CA2334495A1

CA2334495A1 - Computer-aided positioning method and system

Info

Publication number: CA2334495A1
Application number: CA002334495A
Authority: CA
Inventors: Edward Chen; Marwan Sati; Haniel Croitoru; Peter Tate; Liqun Fu
Original assignee: Surgical Navigation Specialist Inc
Current assignee: Cedara Software Corp
Priority date: 2001-02-06
Filing date: 2001-02-06
Publication date: 2002-08-06
Also published as: US20040087852A1; EP1359855A1; WO2002062248A1

Abstract

A method and apparatus are disclosed for ascertaining the trajectory of an object in relation to an anatomical structure for use with computer assisted surgery by identifying at least three landmarks associated with the anatomical structure in an operative area; defining a first plane using at least two landmarks; defining a second plane using at least two landmarks, wherein at least one landmark is different, said second plane being orthogonal to the first plane, such as to define a three dimensional coordinate system; and using the coordinate system to ascertain the trajectory of any object with respect to the anatomical structure.
Exposure to imaging radiation and additional surgical procedure is minimized.

Description

FEB-06-O1 17:37 FROM-COWLING +613-563-9869 T-7B5 P.05/2B F-160 Computer-Assisted Positioning Method and System Inventyrs~ Edward Chen._ Marwan Sati. Haniel Croitg~" deter Tate_ Li~un Fu Field of the Invention '1'hc prtsent invention relates generally tv computer-assisted surgical systems and more particularly, to methods and systems for ascertaining the tr~jectary of an object in relation to an anatomical structure far use with compurcr assisted surgery.
$aokgronad of the Invention Severe damage to the hip joint caused by degeneration, trauma, disease or anatomical abnormalities make total hip joint replacements (THR.) necessary.
A total hip replacement generally comprises four elements, which can be subdivided into two femoral components (femoral prosthesis shall and head) and two acetabular components (acetabular'cup' prosthesis and prosthesis inlay).
~. successful total hip replacement procedure implies the selection of the right implant size through preoperative planning and correct infra-operative prostheses placement. Improper implant size and position can lead to hip joint disloeatiart, decreased range of motion and eventual loosening or failure of both the acetabular and femoral components. The objective for acetabular cup positioning methods is to achieve cup orientation angles of 15 degrees anteversion and 4S degrees abduction for human patients.
Correct canventiortal placement of the acetabular cup can is surgically demanding due to the hemispherical acetabular shape and difficuh anatomical landmark identification for alignment. Limited surgical exposure ofthe patient and anatomical variations of the pelvis add to the complexity of the procedure.

FEB-06-D1 1T:3T FROM-GOWLINC +613-563-9869 T-T85 P.D6/28 F-16D
A large number ofnon-computer-assisted instruments is known to facilitate the correct positioning of the acetabular cup by aligning pasts -which are connected to the positioning rod holding the cup - with anatomical landmarks and external planes. Examples for this approach can be found in (1.S_ Pat. No.
b 4,305,394 ; 4,475,549 ; 4,994,0b4 ; 5,037,424 ; 5,061,270 ; 5,098,437 ;
5,116,339 5,171,243; 5,250,051 : 5,2sa,483 ; 5,320,625 ; 5,364,403 ; 5,527,317 ;
$,571,111 ; 5,584,837 ; 5,583,399 ; 5,755,794 ; 5,$$0,976 ; 5,954,727.
Mare recently, computer-assisted systtms have been developed to facilitate the correct preoperative planning, cup positioning, femoral reaming ere.
Most systems use tamographic patient imaging methods like CT and Mftl to obtain anatomic patient data in digital farm. >;xamples for CT based hip joint planning and positioning systems axe U.S. Pat. No, filed by niGioia et al.:
6,002,859 ; 5,995,738 ; 5,880,976. In the above mexitioned systems, virtual patient models created using CT data axe matched to the patient's anatomy using surface registration techniques in coajunction with an optical tracking system.
In t3.S. Pat. No. 5,251,127 and 5,305,203 issued to Raab, a electro-gaaiometer is used to digitize patient points in the CT data sets.
Other examples far CT based computer-assisted THR procedures are to be found in U-S. list. No. 5.O8fi,401 ; 5,299,288 and 5,408,409 issued to Glassmaa et ~0 al. The above mentioned systems facilitate the robatic reaming of the femoral shaft. The patient-data-to-patient matching process is performed by artificial markers {fiducials) ibserted into the patient's bones prior to the CT imaging and operation. Woolson {U.5. Pat. No. 5,007,936) uses three reference points on the acetabulutn to be visually identified by the surgeon infra operatively to match

