafter, an appropriate screw/implant type may be indicated on the display. 
     Although the device can be used separately from a drill in a freehand manner, as may be the case in confined areas which do not permit access by anything other than the drill bit, the device can be easily adapted to connect integrally with a drill  23 . A mounting device  25  allows the drill bit to move relative to the measuring device  1 . This ensures an accurate displacement reading from the measurement device  1 . The measuring device  1  can also be used with various guide instruments that are well known in the art, such as an aiming arm. 
     It will be appreciated that this description is by way of example only; alterations and modifications may be made to the described embodiment without departing from the scope of the invention as defined in the claims.