Patent Publication Number: US-2007106282-A1

Title: Determination of the position of an anatomical element

Description:
The present invention relates to a method and a device for determining the position of an anatomic element.  
      In a computer-assisted surgical operation, it may be necessary to determine the position of a patient&#39;s anatomic element with respect to a reference coordinate system. As an example, in the case of a computer-assisted arthroplasty of the hip, it is necessary to be able to continuously express, in the reference coordinate system, the equations of characteristic planes associated with the patient&#39;s pelvis.  
       FIG. 1  schematically shows the body of a patient  10  placed in lateral decubitus position for a computer-assisted hip arthroplasty operation. Pelvis  12  of patient  10  has been shown. At a given time, the position of pelvis  12  can be defined from the positions of the three following characteristic anatomic points, represented by crosses: the two anteroposterior iliac spines  14 ,  16 , and the pubic symphysis  18 . The positions of anatomic points  14 ,  16 ,  18  are determined, at an initial time, by means of a probe (not shown) having its position determined by a location system  19 . The system may be an optical, magnetic, ultrasound, optical fiber, etc. location system. A computer (not shown), connected to location system  19 , is capable of determining the position of pelvis  12  from the position of the probe in a reference coordinate system R F  associated with location system  19 .  
      During the operation, it is necessary to continuously track the position of pelvis  12 . For this purpose, a rigid body  20  is attached to pelvis  12 , for example, on iliac crest  22 . The location system continuously provides the position of rigid body  20  in reference coordinate system R F . For this purpose, location system  19  comprises, for example, means (not shown) for emitting an infrared radiation which is reflected by reflective portions  22  of rigid body  20 , the reflected radiation being sensed by cameras (not shown) sized together and supported by location system  19 . Pelvis  12  being attached with respect to rigid body  20 , the position of pelvis  12  can be deduced in reference coordinate system RF throughout the surgical operation based on the position of rigid body  20  in reference coordinate system R F .  
      To obtain the relative position between pelvis  12  and rigid body  20 , it is necessary to determine the positions of pelvis  12  and of rigid body  20  at very close times to avoid a disturbance of the measurements due to the displacements of patient  10  or of location system  19 . For this purpose, the position of pelvis  12  in reference coordinate system R F  can be determined according to the following steps:  
      attaching rigid body  20  on iliac crest  22  of pelvis  12  of patient  10  placed in lateral decubitus position and determining the position of rigid body  20  in reference coordinate system R F ;  
      determining positions of the three characteristic anatomic points  14 ,  16 ,  18  in reference coordinate system R F  and determining the relative position between pelvis  12  and rigid body  20 ;  
      removing rigid body  20 ;  
      preparing the patient for the operation, especially arranging the operating drapes (not shown) to define the operation area;  
      reproducibly placing back rigid body  20 ;  
      continuously determining the position of rigid body  20 , and accordingly, the position of pelvis  12  in reference coordinate system R F .  
      A disadvantage of such a method is that rigid body  20  is arranged on pelvis  12  to determine the relative position between pelvis  12  and rigid body  20  before the preparation of patient  10 . This provides an additional risk of infection of patient  10 . Further, rigid body  20  being temporarily removed to prepare patient  10 , it is difficult to place it back on iliac crest  22  exactly at the position that it had before the preparation of patient  10 .  
      Another conventional method for determining the position of pelvis  12  in reference coordinate system R F  comprises the steps of:  
      preparing patient  10 , especially placing patient  10  in lateral decubitus position, and arranging the operating drapes;  
      performing an incision to have access to the area to be operated on;  
      arranging rigid body  20  on pelvis  12  through the incision previously performed and continuously determining the position of rigid body  20  in reference coordinate system R F ; and  
      palpating the three characteristic anatomic points  14 ,  16 ,  18  and determining the relative position between pelvis  12  and rigid body  20 .  
      A disadvantage of such a method is that the palpating of characteristic anatomic points  14 ,  16 ,  18  is very delicate since the latter are then of difficult access, especially due to the operating drapes covering the body of patient  10 .  
