Abstract:
The described method for determining at least orientation of a pelvic bone in space and for tracking movement of the pelvic bone using a computer assisted surgery (CAS) system includes removably attaching, in a non-invasive manner, at least one reference marker, in communication with the CAS system, to a skin surface covering the pelvic bone. Registering at least orientation readings of the reference marker is then performed with respect to a fixed reference in a reference coordinate system, following which at least an orientation of the pelvic bone in the reference coordinate system is determined using the orientation readings of the reference marker. These steps are repeated to update the orientation readings while tracking the pelvic bone in space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a Continuation of U.S. patent application Ser. No. 11/878,021 filed Jul. 20, 2007, which claims priority on U.S. Provisional Patent Application No. 60/832,151 filed on Jul. 21, 2006, the contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of computer-assisted medical procedures, and more specifically, to bone tracking and positioning in computer-assisted surgery (CAS) systems. 
       BACKGROUND 
       [0003]    Computer-assisted surgery (CAS) makes use of references fixed to the patient using pins inserted into the bones of the limbs or the pelvis. These pins, inserted into the bones before or during the surgery, are of different diameter sizes and can cause pain after the surgery. They are an extra step to the surgery, exclusively because of the navigation system. Also, the insertions of the pins into the bone may cause weaknesses of the bone that can then more easily be fractured. Infections may also occur as for any entry point at surgery. 
         [0004]    Furthermore, the length of the pins is sometimes obtrusive to the surgeon who may cut them to a length better adapted to his movement during the surgery. The cut is also perceived as an extra annoying step; its end may be sharp and hazardous to the personnel working around the surgery table. 
         [0005]    The pins are time-consuming and invasive. Therefore, there is a need for an improvement in this area. 
       SUMMARY 
       [0006]    In accordance with a first aspect of the present invention, there is provided a method for determining at least orientation of a pelvic bone in space and for tracking movement of the pelvic bone using a computer assisted surgery (CAS) system, the method comprising: removably attaching, in a non-invasive manner, at least one reference marker to a skin surface covering said pelvic bone, the reference marker being in communication with the CAS system; registering at least orientation readings of said reference marker with respect to a fixed reference in a reference coordinate system; determining at least an orientation of said pelvic bone in the reference coordinate system using said at least orientation readings of said reference marker; and repeating the steps of registering and determining to update said at least orientation readings while tracking the pelvic bone in space. 
         [0007]    In accordance with a second aspect of the present invention, there is provided a system for determining at least an orientation of a pelvic bone in space and tracking movement of the pelvic bone using a computer assisted surgery (CAS) system, the system comprising: at least one reference marker removably and non-invasively attachable to a skin surface covering the pelvic bone, said reference marker being in communication with the CAS system; a fixed reference positioned in a reference coordinate system and used to identify at least an orientation of said reference marker in said reference coordinate system; and the computer assisted surgery (CAS) system including: a sensing device adapted to register at least orientation readings of said reference marker in the reference coordinate system; and a processing unit receiving said orientation readings from the sensing device and being operable to determine and determining at least the orientation of said bone. 
         [0008]    In accordance with another aspect of the present invention, there is provided a device for use with a computer assisted surgery (CAS) system to determine at least orientation of pelvic bone in a reference coordinate system and to track movement of the pelvic bone, the device comprising at least one reference marker removably and non-invasively attachable to a skin surface covering a pelvic bone, the reference marker being in communication with the CAS system such that the reference marker is detected and tracked in said reference coordinate system by the CAS system with respect to a fixed reference, said reference marker being adapted to provide at least orientation readings thereof to the CAS system such as to determine at least orientation of the pelvic bone in the reference coordinate system and permit the CAS system to track movement of the pelvic bone. 
         [0009]    In this specification, the term “reference marker” is intended to mean an active or passive marker, such as an emitter or a reflector. The term “fixed reference” may also refer to any active or passive device, with a known position in the reference coordinate system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
           [0011]      FIG. 1  is a front view of a pelvic bone with three base units non-invasively attached on the skin surface; 
           [0012]      FIG. 2  is a cross-sectional view of an elongated bone, such as a femur, with three base units attached on the skin surface; 
           [0013]      FIG. 3  is a front view of an elongated bone, such as a femur, with multiple pairs of base units provided thereon; 
           [0014]      FIG. 4  is a cross-sectional view of an elongated bone, such as a femur, with a plurality of reference markers around the bone; 
           [0015]      FIG. 5  is a cross-sectional view of an elongated bone, such as a femur, with a single pair of reference markers around the bone; 
           [0016]      FIG. 6  is an illustration showing a piece of fabric with reference markers thereon attached to a leg; 
           [0017]      FIG. 7  is a block diagram of an embodiment of a system of the present invention used with a set of base units; and 
           [0018]      FIG. 8  is a block diagram of an embodiment of a system of the present invention used with a fabric having reference markers attached thereto. 
