US20050021043A1 - Apparatus for digitizing intramedullary canal and method - Google Patents

Abstract

An apparatus for obtaining an axis of an intramedullary canal of an exposed bone with a position tracking system in computer-assisted surgery, comprising a detectable device trackable in space for position and orientation. A stem portion is secured to the detectable device so as to be tracked for position and orientation. The stem portion has a leading end insertable in an intramedullary canal of the bone through an opening in the bone, and is adapted to be handled by a following end thereof. A tip portion is provided at the leading end of the stem portion. The tip portion is positionable in a determined way with respect to a surface of the intramedullary canal, such that reference points with respect to the intramedullary canal are calculable as a function of the position and orientation of the detectable device. The reference points are related to define an axis of the intramedullary canal. A method is provided for digitizing the intramedullary canal axis.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims priority on U.S. Patent Application No. 60/415,809, filed on Oct. 4, 2002 by the present Applicants, the subject matter of which is incorporated herein by reference. This application also claims priority on U.S. Patent Application No. 60/465,805, filed on Apr. 28, 2003 by the present Applicants, the subject matter of which is incorporated herein by reference.
FIELD OF THE INVENTION
• The present invention generally relates to computer-assisted hip replacement surgery and, more precisely, to hip replacement surgery with minimal preoperative procedures.
BACKGROUND OF THE INVENTION
• Total hip replacement surgery involves the introduction of an artificial hip joint in a patient. The artificial hip joint typically consists of a pelvic implant and a femoral implant. The pelvic implant is a cup received in the acetabulum. The femoral implant consists of a spherical portion received at an end of a longitudinal implant portion. The longitudinal implant portion is introduced into the intramedullary canal of the resected femur, with the spherical portion being generally centered with respect to the previous position of the femoral head. Therefore, the femoral head (i.e., spherical portion of the femoral implant) and the cup (i.e., pelvic implant) coact to create the artificial hip joint.
• Different output values are of concern in hip replacement surgery. In order to reproduce a natural and/or improved gait and range of motion to a patient, the position and orientation of the implants, the offset of the femur and the limb length must be considered during surgery. The work of the surgeon during hip replacement surgery will have a direct effect on these output values.
• Known computer-aided hip replacement surgery techniques presently involve preoperative computerized tomography (CT) steps that enable acquisition of data related to the skeletal configuration of the patient. This data is used, for instance, to determine implant size and to guide the surgeon intraoperatively into reproducing the ideal output values mentioned above.
• There are inconveniences to the use of CT. Firstly, it is desired to reduce preoperative steps to a minimum. Ideally, hip replacement surgery would be free of preoperative steps, as this would be logistically preferred. Cost saving issues are involved in the logistic simplification of hip replacement surgery. Moreover, CT uses X-rays, which are known to be hazardous to health.
• Accordingly, it is believed that computer-aided hip replacement surgery techniques having minimal preoperative steps would be of great appeal to surgeons and to management of health institutions.
SUMMARY OF THE INVENTION
• It is an aim of the present invention to provide an apparatus and method for digitizing a center of a bone canal.
• It is a further aim of the present invention to provide a novel method for guiding an operator in inserting implants in hip joint replacement with computer assistance.
• It is a still further aim of the present invention to provide a method of performing hip joint replacement with minimal preoperative procedures.
• It is a still further aim of the present invention to provide a computer-assisted surgery system for guiding an operator in inserting implants in hip joint replacement.
• Therefore, 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 femur as a function of a limb length and orientation of the femoral implant, comprising the steps of: obtaining a frame of reference of the femur, the frame of reference being trackable in space for position and orientation; providing a digital model of a femoral implant; calculating a desired implant position for the femoral implant with respect to the frame of reference of the femur, as a function of the limb length; and guiding an operator in altering the femur for a subsequent insertion of the femoral implant in the femur by providing information about a current implant position and orientation with respect to the desired implant position, the current implant position and orientation being calculated as a function of the digital model of the femoral implant and of a real-time tracking for position and orientation of at least one surgical tool altering the femur for receiving the femoral implant.
• Further in accordance with the present invention, there is provided a method of doing surgical treatment in computer-assisted surgery for guiding an operator in inserting a pelvic implant of a hip joint implant in an acetabulum of a pelvis as a function of a position and orientation of the pelvic implant with respect to the pelvis, comprising the steps of: creating a frame of reference of a pelvis by registering points on the pelvis, the frame of reference being trackable in space for position and orientation; creating a digital model of a surface of an exposed acetabulum of the pelvis by registering points on the surface of the acetabulum as a function of the frame of reference; providing a digital model of a pelvic implant; calculating an initial center of rotation of the acetabulum as a function of the digital model of the surface of the acetabulum; and guiding an operator in altering the acetabulum for a subsequent insertion of the pelvic implant in the acetabulum by providing information about a current implant position and orientation-with respect to the initial center of rotation of the acetabulum and the frame of reference, the current implant position and orientation being calculated as a function of the digital model of the pelvic implant and of a real-time tracking for position and orientation of at least one surgical tool altering the acetabulum for receiving the pelvic implant.
• Still further in accordance with the present invention, there is provided a method for digitizing an intramedullary canal axis of a bone in computer-assisted surgery, comprising the steps of: performing an opening in a bone to expose an intramedullary canal of the bone; providing a tool trackable in space for position and orientation and a frame of reference on the bone, said tool having a leading end thereof being positionable in a determined way with respect to a surface of the intramedullary canal; and obtaining the axis of the intramedullary canal with respect to the frame of reference by calculating and relating reference points in the intramedullary canal by inserting the leading end of the tool at given depths in the intramedullary canal and calculating a reference point of the intramedullary canal for each said given depths as a function of a position and orientation of said tool having the leading end positioned in said determined way.