      The present invention provides a device for determining the position of a patient&#39;s anatomic element which does not exhibit the above-mentioned disadvantages.  
      The present invention also provides a method for determining the position of a patient&#39;s anatomic element that can be implemented prior to the patient&#39;s surgical operation and in the presence of the operating drapes.  
      For this purpose, it provides a tracking device intended to bear against the skin of a patient in a computer-assisted surgical operation, comprising at least three non-aligned marks intended to be palpated through at least one operating drape and each defining a guiding point.  
      According to another aspect of the invention, at least one mark is a conical recess surrounded with a protruding ring-shaped edge.  
      According to another aspect of the invention, the device comprises at least two distinct mechanical parts intended to bear against the patient&#39;s skin, each distinct mechanical part comprising at least one mark.  
      According to another aspect of the invention, the device comprises a parallelepipedal block, the three marks being arranged on three distinct surfaces of the block.  
      According to another aspect of the invention, two marks are arranged on two parallel surfaces, symmetrically with respect to a plane equidistant to the two parallel surfaces, the third mark being arranged on a surface perpendicular to the two parallel surfaces, closer to one of the parallel surfaces.  
      The present invention also provides a method for determining the position of a patient&#39;s anatomic element comprising the steps of determining, at an initial time, the position of the anatomic element from the positions of characteristic anatomic points of the patient; arranging, on the patient&#39;s skin, a tracking device fixed with respect to the anatomic element; determining the position of the tracking device, the anatomic element being substantially at the position determined at the initial time; determining the relative position of the anatomic element with respect to the tracking device; at least partially covering the tracking device with at least one operating drape; and determining the position of the tracking device at least partially covered with said at least one operating drape and deducing therefrom the position of the anatomic element.  
      According to another aspect of the invention, the positions of characteristic anatomic points of the patient are obtained by palpation of at least three anatomic points of the patient.  
      According to another aspect of the invention, the positions of characteristic anatomic points of the patient are obtained from ultrasound scan images of the patient.  
      According to another aspect of the invention, the positions of the tracking device are obtained by palpation of marks of the tracking device.  
      According to another aspect of the invention, the marks of the tracking device are palpated to determine the position of the tracking device in the absence of said at least one operating drape by means of a first probe comprising a spherical end having a first radius and the marks of the tracking device are palpated to determine the position of the tracking device at least partially covered with said at least one operating drape by means of a second probe comprising a spherical end having a second radius smaller than the first radius.  
      According to another aspect of the invention, the marks are conical recesses.  
      According to another aspect of the invention, the positions of the marks of the tracking device determined in the absence of said at least one operating drape are displayed on a display screen to ease the palpation of the marks of the tracking device to determine the position of the tracking device at least partially covered with said at least one operating drape.  
      According to another aspect of the invention, the displayed positions of the marks of the tracking device determined in the absence of said at least one operating drape are corrected based on the determined position of a first mark of the tracking device at least partially covered with said at least one operating drape. 
    
    
      The foregoing object, features, and advantages of the present invention, as well as others, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which:  
       FIG. 1 , previously described, schematically illustrates a conventional method for determining the position of a patient&#39;s anatomic element;  
       FIG. 2  schematically illustrates an example of a method according to the invention for determining the position of the patient&#39;s anatomic element;  
       FIGS. 3A  to  3 C show views of an example of the forming of a tracking device according to the present invention;  
       FIG. 4A  is a perspective view of a variation of the tracking device of  FIGS. 3A  to  3 C;  
       FIG. 4B  is a perspective view of an element of the device of  FIG. 4A ;  
       FIGS. 5A and 5B  show in further detail two steps of the method illustrated in  FIG. 2 ;  
       FIG. 6  schematically illustrates another example of the method according to the invention;  
       FIG. 7  is a perspective view of another example of the forming of the device according to the invention;  
       FIGS. 8A and 8B  show in further detail two steps of the method illustrated in  FIG. 6 ; and  
       FIGS. 9A and 9B  show information displayed on a display screen on implementation of the method according to the present invention. 
    
    
      In the different drawings, same elements are designated with same reference numerals.  