       
    
    
       [0019]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates a pelvic bone  10  covered by the skin  12 . Three base units  14  are disposed on the pelvis  10  in a non-invasive manner, either being attached directly to the skin  12 , wherein the base units  14  may be non-invasively attached for example using a medical adhesive, i.e. non-toxic for the patient. Alternately, the base units  14  can be provided on a piece of fabric, disposed directly on the skin  12 , that is worn by the patient during the surgery. For example, this piece of fabric may include a pair of snug-fitting shorts, corset or underwear (undergarment) having the base units  14  attached thereto, which is worn by the patient. 
         [0021]    One base unit  14  is attached to each crest of the hip (anterior superior iliac spines), and a third base unit is attached to the pubis area (pubic symphysis) of the pelvis. These locations on the pelvis are chosen for having minimum distance between the out-skin  12  and the bone  10 . The base units  14  may be positioned at other locations on the pelvic bone, without being restricted to these locations in particular. 
         [0022]    Reference markers  16  are present in/on each base unit. These reference markers  16  may be active or passive, optical, RF, (electro-)magnetic, or other. In  FIG. 1 , optical reflective reference markers are illustrated. These three points define the pelvic coordinate system. The position sensing system used with the CAS system will register the position and orientation in space of the pelvic bone with respect to either pre-operative images of the patient, such as CT-scans, fluoroscopy, x-rays, etc, or with respect to any type of intra-operative reconstruction of the bones illustrated on an interactive display device. A fixed reference present in the coordinate system and having a known position is used to position and orient the pelvic bone in space. 
         [0023]    In one embodiment of the present invention, the distance of each base unit  14  on the out-skin  12  to the bone  10  is measured using an ultrasound probe that is applied to each base unit  14 . The ultrasound, which is a transducer, emits an ultrasound wave and measures the time it takes for the wave to echo off of a hard surface (such as bone) and return to the transducer face. Using the known speed of the ultrasound wave, the time measurement is translated into a distance measurement between the base unit and the bone located below the surface of the skin. In another embodiment, an ultrasound device is integrated into each base unit  14 . The measurement is done by either triggering it manually, or automatically. In one embodiment, the measurement is repeated at regular intervals. The measurements are constantly being transferred to the CAS and the position and orientation of the bone  10  in space is updated. The measurement of the distance from the base unit to the bone may also be done using alternative imaging means, such as fluoroscopy. A metal reference is positioned on the skin surface and used with a fluoroscopy system to identify the bone surface. It is possible to make the measurement from the metal reference to the bone surface on the fluoroscopic image 
         [0024]    Once the distance between the base unit  14  and the bone  10  is known, an estimation of the possible displacement of the base unit  14  on the outer-skin relative to the bone  10  can be done and the bone can then be registered to the reference system. In the case of the integrated ultrasound device within each base unit  14 , it becomes possible to measure in real-time variations in distance between the base units  14  and the bone  10  during the surgery. 
         [0025]      FIG. 2  is a cross-sectional view of a bone  18 , such as a femur, a tibia, or any other bone having an elongated form. The base units  14  are positioned around the bone  18  on the out-skin  12 . Similarly to the case of the pelvis  10 , the base units  14  may be glued onto the skin  12  covering the bone  18 , such as using an adhesive tape or adhesive-backed fabric  23  or best  21  as described further below, or a piece of fabric having the base units  14  attached thereto is placed on the body in a snug-fitting manner. For a limb, a sleeve, sock, or belt may be used. The base units  14 , each having reference markers  16  are therefore distributed around the bone. If the reference markers  16  are of the optical type, than only the reference markers  16  in the line-of-sight of the position sensing system will be registered. In the case of a limb, this may mean that only half of the reference markers are visible. The other half can be extrapolated using the readings obtained from the visible markers. For other types of reference markers, such as RF emitters, all reference markers on the bone will generate a reading in order to register the position and orientation of the bone in space. 
         [0026]    When tracking the orientation and position of an elongated bone, the problems encountered due to cutaneous movement can be resolved by placing a single reference marker directly on the bone at the beginning of the surgery. If that reference marker is an ultrasound, the sound emitted by the reference marker can be captured by the base units on the skin. Therefore, the invasive pins are replaced by an ultrasound link, and the only invasive part is the incision that would have been made in any case. 
