Abstract:
A system for automatic calibration of instruments (S) having varying cross-sectional dimensions within a predetermined range and having detectable elements ( 110, 112, 113, 114 ) thereon for computer-aided surgery, comprising a calibration base (C) having detectable elements ( 43, 44, 46, 48 ) secured thereto for detecting a position and an orientation thereof in space by sensors ( 204 ) connected to a position calculator ( 202 ). The calibration base is adapted to receive and to releasably secure a working shaft ( 100 ) of any of the instruments (S) and provides an abutting surface ( 14 ) for a tip ( 102 ) thereof in such a way that a position and orientation of the tip ( 102 ) of the instrument (S) secured therein is calculable when working shaft cross-section dimensions thereof are known. The position calculator ( 202 ) receives instrument data ( 214 ) and calibration data ( 218 ) from an operator through a user interface ( 206 ) and stores the instrument data ( 214 ) and calibration data ( 218 ) for subsequent calibrations.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to computer-aided surgery instrumentation and, more particularly, to the calibration thereof. 
     2. Description of the Prior Art 
     In computer-aided surgery, it is known to use surgical instruments detectable by positioning systems in order to have an on-screen representation of the instrument with respect to an operated part of a patient&#39;s body. It is readily understood that great amounts of precision and accuracy are required in the space positioning of the surgical instruments in order to obtain reliable representation of the operation. A misrepresentation of the instrument with respect to the patient&#39;s body may have dramatic consequences and may even be fatal to the patient. Thus, prior to computer-aided surgery, the instruments must be calibrated. 
     One known method of calibrating is referred to as the axial-conical calibration. This method consists in achieving pre-determined maneuvers with a surgical instrument having detectable devices thereon for it to be located in space by sensors connected to a position calculator. Namely, a first maneuver consists in rotating the surgical instrument with respect to its longitudinal axis, whereby the position of the latter is set. During this rotation, the position calculator receives readings which will allow it to calculate a transform matrix from the positioning system to the axis of the instrument. Thereafter, in a second maneuver, the instrument is rotated according to a conical trajectory having as an apex the working tip thereof. Hence, the positioning system may interpret and find another transform matrix between the positioning system and the tip of the surgical instrument. Although the axial-conical calibration method is simple, the required maneuvers of calibration may take a few minutes to an inexperienced user and the position calculator may require to repeat the maneuvers if they are judged as being unsatisfactory. 
     Calibration systems having permanently calibrated instruments have been provided in order to avoid lengthy steps of calibration. In such systems, a working field is scanned by sensors connected to a position calculator which recognizes the geometry of a given surgical instrument, whereby it is calibrated. 
     Precautions must be taken when using permanently calibrated instruments to ensure that these are not altered or damaged, whether it be in pre-surgery sterilization or during surgery. The instruments are subject to frequent manipulations during surgery, and thus, having sensors or detectable devices thereon involves the possibility that the position of these sensors or detectable devices is altered, whereby precision is lost in the space representation of the instrument. In this case, an inventory of equivalent instruments must be on hand during surgery in case of damage or alteration to an instrument. It would thus be desirable to have a calibration system allowing frequent calibrating by its simplicity and its rapidity of execution, to better suit the surgical room environment. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide a method for automatically calibrating surgical instruments which is simple and rapid in use and which produces a calibration of constant precision to facilitate the calibrating. 
     It is a further aim of the present invention to provide a method for automatically calibrating surgical instruments which includes validating the calibration. 
     It is a still further aim of the present invention to provide an apparatus for automatically calibrating surgical instruments which accommodates a wide range of instruments. 
     It is a still further aim of the present invention to provide an apparatus for automatically calibrating surgical instruments and capable of sustaining sterilization. 
     Therefore in accordance with the present invention, there is provided a calibration base to automatically calibrate instruments having varying cross-sectional dimensions within a predetermined range for computer-aided surgery. Each of the instruments has detectable means, a working shaft and a tip at an end of the working shaft. The calibration base comprises detectable means secured thereto for detecting a position and an orientation thereof in space by sensors connected to a position calculator. The calibration base is adapted to receive and to releasably secure the working shaft of any of the instruments. The calibration base provides a first abutting surface for the tip thereof in such a way that a position and orientation of the tip of any of the instruments secured therein is calculable when working shaft cross-section dimensions thereof are known, whereby any of the instruments is calibrated when the position of the tip of the working shaft thereof is calculated. 