       FIG. 2  shows the body of patient  10  arranged in lateral decubitus position on an operation table (not shown). A tracking device  30  is applied on the skin of patient  10 . According to the present example of embodiment, device  30  comprises a base  32  and a block  34  attached to base  32 . Block  34  comprises at least three marks  36 A,  36 C. By means of a probe with a spherical tip (not shown), it is possible to obtain the position of tracking device  30  in reference coordinate system R F  from the palpation of marks  36 A,  36 C. As will described in further detail hereafter, the palpation of marks  36 A,  36 C of tracking device  30  can be performed even when the latter is covered with operating drapes. Tracking device  30  is attached to patient  10  by applying base  32  on the patient&#39;s pubis. Tracking device  30  is then substantially fixed with respect to pelvis  12 .  
      An example of a method according to the invention for determining the position of pelvis  12  comprises the steps of:  
      arranging on patient  10  tracking device  30  according to the present invention;  
      determining the position of the tracking device with respect to reference coordinate system R F ;  
      determining the position at an initial time of pelvis  12  in reference coordinate system R F , for example by palpation of characteristic anatomic points of patient  10 ;  
      determining the relative position between pelvis  12  and tracking device  30 ;  
      preparing patient  10  for the operation, especially arranging the operating drapes (not shown) to define the sterile operation area;  
      arranging rigid body  20  on pelvis  12  of patient  10  and continuously determining the position of rigid body  12  in reference coordinate system R F ;  
      determining again the position of tracking device  30  through the operating drapes in reference coordinate system R F , and determining the relative position between tracking device  30  and rigid body  20 ; and  
      continuously determining the position of pelvis  12  in reference coordinate system R F .  
       FIGS. 3A, 3B , and  3 C respectively are a side view, a front view, and a top cross-section view of tracking device  30  of  FIG. 2 . Base  32  substantially has the shape of a parallelepipedal plate. Block  34 , substantially parallelepipedal, is arranged along a side of base  32 . On three surfaces, block  34  comprises a mark  36 A,  36 B,  36 C in the form of a conical recess. Each conical mark  36 A,  36 B,  36 C is surrounded with a ring-shaped rounded edge  40 A,  40 B,  40 C protruding with respect to the corresponding surface  41 A,  41 B,  41 C of block  34 . As an example, the angles at the top of conical marks  36 A,  36 B,  36 C are identical and equal to approximately 120° and rounded edges  40 A,  40 B,  40 C have a diameter of approximately 20 mm and a height of approximately 3 mm. Two conical marks  36 A,  36 B are formed on two opposite parallel surfaces  41 A,  41 B of block  34 . The third conical mark  36 C is formed on a surface  41 C of block  34  perpendicular to surfaces  41 A,  41 B and is closer to one of the two conical marks  36 A,  36 B.  
      Side  42  of base  32  may be aligned along the pubic symphysis  18  of patient  10  to ease the palpation of the pubic symphysis on palpation of the characteristic anatomic points of patient  10  to obtain the position of pelvis  12  in reference coordinate system R F  at the initial time. According to a variation, base  32  is not aligned with the pubic symphysis and comprises an opening through which the pubic symphysis is palpated on determination of the position of pelvis  12  at the initial time.  
      Base  32  may be applied against the patient&#39;s pubis via a device usually used to maintain patient  10  on the operation table in lateral decubitus position. According to a variation of the invention, tracking device  30  is directly integrated to the device maintaining the patient in lateral decubitus position. According to another variation, the tracking device is attached to the patient&#39;s body by an adhesive film.  
       FIG. 4A  is a perspective view of a variation of tracking device  30  which is particularly advantageous when the device is integrated to a device maintaining the patient in lateral decubitus position. Tracking device  30  comprises a rigid frame  44 , connected to the holding device, and on which are formed conical marks  36 A,  36 B,  36 C. A foam block  45  is arranged on frame  44  and is compressed against the pubis of patient  10  by the holding device. Foam block  45  enables uniform distribution on the pubis of the forces exerted by the holding device. Frame  44  defines an opening  46  enabling the surgeon to have access to foam block  45 .  