• Still further in accordance with the present invention, there is provided an apparatus for obtaining an axis of an intramedullary canal of an exposed bone with a position tracking system in computer-assisted surgery, comprising: a detectable device trackable in space for position and orientation; a stem portion secured to the detectable device so as to be tracked for position and orientation, the stem portion having a leading end insertable in an intramedullary canal of the bone through an opening in the bone, and being adapted to be handled by a following end thereof; and a tip portion at the leading end of the stem portion, the tip portion being positionable in a determined way with respect to a surface of the intramedullary canal, such that reference points with respect to the intramedullary canal are calculable as a function of the position and orientation of the detectable device, said reference points being related to define an axis of the intramedullary canal.
• 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 femur as a function of a limb length and orientation of the femoral implant with respect to the femur, comprising: a reference tool securable to the femur and trackable in space for position and orientation; a registration tool trackable in space for position and orientation and handled by the operator to register surface information; a bone altering tool trackable in space for position and orientation; a sensing apparatus, for tracking any one of the tools for position and orientation; a controller connected to the sensing apparatus, the controller being provided to: i) register a frame of reference of the femur by at least one of calculating surface information provided by the registration tool as a function of the position and orientation of the registration tool provided by the sensing apparatus, and retrieving in a database a model of the femur; ii) calculate a desired implant position with respect to the frame of reference as a function of the limb length; and iii) calculate a current implant position and orientation in relation to the desired implant position with respect to alterations being performed in the femur with the bone altering tool, as a function of the position and orientation of the bone altering tool provided by the sensing apparatus and of a digital model of a femoral implant provided by the database; and the database connected to the controller for the controller to store and retrieve information relating to an operation of the controller.
• Still further in accordance with the present invention, there is provided a computer-assisted surgery system for guiding an operator in inserting a pelvic implant of a hip joint implant in an acetabulum as a function of an orientation of the pelvic implant with respect to the pelvis, comprising: a reference tool securable to the femur and trackable in space for position and orientation; a registration tool trackable in space for position and orientation and handled by the operator to register surface information; a bone altering tool trackable in space for position and orientation; a sensing apparatus, for tracking any one of the tools for position and orientation; a controller connected to the sensing apparatus, the controller being provided to: i) register a frame of reference of the pelvis by calculating surface information provided by the registration tool as a function of the position and orientation of the registration tool provided by the sensing apparatus; ii) register a digital model of a surface of an exposed acetabulum of the pelvis with respect to the frame of reference by calculating surface information provided by the registration tool as a function of the position and orientation of the registration tool provided by the sensing apparatus; iii) calculate an initial center of rotation of the acetabulum as a function of the digital model; and iv) calculate a current implant position and orientation in relation to the initial center of rotation of the acetabulum and the frame of reference with respect to alterations being performed in the acetabulum with the bone altering tool, as a function of the position and orientation of the bone altering tool provided by the sensing apparatus and of a digital model of an acetabular implant provided by the database; and the database connected to the controller for the controller to store and retrieve information relating to an operation of the controller.
BRIEF DESCRIPTION OF THE 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 front elevation view of leg bones involved in a hip replacement method in accordance with the present invention;
• FIG. 2 is a block diagram of a computer-assisted surgery system in accordance with the present invention;
• FIG. 3 is a flow chart of a method of hip replacement surgery in accordance with the present invention;
• FIG. 4 is a front elevation view of a canal digitizer in accordance with the present invention;
• FIG. 5A is a longitudinal cross-section view of the canal digitizer;
• FIG. 5B is a side elevation view of a handle portion of the canal digitizer;
• FIG. 6 is a front elevation view of fingers of the canal digitizer;
• FIG. 7 is a front elevation view of one of the fingers having a stopper; and
• FIG. 8 is a front elevation view of the fingers of the canal digitizer centering the canal digitizer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
• According to the drawings, and more particularly toFIG. 1, bones of the leg that will be involved in the hip replacement surgery of the present invention are generally shown at1.FIG. 1 is provided as reference for the description of the steps of the hip replacement surgery method described herein. The bones are thepelvis10, thefemur20, thetibia30 and thefibula40. Hereinafter, parts of these bones will each be referenced by numerals from the same numeric decade. For instance, parts of the pelvis (e.g., the acetabulum11) will bear reference numerals between11 and19.
• Referring toFIG. 2, a computer-assisted surgery system is generally shown at50 (hereinafter CAS system50) and generally consists of aCAS controller52 connected tosensor apparatus54. Thesensor apparatus54 tracks for position andorientation tools56, to be described in detail with the description of the hip replacement surgery method of the present invention. Thecontroller52 is typically a PC unit that has user interfaces by which a surgeon will receive or send information that will guide him during the hip replacement surgery. For instance, monitors, keyboard, mouse, and foot pedals are a few of the user interfaces that can be provided with thecontroller52. A database of thecontroller52 is illustrated separately asdatabase58, and is typically the hard disk drive of thecontroller52. A discussion of the preferred system configuration will follow the description of themethod100.
• Referring toFIG. 3, a method for hip replacement surgery in accordance with the present invention is generally shown at100. Although themethod100 is referred to in the singular, various choices of procedure will be given to the surgeon, as will be set forth in the forthcoming description, according to the preferences of the surgeon. A plurality of methods can be derived from themethod100 according to the decisions of the surgeon.
