Abstract:
A device ( 10,10′,20,20 ′) for digitizing a center of rotation of a hip joint implant component (A,F) with respect to a bone element in computer-assisted surgery. The device ( 10,10′,20,20 ′) comprises a detectable member ( 12,22 ) trackable for position and orientation by a computer-assisted surgery system ( 30 ). A body ( 11,21 ) is connected to the detectable member ( 12,22 ) in a known geometry. The body ( 11,21 ) has a coupling portion ( 14,14′,24,25 ) adapted to be coupled to the hip joint implant component (A,F) in a predetermined configuration. The center of rotation of the hip joint implant component (A,F) is calculable in the predetermined configuration as a function of the known geometry and of the position and orientation of the detectable member ( 12,22 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application is a divisional of U.S. application Ser. No. 10/570,630, filed on Mar. 3, 2006, now abandoned which is a United States national-phase entry of International Patent Application No. PCT/CA2004/001638, bearing an international filing date of Sep. 7, 2004, and claiming priority on Canadian Patent Application No. 2,439,850, filed on Sep. 4, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to computer-assisted hip replacement surgery and, more particularly, to a device for positioning hip joint implant components during surgery, and to a system and method associated with the device. 
     BACKGROUND OF THE INVENTION 
     Computer-assisted surgery (CAS) systems provide position and orientation information in different forms throughout the operative steps, to guide the surgeon in his/her decision making. CAS systems are used for instance to assist surgeons in hip replacement surgery. In hip replacement surgery, the hip joint implants being implanted must assure a desired posture to the patient. Accordingly, the position and orientation information provided to the surgeon must be precise and accurate to obtain the desired posture. 
     The femoral implant and the acetabular implant generally form a spherical joint, in which the center of a ball head of the femoral implant coincides with the center of an hemispherical socket of the acetabular implant, at a center of rotation of the hip joint implant. During surgery, the femur is separated from its associated pelvis for the implants to be implanted. Through the separation of the femur from the pelvis, position and orientation information is still provided from the tracking of the femur, the pelvis and the various tools being used. For instance, a rasping tool altering the intramedullary canal of the femur may be tracked such that the center of rotation of the femoral implant (i.e., the center of the ball head) may be calculated as a function of the geometry of the femoral implant and of the altered intramedullary canal. 
     Some types of femoral implants come separate with the ball head being fixable to the femoral implant body. The femoral implant body has a frusto-conical connector end (e.g., a Morse 12/14 taper) upon which the ball head is slid in a friction fit. In calculating the position of the center of rotation of the femoral implant, some precision is lost considering that the fit between the ball head and the frusto-conical connector end is unpredictable to some extent. 
     Alternatively, it may be desired to confirm the position and orientation of the femoral implant. Referring to the above-described example in which the center of rotation of the femoral implant is calculated as a function of the geometry of the femoral implant and of the altered intramedullary canal, it is possible that the femoral implant is not completely fitted as expected in the altered intramedullary canal. In such a case, a confirmation of the position and orientation of the femoral implant would be appropriate. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a device for obtaining position information for hip joint implant components in computer-assisted surgery. 
     It is a further object of the present invention to provide a method and system for obtaining position information for hip joint implant components in computer-assisted surgery. 
     It is a further object of the present invention to provide a device for obtaining the center of rotation of an implant. 
     It is a further object of the present invention to provide a method for obtaining the center of rotation of an implant. 
     Therefore, in accordance with the present application, there is provided a device for digitizing a center of rotation of a hip joint implant component with respect to a bone element in computer-assisted surgery, comprising a detectable member trackable for position and orientation by a computer-assisted surgery system; and a body connected to the detectable member in a known geometry, the body having a coupling portion adapted to be coupled to the hip joint implant component in a predetermined configuration, the center of rotation of the hip joint implant component being calculable in the predetermined configuration as a function of the known geometry and of the position and orientation of the detectable member. 