         [0027]      FIG. 3  is a front view of the bone shown in cross-section in  FIG. 2 . The base units  14 , including the reference markers (not shown), are placed on the out-skin  12  of the bone  18 . The distance between the bone  18  and the out-skin  12  is measured using ultrasound. The possible variation of distance between the base and the bone during the surgery can be measured, either once at the beginning or in real-time during the surgery. 
         [0028]    The anatomical axis  20  of the bone  18  can also be determined. The ultrasound probe is not necessary to determine the anatomical axis  20 . Pairs of reference markers on the out-skin  12  are positioned such that they are substantially facing each other, as illustrated in  FIG. 4 . The mid-point between the measured position of each marker in a pair will constitute a point on the anatomical axis  20 . 
         [0029]    A minimum of one pair, i.e. two reference markers  16  face-to-face, is needed to determine the anatomical axis, as illustrated in  FIG. 5 , if used in combination with a single reference marker placed at a distal end of the bone. Alternatively, two pairs of reference markers positioned face-to-face could also be used to determine the anatomical axis  20  by providing at least two points on the axis  20 . 
         [0030]    In one embodiment of the present invention, a belt having ultrasound emitters thereon is attached around a leg of a patient, as illustrated in  FIG. 6 . A fixed reference is provided in the reference system, for example on a cutting block positioned on the knee, or attached to the pelvis bone (not shown). The reference markers on the belt  21  are referenced to the fixed reference, for example by using a pointer and applying it to the reference markers on the belt  21 . This can also be done automatically without a pointer. The ultrasound emitters measure the distance between the skin and the bone, and the position and orientation of the bone in space is determined. 
         [0031]    In another embodiment, a belt  21  having reference markers (not necessarily ultrasound emitters) is attached around a leg of the patient. The reference markers are positioned such that there are pairs of markers substantially facing each other. The anatomical axis of the bone is determined by locating the middle point between a pair of markers and forming a line from these points along the bone. The position of the belt in space is determined using the reference markers on the belt. At least one pair of reference markers are needed to determine the anatomical axis, if the reading from a single reference marker provides a position from which more than one point on a line can be determined. 
         [0032]      FIG. 7  illustrates a system in accordance with one embodiment of the present invention. A processing unit  30  and a position sensing system  32  are coupled with a set of at least three base units  14 , each base unit  14  having a reference marker  16  thereon. The reference markers  16  on the base units  14  are used to position the bone within a reference coordinate system. The position sensing system, as is known in the art, will use either active or passive devices as markers. The orientation and position of the bone in space can be determined using the information obtained from the reference markers  16  and using the known position of the fixed reference  34  in the coordinate system. In addition, an ultrasound device  36  is used to measure the distance between the surface of the skin and the bone underneath the surface. By updating this measurement, a more precise positioning of the bone is obtained. 
         [0033]    The base units may be provided with a non-toxic adhesive on a surface and stuck directly onto the skin. Alternatively, a fabric or tape strip mounted with the base units is adhered or otherwise attached to the skin, as is described above. The ultrasound device is either integrated into each base unit, or used externally to the base units by applying it manually to each base unit, in the form of a probe, for example. 
         [0034]    An alternative embodiment of the system is illustrated in  FIG. 8 . A processor  40  and a position sensing system  42  are coupled to a fabric  23  which is removably and non-invasively attachable to a bone and having a plurality of reference markers  16  distributed thereon, and a fixed reference  44  positioned in the reference coordinate system and used to identify a position and orientation of the reference markers in the reference coordinate system. 
         [0035]    The reference markers  16  on the fabric  23  may be distributed on the fabric in a variety of ways. One such way is to have at least two reference markers substantially facing each other when the fabric is wrapped around a bone. One or more rows of pairs of markers can be provided on the fabric, as is illustrated in  FIG. 5 . 
         [0036]    The processor is preferably a general purpose computer equipped with software that allows it to compute the location of a bone surface from the information obtained by the reference markers and ultrasound device. The results may be displayed on a screen or monitor to be visualized by a user or operator. The information may be used in conjunction with other known registration techniques to assist in pre-operative or intra-operative procedures. The components of the system may need to be calibrated using standard procedures known to the person skilled in the art. 
         [0037]    While illustrated in the block diagrams as groups of discrete components communicating with each other via distinct data signal connections, it will be understood by those skilled in the art that the preferred embodiments are provided by a combination of hardware and software components, with some components being implemented by a given function or operation of a hardware or software system, and many of the data paths illustrated being implemented by data communication within a computer application or operating system. The structure illustrated is thus provided for efficiency of teaching the present preferred embodiment. The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.