• InStep102, preparative steps for surgery are effected. Namely, general patient information can be entered into the CAS system50 (FIG. 2) for opening a patient file. For instance, a general patient profile can be entered, consisting of the name, birth date, identification number, sex and the like, as well as more specific data pertaining to the surgery, such as leg length discrepancy (with the identification of the longer leg), if applicable. For instance, the leg length discrepancy is measured using X-rays of the hip joint. More precisely, the leg length discrepancy is measured from the vertical comparison between the lesser trochanters. These X-rays are typically taken during the diagnostic stages leading to surgery, so they are usually available for hip joint surgery. The calibration of the various surgical tools to be used is done. For instance, a calibration base and method, as set forth in International Publication No. WO 01/67979 A1 by Jutras et al., can be used for the calibration. Also, correspondence between the tracking of thetools56 and the display on theCAS controller52 can be verified in further calibration steps included inStep102.
• It is pointed out that the general patient information can be entered preoperatively. Moreover, the entering of the general patient information is straightforward such that the surgeon need not be involved. However, in order to minimize the preoperative procedures, all steps ofmethod100 can be performed at the beginning of the surgical session, during short time span preceding the surgery.
• Surgery is initiated betweenStep102 and subsequent Decision104-, by the surgeon exposing the hip joint. No computer assistance is required thereat.
• InDecision104, the surgeon is given the choice of proceeding either with (A) the dislocation of the femur from the pelvis, or with (B) the resection of the femoral head from the femur. Both (A) and (B) will lead to the obtainment of frames of reference for both the femur and the pelvis, as will be explained hereinafter, but both (A) and (B) have advantages over the other. For reference purposes, flow chart lines have been identified as (A) or (B) for themethod100 inFIG. 3, in accordance with the choice of the surgeon. The dislocation of the femur from the pelvis (A) is preferred, as centers of rotation of theacetabulum11 and the femoral head21 (FIG. 1) will be digitized independently from one another. In (B) resecting thefemoral head21 from thefemur20 to then remove thefemoral head21 from theacetabulum11, thefemoral head21 will not be exposed out of the acetabulum11 until it is resected, whereby the center of rotation thereof cannot be digitized with respect to the rest of thefemur20. Therefore, a calculation based on an assumption will enable the calculation of a theoretical femoral center of rotation.
• On the other hand, dislocation (A) of thefemur20 shows some level of difficulty, and involves risks such as fracture of the femur20 (e.g., at the neck22), and damage (e.g., hyperextension) to ligaments and muscle. Resection (B) of thefemoral head21 can be preferred by surgeons as a safer procedure.
• Procedure (A) will initially be described in detail, followed by a description of procedure (B).
• Referring toFIG. 1 (for bone parts reference) andFIG. 3, inStep106, following the choice of (A) dislocating thefemur20 from thepelvis10, thefemoral head21 is removed from theacetabulum11. Therefore, both theacetabulum11 and thefemoral head21 are exposed.
• InStep108, tracking references (included in the tools56) are secured to thepelvis10 and thefemur20. Therefore, both thepelvis10 and thefemur20 can be tracked for position and orientation in space simultaneously as a function of their respective tracking references, by theCAS system50 ofFIG. 2. The tracking references will remain anchored to their respective bones throughout the computer-assisted steps of surgery. It is pointed out thatStep108 could have been performed prior toStep106 in procedure (A), although it is preferred that the tracking references not interfere with the dislocation of thefemur20. TheCAS system50 must thus be adapted to track at least two tracking references simultaneously, and in real-time.
• It is pointed out that the tracking reference may take the form of a marked point on one of the bone elements. For instance, a tracking reference, of the type being screwed to the bone element, can be secured to thepelvis10 to be tracked for position and orientation, while thefemur20 is solely marked with a point (e.g., from a physical or visual marker). In order to relate the marked point on thefemur20 to the tracking reference on thepelvis10, the marked point is registered when thepelvis10 and thefemur20 are in a known and reproducible posture. Thereafter, to update the position and orientation information relating to thefemur20 during surgery, the known and reproducible posture is reproduced and the marked point is digitized with a registration pointer.
• It is also contemplated to provide a portable tracking reference that is positionable on a bone element in a reproducible way. For instance, instead of having a single marked point, thefemur20 may be marked with three nonlinear points to be used to position the portable tracking reference thereon. With such a portable tracking reference, position and orientation information for thefemur20 can be obtained by positioning the portable tracking reference with respect to the three nonlinear points. The portable tracking reference is advantageous in that no screw holes are performed in the bone element.
• Nonetheless, themethod100 will be described with a tracking reference anchored to thefemur20 in accordance with a preferred embodiment of the present invention. Yet it is appreciated that the above described alternatives could be used instead of the anchored tracking reference of thefemur20.
• Referring to FIGS.1 to3, inStep110, digitization of a center of thefemoral head21 is performed. For instance, a registration pointer (from the tools56) having its tip tracked in space is used to register points on the surface of thefemoral head21. Therefore, points of contact between the tip and a given surface can be registered as a function of the tracking reference (Step108). As tracking references have been secured to thefemur20 and thepelvis10 inStep108, the points on the surface of thefemoral head21 are known as a function of the tracking of the respective tracking reference of thefemur20. TheCAS controller52 is equipped with software that will enable it to calculate the center of a sphere (i.e., the femoral head21) from the collected points. Moreover, the center calculation software is adapted to perform a validation of the center calculation with the standard and maximum deviations. A rejection criterion for the validation of the center calculation can be recorded, for instance, during the setting of parameters inStep102.
• Although the above describedStep110 is preferred for obtaining a 3D model from which the femoral center of rotation will be calculated, other equivalent methods are contemplated for obtaining the femoral center of rotation. For instance, photogrammetric scans can be used to rapidly create 3D models, or preoperative computerized tomography images can be gathered, from which the femoral head geometry can be established.
• The diameter of the sphere (i.e., of the femoral head21) can be displayed, as such information can guide the surgeon in the choice of a femoral implant. The femoral center of rotation and the diameter of thefemoral head21 can be used to correct displays on theCAS controller52. Although CT images can be shown on the display unit of theCAS controller52, themethod100 is preferably free of CT images. Accordingly, general visual images of thepelvis10 and of thefemur20 can be displayed for general reference, but these images can be scaled as a function of the center of rotation and diameters, calculated inStep110 for thefemur20 in procedure (A), and to be calculated inStep114 for thepelvis10.