     Also in accordance with the present invention, there is provided a method for calibrating the above described calibration base. The method comprises the steps of (i) detecting a position and orientation in space of the detectable means of the calibration base and of the instrument by the sensors, (ii) receiving instrument data including either identification data relating to instrument cross-sectional dimension data stored by the position calculator or of instrument cross-sectional dimension data to be stored by the position calculator for subsequent calibrations, and (iii) calculating a position of a tip of any one of the instruments secured in the calibration base with respect to the detectable means of the calibration base whereby the instrument is calibrated with respect to the detectable means of the instrument. 
     Further in accordance with the present invention, there is provided a system for automatic calibration of instruments for computer-aided surgery. The system comprises a calibration base as described above. Sensors detect a position and orientation in space of the detectable means of the calibration base and of the instrument. The position calculator as described above is connected to the sensors for calculating a position and orientation of the tip of the working shaft of the instruments secured in the calibration base with respect to the detectable means thereon whereby any of the instruments is calibrated with respect to the detectable means on the instrument when the position of the tip of the working shaft thereof is calculated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: 
         FIG. 1  is a perspective view of an automatic calibration unit with a tool in accordance with the present invention; 
         FIG. 2  is a top plan view, partially cross-sectioned, of the automatic calibration unit with an optically detectable tool; 
         FIG. 3  is a fragmented side elevational view of an example of an instrument tip disposed on the automatic calibration unit; 
         FIG. 4  is a fragmented side elevational view of another example of an instrument tip on the automatic calibration unit; 
         FIG. 5  is a fragmented side elevational view of a further example of an instrument tip on the automatic calibration unit; 
         FIG. 6  is a block diagram illustrating a method of automatically calibrating surgical instruments in accordance with the present invention; and 
         FIG. 7  is a perspective view of the automatic calibration unit with a tool in accordance with a further embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , there is shown a calibration base C supporting a surgical instrument S. The calibration base C comprises a base plate  10  having a bottom flat surface  12 . The base plate  10  also defines an XY plane  14 . For clarity purposes, XYZ axes have been added to the drawings in order to clearly identify the space orientation of planar elements. For instance, the XY plane  14  is planar with respect to the XY axes provided therewith. Also, it is noted that the calibration base C generally consists in a material which must be able to sustain several cycles of autoclave sterilization, such as a stainless steel. Some elements of the calibration base C will require to be made of other materials and will be identified herein as such. 
     A vertical wall  16  extends perpendicularly from the XY plane  14  of the base plate  10 . A first panel  18  projects from a top portion of the vertical wall  16  and shares a top edge surface  20  therewith. A first protrusion  22  and a second protrusion  24  protrude from a proximal surface  26  of the vertical wall  16  and of the first panel  18 , and are adjacent the top edge surface  20  thereof. 
     As best seen in  FIG. 2 , the first panel  18  defines an XZ plane  28  on the back of the proximal surface  26 . The XZ plane  28  is perpendicular with respect to the XY plane  14 . Referring now to both  FIGS. 1 and 2 , a second panel  30  is shown extending perpendicularly from the XZ plane  28  of the first panel  18 , and thereby defines a YZ plane  32 . The YZ plane is perpendicular with the XZ plane  28 , and thus with the XY plane  14 . The XZ plane  28  and YZ plane  32  intersect at an edge line  34 . 
     As best seen in  FIG. 2 , a top bulge  36  and a bottom bulge (not shown) extend horizontally (i.e. in the XY plane) from the top and bottom of the second panel  30 , with a lever  38  pivotally disposed therebetween. The lever  38  has rounded ends  40  and  42  at its extremities. A spring (e.g. torsion spring) is shared between the lever  38  and the second panel  30  such that the lever  38  is biased toward the edge line  34  at the intersection of the XZ plane  28  and the YZ plane  32 . The lever consists in a material able to sustain the high-pressure of an autoclave during the sterilizing of the calibration base C, such as an acetal copolymer. 