     Further in accordance with the present invention, there is provided a method for digitizing a center of rotation of a pelvic implant component with a computer-assisted surgery system, comprising the steps of providing a device being trackable for position and orientation by the computer-assisted surgery system, the device being releasably coupled in a known configuration to the pelvic implant component; tracking a position and orientation of a pelvis implanted with the pelvic implant component and a position and orientation of the device; and calculating a center of rotation of the pelvic implant component with respect to the position and orientation of the pelvis by relating the known configuration of the device with the position and orientation tracking of the pelvis and of the device. 
     Still further in accordance with the present invention, there is provided a method of doing surgical treatment with a position tracking system in computer-assisted surgery for guiding an operator in inserting a femoral implant of a hip joint implant in a resected femur tracked for position and orientation, comprising the steps of positioning a trackable device on the femoral implant in a predetermined configuration, the trackable device being trackable in space for position and orientation; registering implant geometry information for the femoral implant with respect to the trackable device as a function of said predetermined configuration between the femoral implant and the trackable device; and inserting the femoral implant in the femur by obtaining implant position and orientation information, the implant position and orientation information being calculated from said implant geometry information as a function of the tracking for position and orientation of the trackable device with respect to a frame of reference of the femur. 
     Still further in accordance with the present invention, there is provided a computer-assisted surgery system for guiding an operator in inserting a femoral implant of a hip joint implant in a resected femur tracked for position and orientation, comprising a trackable reference device positionable onto the femoral implant in a predetermined configuration and trackable in space for position and orientation; a registration device trackable in space for position and orientation and handled by the operator to register surface information; a sensing apparatus, for tracking any one of the devices for position and orientation; a controller connected to the sensing apparatus, the controller being provided to: i) calculate a position and orientation of the devices as a function of the tracking by the sensing apparatus; ii) digitize surface information of the femoral implant as a function of the tracking of the registration device by the sensing apparatus; and an implant geometry information calculator connected to the controller, for calculating geometry information of the femoral implant from said predetermined configuration with respect to the trackable reference device, as a function of said surface information of the femoral implant; whereby the geometry information is used to provide implant position and orientation information related to a frame of reference of the femur, so as to guide the operator in subsequently inserting the femoral implant in the resected femur. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other features, aspects and advantages of the present invention will become better understood with regard to the following description and accompanying drawings wherein: 
         FIG. 1  is a perspective view of a device for digitizing position and orientation information of a femoral implant, in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a perspective view of a device for digitizing position information of an acetabular implant, in accordance with a preferred embodiment of the present invention; 
         FIG. 3  is a block diagram of a computer-assisted surgery system to be used with the devices of  FIGS. 1 and 2 ; and 
         FIG. 4  is a flow chart illustrating a method of doing surgical treatment for guiding an operator in inserting a femoral implant in a resected femur in hip replacement surgery in accordance with an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings and, more particularly, to  FIG. 1 , a device to be used for obtaining position and orientation information for a femoral implant is generally shown at  10 . A femoral implant is shown at F, and has a body F 10  and a ball head F 20 . The body F 10  has a stem portion F 11 , which is adapted to be received in an intramedullary canal of a resected femur (not shown). A connector end F 12  projects from an end of the stem portion F 11 . The connector end F 12  is illustrated having a frusto-conical shape, for instance having a Morse  12 / 14  taper. 
     The ball head F 20  has a spherical outer surface, and a connector bore F 21 . The connector bore F 21  is illustrated having a frusto-conical shape, so as to correspond to the shape of the connector end F 12  of the body F 10 . When the body F 10  is suitably received in the intramedullary canal of the femur, the ball head F 20  is secured to the connector end F 12 , by the complementary shapes of the connector end F 12  and the connector bore F 21 . 
     As mentioned previously, the position of the center of rotation of the femoral implant F is useful information, even prior to the ball head F 20  being secured thereto. From the calculated center of rotation, it may be required to further alter the intramedullary canal in view of an anticipated leg length discrepancy. Alternatively, the calculated center of rotation may be used to calculate the size of ball head F 20  to be used in the femoral implant F. Femoral implant orientation information is useful in calculating information such as the varus/valgus angle and the offset. 