• InStep112, with the center of thefemoral head21 now identified in procedure (A) as a function of the tracking reference (Step108), resection of thefemoral head21 is performed.
• Referring to FIGS.1 to3, in.Step114, digitization of the acetabular center of rotation is performed, by taking reference points on the surface of the acetabulum11, and using the center calculation software of theCAS controller52, set forth inStep110 to find the acetabular center of rotation. The acetabular center of rotation is therefore known as a function of the tracking reference on thepelvis10. It is noted that in procedure (A) the digitization of the acetabular center of rotation (Step114) is done independently from the digitization of the femoral center of rotation (Step110).
• Step116 consists in the digitization of the acetabular and femoral coordinate systems, i.e., the acetabular and femoral frames of reference.
• The acetabular coordinate system is digitized with the registration pointer. In a preferred embodiment of the invention, three points are taken on thepelvis10 to create the acetabular coordinate system. Referring toFIG. 1, there is one point on theiliac crest12 of the operated side, one point on the contra lateraliliac crest13, and one point on one of the twopubic tubercles14 of thepelvis10. To be generally aligned, the points digitized on the iliac crests12 and13 are taken at the outermost anterior point of the iliac crests12 and13. The points digitized on the iliac crests12 and13 are preferably taken directly on the soft tissue covering the bone pelvis on the iliac crests, as the soft tissue is relatively thin thereon. The point on thepubic tubercle14 completes a first plane, the frontal plane. A second plane, the transverse plane, is perpendicular to the frontal plane and includes the points on the iliac crests. A third plane, the sagittal plane, is perpendicular to the frontal and transverse planes.
• Supplemental information regarding the frontal plane can be obtained for various postures of a patient. For instance, trackable references can be used to gather information about sitting, standing and walking postures. This information can be used to adjust the orientation of the frontal plane, as these postures can provide information not available from the typical lying posture in which a patient is during surgery. This information can influence the anteversion positioning of the implants.
• It is possible to obtain anteversion and/or inclination values of the acetabulum of the patient, to be used as a reference (e.g., comparison basis) later in the surgery. To do so, points are digitized (using a registration pointer from the tools56) on the generally circular edge of theacetabulum11 and a plane is defined from these points. A normal to this plane is projected onto the acetabular transverse plane (as defined in Step116) to give an anteversion angle with the intersection of the acetabular frontal plane and transverse plane. The normal to this plane is projected onto the acetabular sagittal plane (as defined in Step116) to give an inclination angle with a cranial-caudal axis (y) on the acetabular sagittal plane.
• In a preferred embodiment of the present invention, the femoral coordinate system is digitized inStep116 by providing five points of reference on the leg to theCAS controller52, which is equipped with software that will create the femoral coordinate system. Referring toFIG. 1, a first point is taken on the tip of the greater trochanter23 of thefemur20, and will be defined as a starting point of an anatomical axis of thefemur20. Thereafter, points are taken on the medial andlateral epicondyles24 and25 of thefemur20, respectively. A midpoint between the medial epicondyle and lateral epicondyle points, in alignment therewith, is defined as an endpoint of the anatomical axis of the femur. The fourth and fifth points are taken on themedial malleolus31 of thetibia30 and on thelateral malleolus41 of thefibula40, with the leg being bent at the knee. By having the leg bent at the knee, thetibia30 stands on theposterior condyles26 of thefemur20. Therefore, an assumption is made wherein an aligned midpoint of the medial and lateral malleoli points is said to define a plane (i.e., sagittal plane) with the anatomical axis, with an axis of the knee being normal to the sagittal plane. The frontal plane is perpendicular to the sagittal plane, with the anatomical axis lying therein. The transverse plane is perpendicular to the sagittal and frontal planes, and can be positioned at any height. With the anatomical axis and the midpoint of the malleolus region digitized, the femoral coordinate system, i.e., the femoral frame of reference, is complete. It is noted that it is not required to measure two points to obtain a midpoint of the malleolus region. As this latter point will be in the sagittal plane, the only requirement is that a point is taken at a midpoint of the malleolus region, and may thus be placed approximately by the operator.
• It is pointed out that the projection values described herein (e.g., inclination, anteversion, etc.) are based on the acetabular and the femoral coordinate systems. As it is contemplated to use alternative methods of digitizing the acetabular and the femoral coordinate systems, in addition to the preferred methods ofStep116, the projection values would be related to the alternative acetabular and femoral coordinate system.
• Now that the procedure (A) has been described, the procedure (B), involving the resection of thefemur20 without the dislocation thereof, will be described. It is pointed out that some of the steps described for procedure (A) are also performed in procedure (B), as readily seen inFIG. 3. Therefore, unless stated otherwise, corresponding steps will involve the same features and will not be described in detail for procedure (B). As shown inFIGS. 1 and 3, the first step of the surgical procedure following the decision to proceed with the resection (B) isStep108, wherein tracking references are secured to thepelvis10 and to thefemur20.
• Referring toFIGS. 1 and 3,Step200 consists in a registration of a relative position between thepelvis10 and thefemur20, as a function of the tracking references on each. The leg is simply left in a straight position, and a relative position is acquired between tracking references secured to their respective bones.
• In procedure (B),Step112 of resecting thefemoral head21 from the femur followsStep200. Accordingly, theacetabulum11 of thepelvis10 is exposed, andStep114, consisting in the digitization of the acetabular center of rotation, follows. As mentioned previously, the digitization of the acetabular center of rotation is as a function of the tracking reference secured to thepelvis10.
• Step202 includes the calculation of the femoral center of rotation. In this calculation, an assumption is made that the acetabular center of rotation, calculated in theprevious Step114 in procedure (B), coincides with the femoral center of rotation. However, as thepelvis10 and thefemur20 are separated due to the resection of thefemoral head21 inStep112, the position of the femoral center of rotation is calculated as a function of the relative position between thepelvis10 and thefemur20, obtained inStep200.
• Step116 follows, with the digitization of acetabular and femoral coordinate systems, i.e., acetabular and femoral frames of reference.
• Therefore, procedures (A) and (B) have now generally gathered the same data. At this point,Decision150 follows, wherein the surgeon is given the choice of proceeding first with (C) the insertion of the pelvic implant, or with (D) the preparation of the femur for the insertion of the femoral implant.