       FIG. 4B  is a perspective view of foam block  45 . Foam block  45  is crossed by a slot  47  to which the surgeon has access through opening  46  of tracking device  30 . With an adapted tool, the surgeon can space apart the walls of slot  47  to clear a pathway to the pubis. The surgeon can then palpate the pubis, through the pathway thus cleared, to determine the position of the pubic symphysis, either directly with a probe, or with an echographic probe located in reference coordinate system R F .  
       FIG. 5A  illustrates a step of the method for determining the position of tracking device  30  illustrated in  FIG. 2 . A probe, having a single end  42  shown by a sphere in  FIG. 5A , is placed successively at the level of each conical mark  36 A,  36 B,  36 C. The probe is equipped with a rigid body, similar to rigid body  20  shown in  FIG. 2 , so that location system  19  determines the probe position and accordingly the position of the center of spherical end  42  of the probe in reference coordinate system R F . Points C 1 , C 2 , C 3 , represented by crosses in  FIG. 5A , correspond to the positions taken by the center of spherical end  42  of the probe when it is successively placed in marks  36 A,  36 B,  36 C. The two conical marks  36 A,  36 B are arranged symmetrically with respect to each other so that points C 1  and C 2  define a straight line D 1  perpendicular to surfaces  41 A and  41 B. Point C 3  projects on straight line D 1  at a point H and the straight line connecting C 1  to H defines a straight line D 2  perpendicular to D 1 . Straight lines D 1  and D 2  altogether completely define a three-dimensional reference frame characteristic of the position of the tracking device. Thus, from lines D 1  and D 2 , the computer connected to location system  19  defines the position of the tracking device in reference coordinate system R F .  
      To determine straight lines D 1 , D 2 , the computer may number points C 1 , C 2 , C 3 . To avoid ambiguities in the numbering, conical mark  36 C is not placed at an equal distance from conical marks  36 A,  36 B but is closer, for example, to conical mark  36 B.  
       FIG. 5B  illustrates a subsequent step of the method for determining the position of tracking device  30  when operating drapes  44  cover patient  10 , and especially tracking device  30 . A probe comprising a spherical end  46 , shown by a sphere in  FIG. 5B , is then used. The recessed conical shape of marks  36 A,  36 B,  36 C and the protruding shape of rounded edges  40 A,  40 B,  40 C define characteristic surfaces which enable the surgeon to easily identify when the probe is in a proper position on the axis of the cone of a conical mark  36 A,  36 B,  36 C. Indeed, by sliding the probe sphere along the cone walls as deep as possible, the sphere center is on the cone axis.  
      The conical shape of marks  36 A,  36 B,  36 C and the presence of edges  40 A,  40 B,  40 C enables, despite the presence of operating drapes  44 , defining points C 1 ′, C 2 ′, C 3 ′, represented by crosses in  FIG. 5B , occupied by the center of spherical end  46  of the probe when it is placed at the level of conical marks  36 A,  36 B,  36 C. Conical marks  36 A,  36 B being arranged on opposite surfaces  41 A,  41 B symmetrically with respect to each other, straight line D 1 ′ defined by the two points C 1 ′, C 2 ′ is identical to straight line D 1  of  FIG. 5A . Similarly, the symmetry of revolution of mark  36 C ensures that straight line D 2 ′, defined from point C 2 ′ and from straight line D 1 ′, is identical to straight line D 2 . The computer thus determines a same position of the tracking device in reference coordinate system R F  as that obtained at  FIG. 5A  if the tracking device has not moved. In the present example of embodiment, spherical end  46  is shown, as an example, with a radius smaller than the radius of spherical end  42  of the probe of  FIG. 5A . An advantage of tracking device  30  according to the present example of embodiment is that it enables correct determination of the position of tracking device  30  independently from the radius of the end of probe and from the thickness of operating drapes  44 .  
      According to a variation of the present invention, instead of conical marks  36 A,  36 B,  36 C, marks formed of raised areas which project out of surfaces  41 A,  41 B,  41 C of tracking device  30 , are provided. These are for example, pyramidal, conical, tapered, etc. surfaces distributed on block  34  and having their position determined by a probe of complementary shape. As compared with the previously-described tracking device, this amounts to inverting the geometrical shapes of the mark and of the probe.  