     Accordingly, the device  10  is to be used in digitizing the center of rotation of the femoral implant F and/or the orientation of the femoral implant F. The device  10  has a tubular body  11 . A tracker base  12  projects from the tubular body  11 . The illustrated tracker base  12  is of the type that receives the passive type of tracker, i.e., in the form of three detectable devices  13  in a known geometrical pattern. Alternatively, the tracker base  12  could be used to secure an active tracker to the tubular body  11 . The tubular body  11  defines a cylindrical bore  14  (i.e., cylindrical receptacle), having a circular edge  15  at its opening in the tubular body  11 . The circular edge  15  has a known diameter, and a known position and orientation with respect to the tracker on the tracker base  12 . 
     The device  10  is to be positioned onto the connector end F 12  of the femoral implant F. More specifically, the connector end F 12  is received in the cylindrical bore  14 , such that the circular edge  15  abuts against an outer surface of the connector end F 12 . In such a position, the cylindrical bore  14  and the connector end F 12  will axially align themselves, considering that the connector end F 12  is frusto-conically shaped. 
     Furthermore, the geometric interrelation (i.e., predetermined known configuration) between the connector end F 12  and the cylindrical bore  14  allows the calculation of the position and orientation of the taper of the connector end F 12  with respect to the tracker of the device  10 . This position and orientation information of the connector end F 12  may then be used to calculate the anticipated center of the ball head F 20  as a function of the size and geometry of the ball head F 20 . This position and orientation information of the connector end F 12  may alternatively be related to a reference tracker on the femur to allow the calculation of navigation information (e.g., offset, varus/valgus angles, limb length discrepancy, etc.) 
     An alternative method of calculating the center of the ball head F 20  is contemplated. A device  10 ′, having the tracker base  12  with the three detectable devices  13  with a hemispherical hole  14 ′ can be positioned directly on the ball head F 20  once the ball head F 20  is secured to the connector end F 12  of the femoral implant F. Ball heads typically come in 3 defined sizes of 22, 28 and 32 mm, whereby the device  10 ′ is typically provided with corresponding diameters for the hemispherical receptacle  14 ′. Therefore, when the device  10 ′ is mounted onto the ball head F 20 , the relation between the center of the hemispherical receptacle  14 ′ and the center of the ball head F 20  is known (e.g., the centers are coincident), such that the center of the ball head F 20  may be established with respect to a frame of reference on the femur. The determination of the position of the center of rotation of the femoral implant F (through the above described procedure) can be accomplished on trial ball heads for the calculation of other parameters (e.g., limb length), as well as on the definitive ball head F 20  installed on the femoral implant F. 
     It is also contemplated to provide an alignment mechanism between the implants F and/or A and the devices  10  ( 10 ′) and  20  ( 20 ′), respectively, for the interconnection between the implant and its associated device to be reproducible in position and orientation. 
     Referring to  FIG. 2 , an alternative embodiment of the device, to be used to obtain position and orientation information for an acetabular implant is generally shown at  20 . An acetabular implant is shown at A and has a shell A 10  and a liner A 20 . The shell A 10  has a cup-shaped body having an outer surface A 11  and a receiving cavity A 12 . The acetabular implant A is to be fitted into an acetabulum (not shown), with the outer surface A 11  being in contact with a surface of the acetabulum. The receiving cavity A 12  is equipped with connector holes such that an impactor (not shown) can be used to insert the shell A 10  into the acetabulum and adjust its position and orientation. 
     The liner A 20  also has a cup-shaped body. The liner A 20  is sized so as to fit into the receiving cavity A 20  of the shell A 10 . More specifically, the liner A 20  has an outer surface A 21  and a socket A 22 . The outer surface A 21  contacts the surface of the receiving cavity A 12  when the liner A 20  is fitted into the shell A 10 . The socket A 22  will house the ball head F 20  ( FIG. 1 ) of the femoral implant F to form the hip joint implant. 