     Returning to  FIG. 1 , detectable spheres  44 ,  46  and  48  are secured to the proximal surface  26  of the vertical wall  16  and the first panel  18 . The detectable spheres are coated with a retro-reflective layer in order to be detected by, for instance, an infrared sensor using axial illumination. It is pointed out that other shapes are known and could also be used as alternative embodiments to retro-reflective spheres. As an example, straight cylinders, corner reflectors or the like having retro-reflective properties could also be used. It is also noted that the detectable spheres  44 ,  46  and  48  may be removed by providing snap-fit mating adapters such that single-use spheres may be used. This allows for the spheres to be sterilized with processes milder than autoclave sterilization, whereby a coating does not need to be characterized by its capability to sustain high temperatures or pressures. 
     Still referring to  FIG. 1 , the surgical instrument S is shown having a shaft  100  of circular cross-section. A tip  102  is disposed at a working end thereof, whereas a handle  104  is disposed at a handling end thereof. Referring to  FIGS. 3 ,  4  and  5 , tips of various tools in accordance with the present invention are shown at  102 . Returning to  FIG. 1 , an arm  106  extends from the handle  104  of the surgical instrument S. A blade  108  is disposed at a free end of the arm  106  and comprises detectable spheres  110 ,  112  and  114  secured thereto. The detectable spheres  110 ,  112  and  114  are similar in construction to the above mentioned detectable spheres  44 ,  46  and  48 . 
     The calibration base C is adapted for receiving and releasably securing surgical instruments having working shafts of a wide range of cross-section shapes and diameters (e.g. 3 to 37 mm). In the preferred embodiment, instruments having circular cross-sections are used with the calibration base C. As seen in  FIG. 2 , the surgical instrument S is abutted against the XZ plane  28  and the YZ plane  32  and is biased in this position by the lever  38 . By knowing the diameter of the working shaft  100 , it is possible to calculate the positioning of the longitudinal axis thereof (i.e. at the center of the circular cross-section) which is possible with respect to the edge of the calibration base C. Moreover, the surgical instrument S is disposed in the calibration base C with the working tip  102  thereof touching the XY plane  14  of the base plate  10 . 
     As the position and orientation of the detectable spheres  44 ,  46  and  48  may be determined by sensors, and the position of these spheres on the calibration base C is known as they are secured thereto, the position and orientation of the working tip  102  of the surgical instrument S is calculable as it is located at the intersection of the longitudinal axis of the working shaft  100  and the XY plane  14  of the calibration base C. 
     Although the preferred embodiment discloses planes  14 ,  28  and  32  all being in a perpendicular relation, it is pointed that the planes  14 ,  28  and  32  may be in any relation with respect one to another so long as the position of a given portion of the instruments is calculable. For instance, the planes  28  and  32  may define a V-shaped channel of obtuse or acute angles for receiving the working shaft  100  thereagainst, even though the preferred embodiment discloses a right angle therebetween. 
     Referring now to  FIG. 6 , a positioning system for calibrating the surgical instrument S disposed in the calibration base C is generally shown at  200 , and comprises a position calculator  202 . The position calculator  202  is a computer program, which may be, for instance, installed on the computer-aided surgery platform. The position calculator  202  receives space locations, including the position and orientation, of the detectable spheres  44 ,  46  and  48  of the calibration base C, and the detectable spheres  110 ,  112  and  114  mounted on the surgical instrument S through sensors  204 . The position calculator  202  also receives instrument identification from an operator through user interface  206 . It is noted that the cross-sectional dimensions of instruments to be used with the position calculator  202  are stored thereby such as to be retrieved upon operator instrument identification. For instance, the operator may indicate that the instrument whose working tip is illustrated in one of  FIGS. 3  to  5  is to be calibrated. In the event where the instrument information is not stored by the position calculator  202 , it is possible for the operator to enter new information amongst the instrument data  214  through the user interface  206 . The user interface  206  may comprise typical keyboard, mouse and monitor. 