• Once more, procedures (C) and (D) will each have their respective advantages. Procedure (C) is the preferred embodiment and will most likely involve at least one fewer step. Some surgeons may prefer the procedure (D) as it involves work on thefemur20, which has just been resected to lose its femoral head21 (Step112).
• Procedure (C) will initially be described, followed by a description of procedure (D).
• Step152 is the first step of procedure (C), and consists in the preparation of the acetabulum for insertion of the pelvic implant. Typically, the preparation of the acetabulum is performed by a reamer (from amongst thetools56 of the CAS system50). The previous acetabular center of rotation is known as a function of the tracking reference secured to thepelvis10, as it was acquired in previous.Step114. Preferably, the reamer is tracked for position and orientation, such that an axis of actuation of the cup tool on the reamer is displayed on theCAS controller52.
• InStep152, the diameter of the pelvic implant chosen by the surgeon will be used to display a position of the new acetabular center of rotation in comparison to the digitized acetabular center of rotation (Step114). For instance, the distance between the centers of rotation can be displayed numerically (e.g., in mm) as a function of the acetabular coordinate system digitized inprevious Step116. Also, the anteversion and inclination of the actuation axis of the reamer, both as a function of the acetabular coordinate system, can be given numerically (e.g., in degrees) to guide the surgeon in the reaming. More precisely, the anteversion is calculated as the angle between an intersection of the acetabular frontal plane and transverse plane and the projection of the axis of the reamer on the acetabular transverse plane, and the inclination is the angle between the reamer axis and a cranial-caudal axis (y) on the sagittal plane of the acetabular coordinate system (Step116).
• Step154 consists in the insertion of the pelvic implant in theacetabulum11. A tracked impactor (from amongst the tools56) is preferably used. As the pelvic implant size has been chosen, the diameter thereof and the known relation between the impactor and the pelvic implant is used with the tracking of the impactor to give the anteversion and the inclination of the pelvic implant. Also, the distances between the current and the digitized centers of rotation can be displayed. Therefore, the surgeon is guided during the use of the impactor so as to position the pelvic implant to a given position of the center of rotation thereof, and to a given orientation [with respect to anteversion and inclination (Step152)] to provide a maximal range of motion and stability of the leg.
• Although the pelvic implant is secured at this point to thepelvis10, it is possible to adjust the position and orientation of the pelvic implant. Firstly, the tracked impactor may be reconnected to the pelvic implant to serve as a lever in manipulating the pelvic implant with the tracked impactor, allowing position and orientation information (e.g., anteversion and inclination) to be calculated from the tracking of the impactor. Alternatively, points on the circular edge of the pelvic implant may be digitized to define a plane, with the normal to this plane being used to calculate the anteversion and the inclination, as suggested previously to obtain this information for the acetabulum. This alternative approach is well suited for pelvic implants having screw holes for additional fixation, through which the implants can be altered in position and orientation.
• Step156 consists in the creation of a femoral target height. The target height is a desired position for the femoral center of rotation, and is calculated as follows:
  (target height)=(Δ_{PELVIC COR})−(initial Δ_LL),
  where (Δ_{PELVIC COR})is the deviation of the implant center of rotation (i.e., Step154) with respect to the digitized acetabular center of rotation (i.e., Step114), in cranial-caudal (y) direction (with a cranial deviation having a positive value), and (initial Δ_LL)is the initially acquired limb length discrepancy (Step102).
• Referring toFIGS. 1 and 3,Step158 consists in the preparation of thefemur20 for the insertion of the femoral implant. More precisely, an axis of theintramedullary canal27 is digitized as a function of the tracking reference secured to thefemur20 by relating points in thecanal27. Theintramedullary canal27 is exposed by the resection of thefemoral head21 performed inStep112. In an embodiment of the present invention, the points in thecanal27 are digitized by the insertion and tracking of a pointer (from amongst the tools56) in thecanal27, to various depths. It is also contemplated to use a tool, such as an awl, that is trackable in space for position and orientation, and whose axis is known with respect to the tracking. The awl can be inserted in the intramedullary canal to a depth wherein the inner diameter of the intramedullary canal is generally equivalent to the awl. At such a depth, the awl can be considered to be generally centered in the intramedullary canal, whereby the axis of the awl can be registered as being the axis of the intramedullary canal.
• Referring toFIG. 4, a canal digitizer in accordance with a preferred embodiment of the present invention is generally shown at300. Thecanal digitizer300 can be used to accurately position a tip thereof in the center of theintramedullary canal27, and hence an axis centered in thecanal27 can be digitized. Thecanal digitizer300 will be described in further detail hereinafter.
• Step158 also includes the rasping of theintramedullary canal27 in view of the insertion of the femoral implant therein. The rasp is part of thetools56 of theCAS system50, and is therefore tracked for position and orientation. The tooling portion of the rasp, which will alter theintramedullary canal27, and the femoral implant both have a predetermined geometry. It is preferred to have rasps each having a tooling end with a generally similar geometry to the bone-engaging portion of the femoral implants. Moreover, the rasps can be provided in different sizes, with an equivalent femoral implant for each size of rasp. Therefore, the tracking of the rasp for position and orientation relative to the femoral coordinate system during alteration of the canal is used to calculate the current position and orientation of the femoral implant. A plurality of guiding parameters are displayed to the surgeon to quantify the current position and orientation of the femoral implant, and are listed below.
• The anteversion of the femoral implant as calculated from the tracked rasp can be displayed numerically (e.g., in degrees) based on the femoral coordinate system calculated inStep116. It is represented by the angle between the intersection of the frontal plane and the transverse plane and a projection of the neck axis (anticipated for the femoral implant) onto the transverse plane (Step116).
• Another guiding parameter to be provided to the surgeon is the varus/valgus angle of the femoral implant, which is equivalent to the varus/valgus angle of the tracked rasp. The angle is measured between the projection of the intramedullary canal axis and the projection of the longitudinal rasp axis onto the femoral frontal plane (Step116), and is displayed to the surgeon in degrees.