     As mentioned previously, the position of the center of rotation of the acetabular implant A (i.e., the center of rotation of the socket A 22 ) is useful information prior to the liner A 20  being received in the shell A 10 . The center of rotation of the acetabular implant A is dependent on the socket size of the liner A 20 , and on the geometry of the liner A 20 . The calculated center of rotation of the acetabular implant A can be used for calculating navigation information such as the offset and the limb length discrepancy. 
     The device  20  is to be used in digitizing the center of rotation of the acetabular implant A. The device  20  has a generally hemispherical body  21 . A tracker base  22  projects from an underside of the hemispherical body  21 . The illustrated tracker base  22  is of the type that receives the passive type of tracker, i.e., for instance three detectable spheres in a known geometrical pattern. The tracker base  22  could be used to secure an active tracker to the body  21 . The hemispherical body  21  defines an outer surface  24 . 
     The device  20  is to be positioned into the receiving cavity A 12  of the shell A 10  of the acetabular implant A. More specifically, the hemispherical body  21  is sized to fit the receiving cavity A 12  of the shell A 10 , such that the center of rotation of the receiving cavity A 12  of the shell A 10  may be determined. From the center of rotation of the receiving cavity A 12 , the center of rotation of the liner A 20  may be calculated, knowing the geometry of the liner A 20  (e.g., the CAS system being provided with geometry data of various sizes of liners). It is also possible that the liner A 20  is of the type having its center coincident with the center of the shell A 10 . Therefore, the anticipated center of the socket A 22  is calculable as a function of the center of the receiving cavity A 12  and of the geometry of the liner A 20  (stored in the CAS system). 
     Thereafter, the anticipated center of the rotation of the socket A 22  can be related to a reference tracker on the acetabulum to allow the calculation of navigation information, such as the offset and the limb length discrepancy. 
     It is pointed out that the device  20  may be used to determine the center of rotation of the liner A 20  directly. More specifically, the hemispherical body  21  may be sized so as to be received directly in the socket A 22  of the liner A 20 , with the liner A 20  having beforehand been secured in the receiving cavity A 12 . Moreover, an alternative configuration of the device  20 , herein illustrated as device  20 ′, is provided with a flange  25  at a periphery of the outer surface  24 , so as to enable the calculation of a plane associated to the center of rotation of the acetabular implant A. 
     The setting of the femoral implant F in the intramedullary canal of the femur is an operation that involves a plurality of factors that will have a direct impact on the success of the hip replacement surgery. Therefore, the setting of the femoral implant F advantageously involves the creation of reference systems that will be used to provide numeric data throughout the surgery to the surgeon for such anatomical references as varus/valgus angle, limb length discrepancy and femoral anteversion. These values are calculable using position and orientation data of the femoral implant, which will be available during the setting of the femoral implant F in the femur. 
     Therefore, referring to  FIG. 4 , a method for doing surgical treatment with a tracking system in computer-assisted surgery, for guiding an operator in inserting a femoral implant in a femur as a function of the limb length and the orientation of the femoral implant is generally shown at  50 . 
     The insertion of the femoral implant in the femur takes place after the femoral head has been resected, and the intramedullary canal has been altered in view of the insertion of the implant therein. Such steps are described in International Publication No. WO 2004/030556, published on Apr. 15, 2004, by Jansen et al. At this point, a generic digital model of the implant F is available through the CAS assisting the operator. 
     In Step  52  of the method  50 , the device  10  ( FIGS. 1 and 3 ) is positioned on the connector end F 12  of the implant F. If the ball head F 20  is already secured to the implant body F 10 , the device  10 ′ is used ( FIGS. 1 and 3 ). 
     In Step  54 , the orientation of the neck axis of the connector end F 12 , and the center of rotation of the ball head F 20 , are calculable as a function of the position and orientation of the tracker base  12 . 
     In Step  56 , a plane is digitized for the implant F. More specifically, three non-linear points are digitized using a registration pointer, whereby a plane may be digitized with respect to the device  10  in which all three points lie. For instance, points are taken at P 1 , P 2  and P 3  in  FIG. 1 . With these points and with the neck axis calculated in Step  54 , the position and orientation digitized and calculated in Steps  54  and  56  may be associated to the digital model of the implant. 