      According to a variation of the present invention, block  34  comprises more than three conical marks so that at least three conical marks are continuously accessible to the probe.  
       FIG. 6  illustrates another example of the method according to the invention for determining the position of pelvis  12  in reference coordinate system R F . According to this example, a tracking device  50  formed of an assembly of markers  52 , three markers being shown in  FIG. 5  is provided. Each marker comprises a conical mark  54  which substantially has the same shape as conical marks  36 A,  36 B,  36 C of device  30  of  FIGS. 3A  to  3 C. Tracking device  50  comprises at least three markers  52  and preferably from five to six markers. Markers  52  are distributed on the patient at locations such that the relative positions between the markers remain unchanged even if patient  10  makes slight motions. Similarly to what has been previously described, the relative position between tracking device  50  and pelvis  12  is determined from the palpation of markers  52 . After preparing patient  10  and arranging rigid body  20 , the position of tracking device  50  is determined in reference coordinate system R F , by palpation of markers  52  in the presence of the operating drapes covering patient  10 . The relative position of tracking device  50  with respect to rigid body  12 , and thus the relative position of pelvis  12  with respect to rigid body  20  can then be determined, and the position of pelvis  12  can then be determined in reference coordinate system R F .  
       FIG. 7  shows a perspective view of an example of the forming of a marker  52  of  FIG. 6 . Marker  52  comprises a parallelepipedal or circular base  56 . Conical mark  54  is formed at the center of a surface of the base. As for tracking device  30  of  FIGS. 3A  to  3 C, a ring-shaped border  58  surrounding conical mark  54  protrudes from base  56  and eases the palpation of a point located on the axis of the cone of mark  54  when marker  52  is covered with an operating drape. Base  56  comprises an adhesive film  60  on the surface opposite to conical mark  54  for attaching marker  52  to patient  10 , similarly to a cardiology electrode. According to a variation of the present invention, a mark formed of a raised area which protrudes from base  56  is provided instead of conical mark  54 . It may for example be pyramidal, conical, tapered, etc. surfaces.  
       FIG. 8A  illustrates a step of the method for determining the position of tracking device  50  illustrated in  FIG. 6 . A probe, a single end  62  of which is represented by a sphere of radius R in  FIG. 8A , is successively placed on the axis of the cone of mark  54  of each marker  52 . The recessed conical shape of mark  54  and the protruding shape of the rounded edge surrounding mark  54  define characteristic surfaces which enable the surgeon to easily identify when the probe is in a correct position at the level of conical mark  54 . Location system  19  is capable of determining the position of center C of spherical end  62  placed in conical mark  54 . Based on the positions of the centers determined for all markers  52 , the computer determines the position of tracking device  50  in reference coordinate system R F .  
       FIG. 8B  illustrates a subsequent step of the method for determining the position of tracking device  30  when operating drapes  44  cover patient  10  and, more specifically, markers  52 . A probe comprising a spherical end  64 , represented in  FIG. 8B  by a sphere of radius R′ smaller than R, is then used. The difference between radiuses R and R′ substantially corresponds to the thickness of operating drape  44  covering marker  52  so that the position of center C′ of spherical end  64  with respect to base  56  is identical to the position determined for center C of spherical end  62  with respect to base  56  in  FIG. 8A . Such a thickness may be empirically determined and be memorized by the computer. The thickness may also correspond to a parameter provided to the computer before preparation of patient  10  and which depends on the type of operating drape and of probe used. Based on the positions of the centers determined for all markers  52 , the computer determines the position of tracking device  50  in reference coordinate system R F .  
      According to another example (not shown) of embodiment of the tracking device according to the present invention, the device is formed of two bars. At the end of each bar are provided conical marks similar to previously-described conical marks  36 A,  36 B,  36 C,  54 . The two bars are placed independently from each other on the patient. Similarly to what has been previously described for tracking device  30 , the computer determines a straight line for each pair of symmetrical conical marks of a bar. The computer also determines for each pair of conical marks of a bar a midpoint equidistant from the conical marks. An advantage of such a tracking device is that for each bar, the determination of the straight line and of the midpoint does not depend on the presence or on the absence of the operating drapes and thus on their possible thickness. Based on the two straight lines and on the two midpoints, the computer can then determine the position of the tracking device in reference coordinate system R F .  