• Another guiding parameter to be provided to the surgeon is the distance between the previous femoral center of rotation [i.e., digitized inStep106 for the dislocation procedure (A), or calculated inStep202 for the resection procedure (B)] and the current femoral center of rotation. The current femoral center of rotation is calculated as a function of the femoral implant geometry (e.g., the ball head size) and the tracking of the rasp. The distance can be given in X, Y and Z values (e.g., in mm) according to the femoral coordinate system (Step116).
• Another guiding parameter to be provided to the surgeon is the current leg length discrepancy. The current leg length discrepancy, (current Δ_LL), is calculated as follows:
  (current Δ_LL)=(initial Δ_LL)−(Δ_{PELVIC COR})+(Δ_{FEMUR COR}),
  where (Δ_{FEMUR COR}) is the Y value calculated above inStep158, and where (Δ_{PELVIC COR}) and (initial Δ_LL) have been calculated inStep156. The current leg length discrepancy can be displayed by theCAS system50 as an overall leg length, or as a relative value between leg lengths, with the value 0 representing legs of equal length.
• Another guiding parameter is the offset of the femoral implant. The offset is the distance between the acetabular center of rotation and the axis of the implant (i.e., the anatomical axis of the femur as defined previously) on the transverse plane, and is thus directly related to the size of the femoral implant. It is pointed out that a type of femoral implant contemplated for the above describedmethod100 can be provided in various sizes, and a size of femoral implant can have various femoral centers of rotation along the neck of the implant. A proper identification of the size of the femoral implant must therefore be provided as it will have an effect on both the offset and the limb length.
• It may also be desired to display the sum of the measured femoral and acetabular distances in the X-axis.
• Once the surgeon is satisfied with the raspedintramedullary canal27 in procedure (C), the femoral implant is inserted. Step160 consists in the insertion of the femoral implant in theintramedullary canal27. It is anticipated that the femoral implant will be positioned and orientated substantially as calculated from the tracking of thetool56 that has previously altered the intramedullary canal. However, it is also contemplated to track the femoral implant for position and orientation with a position of the center of rotation of the implant being calculable by theCAS controller52 as a function of the tracking of the femoral implant. Such a step would ensure a precise position of the implant. TheCAS controller52 can change its display of thefemur20 upon the insertion of the femoral implant, or of thepelvis10 upon the insertion of the pelvic implant (Step154).
• Step162 consists in the analysis of the range of motion of the hip joint. The range of motion can be assessed by the following parameters: the angles of flexion/extension, of adduction/abduction, and the internal/external rotation of the leg. These angles are measured based on both coordinate systems digitized inStep116, and the minimal and maximal angle values can be recorded as part of the patient profile.
• Step164 signals the end of the computer assistance to the hip replacement surgery.
• Now that procedure (C) has been described in detail, procedure (D), involving the femur preparation prior to the pelvic implant insertion, will now be described. In procedure (D),Step250 followsDecision150, and consists in the same operations effected inStep158. Namely, thefemur20 is prepared for the insertion of the femoral implant. However, an assumption is made that the actual pelvic implant center of rotation will be coincident with the digitized acetabulum center of rotation (Step114). The digitized acetabulum center of rotation will thus be used in providing the various guiding parameters to the surgeon.
• FollowingStep250, procedure (D) comprisesSteps152,154 and156, as performed in procedure (C). It is pointed out that inStep154, the actual pelvic implant center of rotation is obtained.
• InDecision252, a comparison is made between the actual pelvic implant center of rotation and the digitized acetabulum center of rotation, that has been used for the preparation of the femur inStep250. If there is a difference between these values, femoral correction may be required, as shown inStep158. Otherwise, the femoral implant can be inserted in the femur.
• Thereafter, Steps162 and164 follow to end procedure (D).
• Now that themethod100 has been described in detail, theCAS system50 will be described in accordance with the preferred embodiment of the present invention.
• Referring toFIG. 2, an operator (e.g., surgeon) is illustrated at S and is guided in performing surgery by theCAS system50. More specifically, the operator S interacts with thecontroller52 of theCAS system50 using the user interfaces of the controller52 (e.g., mouse, display unit, keyboard, sound emitter). As shown inFIG. 2, thecontroller52 will provide guiding information on themethod100 to the operator S throughout CAS. The guiding information is for instance retrieved by thecontroller52 from thedatabase58, and will guide the operator S in handling thetools56.
• Thetools56 are each trackable in space for position and orientation by thesensing apparatus54, such that a position and/or orientation of given components thereof are calculable. As a general basic requirement, thetools56 include the reference tools, such as the trackable references securable to the bones (Step108), for the creation of frames of reference of the bones. Another one of the requiredtools56 is a registration tool that will enable to gather surface information about the bones (e.g., Steps110,114,116, etc.). As mentioned previously, the registration tool can be a registration pointer, a tracked photogrammetric sensor, or the like. Finally, a bone altering tool is included in the requiredtools56, such as a reamer and a rasp, for which uses have been described previously inSteps152 and158, respectively. Also, thetools56 include the pelvic (impactor) and the femoral implant, that can be tracked for position and orientation, to guide the operator during the insertion. It is pointed out that information relating to the tools (e.g., geometry, position of tip) is either known by the controller52 (or retrievable from the database58) or determinable using various steps of calibration.
• Thesensing apparatus54 is connected to thecontroller52, and transfers position and orientation tracking to thecontroller52. The position and orientation trackings are used by thecontroller52 to calculate parameters pertaining to the CAS. More precisely, the position and orientation trackings of the reference tool and registration tool are used to create frames of reference of the pelvis and the femur, as described inSteps110,114 and116. As shown inFIG. 2, the frame of reference information is provided to the operator S, for instance using the display unit of thecontroller52.
• For the pelvic implant, an initial center of rotation is calculated with respect to the frame of reference, as described inStep114. The acetabular center of rotation will be used with the pelvic frame of reference as references for the alteration of the acetabulum in view of the insertion of the pelvic implant therein. A desired femoral implant position is calculated by thecontroller52 in accordance with theStep156, and will be used as a reference for the alteration of the intramedullary canal in view of the insertion of the femoral implant therein. Thedatabase58 stores information that is retrieved by thecontroller52 to make the calculation.