     In Step  58 , a tip of the implant is digitized with respect to the device  10 , using the registration pointer. The tip is illustrated at P 4  in  FIG. 1 . 
     In Step  60 , a longitudinal axis of the implant F is digitized with respect to the device  10 . More specifically, the CCD angle of the implant F is generic information provided with the digital model of the implant F. Accordingly, using the neck axis calculated in Step  54  and the CCD angle, a line parallel to the longitudinal axis is defined. The longitudinal axis is then calculated with respect to the device  10  or  10  as being parallel to this line, while lying in the plane digitized in Step  56  and passing through the tip of the implant digitized in Step  58 . 
     In Step  62 , now that the required geometry information pertaining to the implant F is known (i.e., longitudinal axis, neck axis, center of rotation, with respect to the device  10 ), the implant F is inserted in the altered intramedullary canal of the femur F. 
     Real-time information may be provided to the operator, whereby the device  10  ( 10 ′) must be kept onto the implant F during the insertion of the implant F in the intramedullary canal. Accordingly, a locking mechanism should be used to secure the device  10  to the implant F in position and orientation. 
     In Step  64 , the geometry information gathered for the implant F is associated to the frame of reference of the femur. By positioning the device  10  (or  10 ′) on the implant F, the position of the center of rotation of the implant F is known, as well as the position of the neck axis. 
     The orientation of the implant F may be calculated by knowing the interconnection between the implant F and the device  10  (or 10′) (through an alignment mechanism, as mentioned previously). 
     Alternatively, the orientation of the implant F may be calculated using the digital model of the altered intramedullary canal with respect to the frame of reference of the femur, in association with the position and orientation of the device  10  (or 10′). The digital model of the altered intramedullary canal is information available as calculated during the alteration of the intramedullary canal, as described in International Publication No. WO 2004/030556, published on Apr. 15, 2004, by Jansen et al. 
     Therefore, when the geometry information of the implant F is associated to the frame of reference of the femur, the geometry information can be used to calculate position and orientation information of the implant F with respect to the femur. 
     For instance, the longitudinal axis of the implant F, as obtained through the method  50 , can be used in the calculation of the varus/valgus angle of the femoral implant F. More specifically, the longitudinal axis of the femoral implant is projected onto a frontal plane of the patient along with an axis of the intramedullary canal (as described in International Publication No. WO 2004/030556), with the angle between these two projections representing the varus/valgus angle. 
     Also, the neck axis of the implant is projected onto the transverse plane (as described in International Publication No. WO 2004/030556), whereby the femoral anteversion is calculable as the angle between this projection and the intersection of the transverse and frontal planes. 
     Referring to  FIG. 3 , a CAS system in accordance with the present invention is generally shown at  30 . The CAS system  30  has a controller  31  that is connected to the sensing apparatus  32 . 
     The sensing apparatus  32  tracks the devices  10 ,  10 ′,  20  and  20 ′, as well as a registration device  35  (e.g., registration tool), and frames of reference  36  associated to bones (e.g., femoral and pelvic frames of reference as described in International Publication No. WO 2004/030556). For instance, the sensing apparatus  32  is an optical sensing apparatus that visually detects the position of the passive detectable devices, such as those illustrated at  13  in  FIG. 1 ). The tracking output of the sensing apparatus  32  is calculated as position and orientation of the devices by the controller  31 , whereas registered points, as described in Steps  56  and  58  ( FIG. 4 ), are digitized as surface information of the implants. 
     The CAS system  30  has an implant geometry information calculator  33 , that will receive the position and orientation of the devices  10 ,  10 ′,  20 ,  20 ′, as well as the surface information, so as to calculate geometry information, as mentioned in Steps  54  and  60 , and transfer this data in the form of implant position and orientation information, as described in Step  62 , to an operator through operator interface  34 . 
     The controller  31  typically has a controller calculator  37  consisting of a processor that will calculate the above described information, and a database  38  that will hold some information that may be required in the calculation, such as digital model of implants, to which the geometry information and the implant position and orientation information may be associated, as mentioned in the method  50 .