      According to another example (not shown) of embodiment of the tracking device according to the present invention, the tracking device comprises a cylindrical block of axis D, possibly hollow, comprising a cylindrical outer wall on which are distributed three marks. An advantage of such a tracking device is that the determination of axis D and of the plane comprising the three tracking points associated with the marks does not depend on the presence or on the absence of operating drapes, and thus on the thickness of such operating drapes. From the determination of axis D and of the plane associated with the marks, the computer can then determine the position of the tracking device in reference coordinate system R F .  
      More generally, the tracking device comprises a lateral wall on which are distributed at least three marks. The shape of the marks is such that the guiding points obtained by palpation of the marks in the presence or in the absence of the operating drapes are in a same plane and that at least one point determined from the tracking points is invariant when it is determined in the presence or in the absence of operating drapes. For the tracking device shown in  FIGS. 5A and 5B , such an invariant point is for example the point corresponding to the projection of C 3  (respectively, C 3 ′) on the straight line joining points C 1  and C 2  (respectively C 1 ′ and C 2 ′). For a cylindrical tracking device, an invariant point is for example the point equidistant to the guiding points.  
      In the previously-described examples of embodiment, a probe comprising a spherical end has been used. However, an intermediary pliers-shaped element may be used for the palpation of the tracking device. The pliers-shaped element comprises legs that can cooperate with the marks of the tracking device to ensure the attachment of the pliers-shaped element on the tracking device. The pliers-shaped element can be attached to the tracking device in the absence of operating drapes or with one or several operating drapes interposed between the tracking device and the pliers-shaped element. Such a pliers-shaped element itself comprises marks intended to be palpated, for example, by a probe comprising a spherical end. The pliers-shaped device is designed so that the marks of the pliers-shaped element are at the same relative positions with respect to the tracking device when the pliers-shaped element is attached to the tracking device in the presence or in the absence of operating drapes. The position of the tracking device in reference coordinate system R F  obtained from the position of the pliers-shaped element is then identical in the absence or in the presence of operating drapes. As an example, for a cylindrical tracking device comprising three marks on the outer cylindrical lateral wall, the pliers-shaped element comprises three legs connected to a base on which are arranged the marks of the pliers-shaped element. An actuation mechanism enables simultaneously and identically displacing the legs so that each end of a leg bears against a mark of the tracking device, possibly with interposed operating drapes, the cooperation of the three legs with the marks of the tracking device ensuring the attachment of the pliers-shaped element on the tracking device.  
       FIG. 9A  shows a display screen  70  connected to the computer on which the computer can display information to help the surgeon find conical marks  36 A,  36 B,  36 C,  54  covered with the operating drapes. As an example, the computer displays an image  72  representing a portion of pelvis  12  or of any other anatomic element of the patient on which are superposed the positions, represented by circles  74 , of conical marks  36 A,  36 B,  36 C,  54  such as determined in reference coordinate system R F  on palpation of tracking device  30 ,  50  in the absence of the operating drapes. The computer also displays an image  76  representing the position of the probe with respect to conical marks  36 A,  36 B,  36 C,  54  in reference coordinate system R F , according to any viewing angle. The surgeon can then more easily find the positions of conical marks  36 A,  36 B,  36 C,  54  when tracking device  30 ,  50  is covered with operating drapes. It is assumed that location system  19  and thus reference coordinate system R F  is substantially fixed. Further, the computer can assign a number to the conical marks determined on palpation of tracking device  30 ,  50  in the absence of the operating drapes. The next conical mark to be palpated by the surgeon according to the numbering order can then be represented by a symbol  78  different from symbol  74  used for the other conical marks. More detailed information may be displayed in a window  80  inserted in display screen  70 .  