2~ patient C'f data.
The computer-assisted acetabular cup positiatting devices described in the above references, have the fohowing disadvantages:
1. Conventional cup positioning instruments, while being cost-effective, offer great risk of inaccuracy due to the mere dependence on visual FEB-O6-01 17:36 FROM-COWLING +613-563-9669 T-785 P.07/28 F-160 alignment by the surgeon.
2. Most procedures require additional pre-operative imaging necessary for trajectory-guidance purposes. Further, procedures requiring pre-operative plasemern of fiducials are an additional surgical operation.
g Such approaches result in increased costs, while additional operations and radiation also bear health risks far the paricnt.
Accordizzgly, there is a need for a method and system which provides sufficient acetabular cup placement accuracy, without the need for additional pre-operative imaging andlor fiducial placement.
SUMMARY ~F THE INVFN~'It~I~T
The present invention seeks to provide a method and system which minimizes the above problems.
According to one aspect of the invention, there is provided a method of ascertaining the trajectory of an object in relation to an anatomical structure for '~ 5 use with computer assisted surgery, the method including the steps of identifying at least three landmarks associated with the anatomical structure in an operative area: defining a first plane using at least two landmarks; defining a second plane using at least two landmarks, wherein at least one landmark is different, said second plane being orthogonal to the fast place, such as to define a 2Q three dimensional coordinate system; and using the coordinate system to ascertain the trajectory of any object with respect to the anatomical structure.
The invention defined above extends to all imaging modalities in computer-assisted image-guided surgical navigation systems.
In another aspect of the invention there is provided a system for Zb ascertaining the trajectory of an object in relation to an anatomical structure far use with computer assisted surgery, including means for identifying at least three FEB-06-01 17:36 FRAM-COWLING +613-563-9669 T-765 P.08/26 F-160 _g_ landmarks associated with the anatomical structure in an operative area; means for defining a first plane using at least two landmarks; means for defining a second piano using at least two landmarks, wherein at least one landmark is different, said second place being orthogonal to the first plane, such as to define a three dimensional coordinate system; and means for using the coordinate system to ascertain the trajectory of any object with respect to the anatomical structure.
In another yet another aspect of the invention there is provided a computer readable medium having computer executable software code stored thereon, the code for ascertaining the trajectory of an object in relation to an anatomical 1 D structure for ase with computer assisted surgery, comprising a code for identifying at least three landmarks associated with the anatomical structure in an operative area; a cede for defining a first plane using at least two landmarks;
means for defining a second plane using at least two landmarks, wherein at least one landmark is different, said second plane being orthogonal to the first plane, such as to define a three dimeztsional coordinate system; and a code for using the coordinate system to ascertain the trajectory of any object with respect to the anatomical structure.
BRIEF D~SCIrIPTION OF DRAWINGS
The present i»vention, by way of example only, will be further understood Z~ from the following description with references to the drawings in which:
Figure 1 is a schematic layout of the system in accordance with an embodiment Qf the invention.
Figure 2 is a representation of a frontal view of a pelvis.
Figure 3 is a diagram of the coordinate system in accordance with au embodimem ofthe invention.