• The current pelvic and femoral implant positions and orientations are calculated as a function of the position and orientation tracking of the bone altering tools, and of the geometry of the respective implants. This is performed in accordance withSteps154 and158/252 of themethod100. Once more, thecontroller52 uses the output of thesensor apparatus54 and information stored in thedatabase58 for the calculations, that will be displayed for guiding the operator S.
• TheCAS system50 can operate with active or passive tracking. In a preferred embodiment of the present invention, thesensor apparatus54 is a NDI Polaris® optical tracking apparatus, with appropriate operating software in thecontroller52. With the Polaris® optical tracking apparatus, passive detectable devices, such as retro-reflective spheres, are used in patterns to be tracked in space for position and orientation. Each one of thetools56 that requires to be tracked has an own detectable pattern.
• TheCAS system50 must guide the surgeon throughout themethod100, and relevant information is displayed to ensure the surgeon follows the proper Steps of operation. For instance, when leg length discrepancy values are given, the cranial-caudal convention can be displayed to explain the reading obtained. Animations can be initiated automatically to guide the surgeon, for example, in taking reference points on the various bones, such that the reference points are taken in a given order, or at the right locations.
• Referring toFIGS. 4, 5A and5B, thecanal digitizer300 is shown having anelongated shaft302, ahandle portion304, apiston306, adetectable device base308, and a centeringmechanism309.
• Theshaft302 is hollow, and has anouter surface310 and a freeopen end312. Theshaft302 is graduated such that when thedigitizer300 is inserted in a canal, the tool handler has an indication of the depth of insertion of thedigitizer300. As shown inFIG. 5A, asplitter314 is provided at the freeopen end312, and is held between a pair ofextensions316.
• Thehandle portion304 has a pair ofradial flanges320. Thedetectable device base308 projects outwardly from a distal one of theradial flanges320. Theproximal flange320 is adjacent to thepiston306, and is used as leverage by the tool handler to push thepiston306 inwardly. A flaredtip322 is adjacent to thedistal flange320, and will enable thedigitizer300 to be centered when abutting against walls of a canal, if theelongated shaft302 is completely inserted in the canal. Thehandle portion304 also defines aninner cavity324, having a guidingchannel325 adjacent to theproximal flange320, for a purpose to be described hereinafter.
• As shown inFIG. 4, a flaredadapter323 can be optionally provided in sliding engagement on theshaft302. The flaredadapter323 serves the same purpose as the flaredtip322 of thehandle portion304, but is displaceable on theshaft302 so as be used at various depths of insertion of theshaft302 in the canal.
• The centeringmechanism309 has aslender rod326, concentrically disposed in theshaft302. Therod326 is connected to thepiston306 at a proximal end thereof, and has a pair of fingers,328 and330, pivotally mounted thereto at a distal end thereof, as best seen inFIG. 6. Returning toFIGS. 5A and 5B, a spring332 surrounds a proximal portion of therod326, and interacts with a surface of theinner cavity324 of thehandle portion304 such that therod326 is biased in the proximal direction, i.e., thepiston306 is held away from thehandle portion304. Accordingly, when thepiston304 is pushed in by the tool handler, therod326 moves in the distal direction, but retracts back proximally upon release of thepiston306. The spring332 is fixed at a proximal end thereof to thepiston306, for instance, by welding therebetween.
• Thepiston306 has apin327 at a distal end thereof. Thepin327 cooperatively engages into the guidingchannel325 of thecavity324 of thehandle portion304. Accordingly, the assembly of thepiston306/centeringmechanism309 can be readily separated from theshaft302/handle portion304 assembly by guiding the former from the latter until thepin327 is out of the guidingchannel325. The guidingchannel325 has anelbow portion325′ (FIG. 5B) and astraight portion325″, such that once thepin327 is past theelbow portion325′ (FIG. 5B) in thecavity324, thepiston306/centeringmechanism309 assembly is captive and free to translate in thestraight portion325″, being held captive with respect to thehandle portion304. This assembly is preferred to facilitate the sterilization of theinner cavity324 and of the interior of theshaft302.
• Referring to FIGS.6 to8, thefingers328 and330, at the distal end of therod326, are kept with theirrespective tips329 and331 separated from one another by astopper334 protruding from thefinger328. Therefore, thefingers328 and330 define aslot336. Thefingers328 and330 are held bypivot338. Thetips329 and331 are both at a same distance from thepivot338.
• When thepiston306 is pushed inwardly, therod326 moves towards the freeopen end312 of theshaft302, such that thesplitter314 engages with theslot336. In doing so, thefingers328 and330 are guided away from one another, as best seen inFIG. 8. If thedigitizer300 is in a canal, as shown at340 inFIG. 8, thefingers328 and330 will center thedigitizer300 in the canal. Other configurations are possible. For instance, a pivotless solution can also be imagined wherefingers328 and330 would be two blades protruding from a same solid and the flexion of the material would split the blades apart when engaged on thesplitter314. It has also been thought to equip theelongated shaft302 with an inflatable end that will be self-centered in the intramedullary canal upon being inflated.
• Therod326 has a given length, such that thefingers328 and330 at an end thereof do not interfere with thesplitter314 when the assembly of thepiston306/centering mechanism309 (including the rod326) is screwingly inserted in theshaft302/handle portion304. Such interference would prevent thepin327 from going past theelbow portion325′ of the guidingchannel325 in theinner cavity324. Another possible configuration is to provide an axial rotational degree of freedom between thepiston306 and therod326. Therefore, an engagement of thefingers328 and330 with thesplitter314 would not prevent the engagement of thepin327 of thepiston306 in the guidingchannel325 of theinner cavity324. This can be achieved by providing an annular groove339 on an end of therod326, and corresponding engagement pins341 between the grooves339 and thepiston306.
• Thedigitizer300 is used with a tracking system, such as CAS system50 (FIG. 2). AsCAS system50 is preferably equipped with an optical tracking system, thedigitizer50 is shown having thedetectable device base308 consisting of a support for detectable devices. More precisely, thedetectable device base308 has anarm342 with asupport plate344 at a free end thereof. Snap-fit fingers346 are provided for receiving detectable devices in snap-fit engagement therewith. For instance, the detectable devices can be retro-reflective detectable spheres, one of which is shown at348 inFIG. 4 on the verge of being snap-fitted to one of thefingers346. It is obvious that thedigitizer300 may also be equipped with an active detectable device, provided that the tracking system is configured therefor.
• The tracking system used with thedigitizer300 must know the relation between the detectable device on thebase308 and the tip of theshaft302. The relation can be determined in calibration using a calibration base (as explained previously for thetools56 ofFIG. 2). Accordingly, when thedigitizer300 is stabilized in the canal, the position and orientation of the detectable device can be registered, and a center point of the canal can be calculated thereafter, where the tip of theshaft302 is located.