     It is noted that the space relation  208  of the detectable spheres  110 ,  112  and  114  is stored by the position calculator  202 , such that the latter will recognize them through the sensors  204 . Other information stored by the position calculator  202  include the space relation  210  of the detectable spheres  44 ,  46  and  48 , the space geometry  212  by the calibration base C, including the space relation between these spheres and the calibration base C. Also, the cross-section shapes and dimensions  214  of the various tools to be used is stored by the position calculator  202 . Once the position and orientation of the detectable spheres  44 ,  46  and  48  and the detectable spheres  110 ,  112  and  114  are detected by the sensor  204 , and the instrument is identified by the operator through the user interface  206 , the position and orientation of the working tip  102  of this instrument is calculated with respect to the detectable spheres  110 ,  112  and  114  attached thereto, as explained above. This results in the calibration of the instrument, as prompted by the position calculator  202  to the user interface  206  and as signaled to the computer-aided surgery system  216 . 
     The position calculator  202  also stores the prior calibration data  218 , which consists in the calculated position and orientation of the tips of all the instruments which have been calibrated previously. This allows for a validation of the calibration of the instruments. For instance, the instrument S depicted in  FIG. 1  is calibrated and used for surgery. The position calculator  202  will automatically store the position and orientation of the working tip  102  of the instrument S with respect to the position of the detectable spheres  110 ,  122  and  114  thereon. Thereafter, at the next calibration of the same instrument S, the position calculator  202  will compare the new calculated position and orientation of the working tip  102  to the stored reference position and orientation. If the new calculated position and orientation are not within an allowable range, the operator will be prompted to verify the state of the instrument and the positioning thereof in the calibration base C. If, after a second reading of the sensors  204  the position and orientation is the same as the previous one, the operator will be prompted to either accept the new space position and orientation, or to retry calibrating until the stored reference position and orientation are attained. 
     In the preferred embodiment, the calibration base C is constructed in accordance with high standards of precision such that the XY plane  14 , the XZ plane  28  and the YZ plane  32  are all planar and in perpendicular relationship. Although other configurations are possible, the above described geometry of the calibration base C provides a simple solution. 
     The calibration base C is permanently calibrated as it does not change shape. As mentioned above, the calibration C is made of a material which can sustain great impacts (i.e. stainless steel). Also, as seen in  FIG. 2 , the first protrusion  22  and the second protrusion  24  are provided in order to protect the detectable spheres  44 ,  46  and  48  in case of a fall of the calibration base C. 
     Although the use of retro-reflective spheres has been described above, it is pointed out that the detection of the position and orientation of the instrument S and the calibration base C may be achieved by other devices such as magnetic sensors, ultrasound sensors and infrared LEDs. Referring to  FIG. 7 , a housing  43  is shown mounted to the calibration base C, whereas a housing  113  is shown secured to the surgical instrument S. The housings  43  and  113  may contain either magnetic sensors, ultrasound sensors or the like. 
     The use of the lever  38 , for releasably securing the instrument S ensures the precise positioning of the latter with respect to the XZ plane  28  and the YZ plane  32  and is adapted for receiving shafts of various diameters (e.g. 3 to 37 mm) with its rounded end  40 . It is pointed out that alternative mechanisms may be used instead of the spring-biased lever  38 , so long as the working shaft is pressured against the planes  28  and  32 . Gravity forces the tip  102  of the instrument S against the XY plane  14  in the preferred embodiment, thus rendering the above-described releasable connection virtually instantaneous. 
     The arm  106  of the surgical instrument S is of a material which is substantially less resistant to impacts than the blade  108 . Therefore, in the event of a great impact on the surgical instrument S, the arm  106  would get deformed before the blade  108 , thereby protecting the geometry thereof which defines the space positioning of the detectable spheres  110 ,  112  and  114  and is stored at  208  by the position calculator  102 . Thus, if the arm  106  is damaged, the surgical instrument S may be quickly recalibrated according to the above described method in order to set the location of its tip  102  in space.