       FIG. 9B  illustrates an example of information displayed on window  80 . A first symbol  82 , for example, a circle, placed at the center of window  80 , corresponds to the position of the next conical mark to be palpated by the surgeon according to the numbering order. A second symbol  84 , for example, a cross, corresponds to the position of the projection of the center of the end of the probe on a specific plane, for example, the plane perpendicular to the axis of the conical mark associated with first symbol  82  and containing the top of the conical mark. Graduated axes  85 ,  86  enable estimating the distance in the plane between the probe end and the conical mark to be palpated. The distance separating a cursor  87  from a reference bar  88  is representative of the distance separating the probe end and the conical mark to be palpated along the direction perpendicular to the previously-defined plane. The surgeon can then more easily find the next conical mark to be palpated.  
      After having found first conical mark  36 A,  36 B,  36 C,  54 , the computer may apply a translation to the symbols displayed on display screen  70  corresponding to the other conical marks  36 ,  54  not yet palpated, equal to the translation between the position of the first conical mark obtained in the absence of operating drapes and the newly-determined position of the first mark in the presence of the operating drapes. This compensates for a possible displacement of location system  19  or of patient  10 .  
      When a number of conical marks greater than three is used, the computer can implement conventional point-aligning methods (for example, according to a lesser square method) to eliminate the points obtained by palpation of conical marks, the positions of which are the most distant from the provided positions or to enable the surgeon to palpate again the conical marks corresponding to the points considered as incorrect.  
      According to a variation of the present invention, the position of the characteristic anatomic points are obtained with a local ultrasound scan probe which transmits two-dimensional images to the computer and the position and the orientation of which are constantly measured in reference coordinate system R F  by means of a rigid body added to the probe and which is similar to the rigid body shown in  FIG. 2 . The images can be displayed on a screen. The positions of the characteristic anatomic points are, for example, obtained, as follows:  
      the surgeon visually analyzes the two-dimensional images provided by the ultrasound scan probe and designates points by means of a mouse or of a touch-sensitive display unit;  
      the computer analyzes the two-dimensional images and automatically locates points having remarkable properties; or  
      the computer determines from the two-dimensional images a set of points in a three-dimensional space and deforms a three-dimensional pelvis model from the set of determined points to obtain a three-dimensional surface representative of the pelvis of patient  10 . The positions of the characteristic anatomic points can then be obtained by searching points of the three-dimensional surface having specific geometric properties or simply resulting from the deformation of the corresponding anatomic points on the deformed model.  
      According to another variation of the present invention, the characteristic anatomic points of patient  10  are clouds of points that may form portions of curved lines or of surfaces. These points, these lines, and these surfaces may be aligned with three-dimensional images of the patient of scanner or magnetic resonance type according to conventional alignment methods. By combination, the position of these three-dimensional images will be obtained in the coordinate system linked to rigid body  20  once the fields have been arranged.  
      The present invention has many advantages:  
      First, it enables easily palpating anatomic points or surfaces on a patient in the absence of the sterile fields, and finding these points or surfaces in a reference frame associated with a rigid body attached to the patient once the sterile fields have been arranged, even if the patient has moved between the two phases.  
      Second, it enables determining the position of a patient&#39;s anatomic element, which position is used in a computer-assisted surgical operation, while limiting risks of additional infection of the patient.  
      Third, it enables determining the position of the anatomic element even if the patient is covered with operating drapes.  
      Fourth, the tracking device according to the present invention is arranged on the patient at the level of a region against which the device for maintaining the patient in lateral decubitus position generally bears so that the device according to the present invention does not disturb the surgeon&#39;s motions during the operation.  
      Of course, the present invention is likely to have various alterations and modifications which will readily occur to those skilled in the art. In particular, the present invention has been described in for a computer-assisted arthroplasty of the hip. It should be clear that the present invention can find an application for any computer-assisted operation in which the position of an anatomic element of the patient must be continuously determined even if the patient is covered with operating drapes. As an example, the present invention may apply to a skull surgery operation in which anatomic points or surfaces can be located on the patient&#39;s skin before arranging operating drapes and in which the method and the device according to the present invention are used to find these positions after arranging the operating drapes, and thus compensate for a possible motion of the patient between the two phases.