Claims (8)

1. A method for digitizing an intramedullary canal axis of a bone in computer-assisted surgery, comprising the steps of:

performing an opening in a bone to expose an intramedullary canal of the bone;

providing a tool trackable in space for position and orientation and a frame of reference on the bone, said tool having a leading end thereof being positionable in a determined way with respect to a surface of the intramedullary canal; and

obtaining the axis of the intramedullary canal with respect to the frame of reference by calculating and relating reference points in the intramedullary canal by inserting the leading end of the tool at given depths in the intramedullary canal and calculating a reference point of the intramedullary canal for each said given depths as a function of a position and orientation of said tool having the leading end positioned in said determined way.

2. The method according toclaim 1, wherein the method is performed on an anatomical bone model or on a cadaver.

3. An apparatus for obtaining an axis of an intramedullary canal of an exposed bone with a position tracking system in computer-assisted surgery, comprising:

a detectable device trackable in space for position and orientation;

a stem portion secured to the detectable device so as to be tracked for position and orientation, the stem portion having a leading end insertable in an intramedullary canal of the bone through an opening in the bone, and being adapted to be handled by a following end thereof; and

a tip portion at the leading end of the stem portion, the tip portion being positionable in a determined way with respect to a surface of the intramedullary canal, such that reference points with respect to the intramedullary canal are calculable as a function of the position and orientation of the detectable device, said reference points being related to define an axis of the intramedullary canal.

4. The apparatus according toclaim 3, wherein the tip portion has two fingers actuatable from the following end of the stem portion to extend radially from the stem portion to center the leading end of the stem portion in the intramedullary canal.

5. The apparatus according toclaim 4, wherein the fingers are pivotally mounted to one another.

6. The apparatus according toclaim 4, wherein the fingers are biased to be retracted radially, so as to facilitate an insertion of the stem portion in the intramedullary canal.

7. The apparatus according toclaim 3, further comprising a flared adapter slidingly mounted on the stem portion, the flared adapter being flared toward the following end of the stem portion to engage with a surface of the intramedullary canal at the opening of the intramedullary canal, to center the stem portion in the intramedullary canal.

8. The apparatus according toclaim 3, wherein the stem portion is graduated on an outer surface thereof to indicate a depth of insertion of the stem portion in the intramedullary canal.

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