FEB-O6-O1 1'f;36 FROM-GOWLING +613-563-9869 T-Z65 P.09/28 F-160 _g_ Figure 4 is a zepresentatiori of a tracked probe in accordance with an embodiment of the invention.
Figure S is a diagram of a acetabular cup positioner in accordance with an embodiment of the invention Figure 6 is diagram of three orthogonal planes in accordance with an embodiment of the inveurion Figure 7 is a diagram of a plane normal coordinate system in accordance with an embodiment of the invention.
DET~.ED DESCRxPT~bN OF THE PRTFERIx.ED EMBQDIMEN'Y' Referring to Figure l, a cpmputcr-aided acetabular cup positioning apparatus includes a mobile fluoxoscapic C-arm x-ray imaging device 20. Mobile x-ray devices used in the operating room are generally known as C-arms due to their shape. The imaging xnethad is referred t4 as 'fluoroscopy' since no x-ray f lm is being used. Fluoroscopy-based navigation systems ors commercially available.
While the embodiment of the invention described is iri reference to fluoroscopic-based navigation, it can be appreciated that other imaging modalities may be used, for example, computerized tomography (CT), magnetic resonance imaging (MRI), ultrasound, bi-planar x-ray. Howevex, tar CCrtam OI tnesC
2D devices, for example, there is additionally a need for pre-operative tamographie imaging, fiducial placement or infra-operative matchuag of tomographic datasets imaging device 20 includes a C-arm 22 slidably and pivotally attached to a downwardly-extending L-atm 23 at an attachment point 28. The L-arrn 23 is held in suspension by a mobile support base 24. 'I"he C-arm 22 is orbitable about ari axis of orbital rotation. while the L-arm 23 is rotatable about ari axis of lateral FEB-O6-D1 1T;38 FROM-COWLING +613-563-9869 T-785 P.10/28 F-160 _7_ ratatiarl to thereby rotate the C-arm 22 laterally.
X-ray source 30 is located at one end of C-arm 22 and X-ray image receptor assembly 3Z is located at the other end of C-arm 22. ~'he X-ray source 30 is capable of a generating continuous or pulsed stream of X-ray photons.
The C-aixtt 22, X-ray source 3o and image receptor assembly 32 are rotatable about, and defines, a free space 34. Within the free space 34, a opcratitlg table 50 and patient SZ are positioned. X-rays emitted from the X-ray source passes through the free space 34 to the image receptor assembly 32, intersecting the patient 52, and generating a planar two-dimensional image of the patient.
By arbitally and laterally rotating the C-arm 22 about the free space 34, X-rays may be directed to pass through the patient 52 along multiple planes to generate cwo dimensional images from different perspectives.
The itn~age receptor assembly 32 generates an image representing the intensities of recoived X-rays. In the preferred embadimenl, the image receptor asserrfbly 32 comprises an image intensifier 3b that converts the received x-ray photons to visible light. The image intensifier 36 is electronically coupled to a digital charge coupled device (CC~) camera (not shown) that converts the visible light to an analog video signal.
The image receptor assembly 32 may be additionally provided with an X-ray off detector (not shown) to detect when a new image has been inquired. For exaiuple, the X-ray detector may be in the form of a detector diode that directly absorbs received X-ray radiation or be a photodiode with a scintillator. The X-ray off detector may be used to synchronize the fluotroscopic image with the optical position tracking data as detailed below.
'the image receptor assembly 32 is interfaced via electronic cables 33 id a computer system 40 to which imaging data is communicated.
The computer system ~0 includes a computer 42 with a graphics processor. Preferably, the graphics processor is a video capture and display circuit board such as Matrox Meteor-IITM that is capable of capturing, digitizing and displaying an analog ~d~ ~J~al. The computer 42 is electronically FEB-06-O1 17:39 FROM-GOWLING +613-563-9669 T-765 P.11/28 F-160 _g_ interfaced with at least ono video display monitor 44 or other display, for use ixi interactive viewing and display of images.
The computer system 40 is provided with a plurality of data input interfaces for the receipt, storage and processing of data received from external sources, as more particularly described below. Without limitasion, input interfaces includes eleeironic interfaces (far exampic port connections to external source devices, modems, keyboard, mouse, etc.), optical interfaces, or radio frequency interfaces.
T he computer system 40 is solected to be suiTablc for image guidod surgery and surgical navigation. For example, the computer system 40 is provided with sufficiont memory, data storage, resolution, and proccssi~ag spends sufficient to calculate, process, store cad display high quality, high volumo, real-time images. The computer 42 rttay also be provided with a network card to interface with a network. Example of computer systems are ~ellTM PrccisionTM
~ b Workstations 33Q, 420 or 620.
The computer system 40 is furthor provided with surgical navigatinrt software that allows for the acquisition and registration of fluoroscopic images cad superimposition of optically-tracked instruments, such as aNN Fluoro~
software.
2p The image receptor assembly 32 is Iitrther fated with two calibration plates 46, which are clamped onto the image intensifier 3b. The calibration plates 46 contains radio-aQaquc beads spaced in a well-defined goometry and arc positioned adjacent to the image intensifier 36 in the path of incoming X-ray photons emitted from the X-ray source 30. The raw, unprocessed images as 25 captured by the image intensifier 36 axe overlaid with the irsiages of the radio-opaque beads. The images of the beads will appear distorted from their true geometry following x-ray transmission through the calibration plate 46.
Information regarding the actual positioning of the radio-opaque beads previously stored in the computer 42 is used in a mathematical model to compute imago 30 distortion.

FEB-06-01 17:39 FROM-COWLING +613-563-9669 T-785 P.12/26 F-160 Tkte mathematical model is derived using conventional means and may be applied to process captured raw image for display in substantially distortion-free f4rxrt. h'istortion computation is described, for example, by Thomas S. Y.
Tang, "Calibration and Point-Based registration of Fluoroscopic Iulages", Queen's 'University, Kingston" Ontario, Canada, fan. 1999 and Champlebaux G, Lavallee S, Cinquin P "Accurate Calibration of Cameras artd Mange Imaging Sensors: The 1"~IpBS Method", proc. IEEE of Int. Couf on ~abotics arid Automatiart, Nice France, 1992. The mathematical model may be embodied in software such as navigation or imaging software applications.
Altcrnaiively, distortion may be corrected using alternate methods including methods dispensing with the need for one or both calibration plates.
Infra-operatively, a patient 52 is prepared for surgery within the &ee space 34, with the area of surgical interest exposed. When surgery commences, the patient 52 is provided with a patient sacker a8.
The patient tracker 48 is an active or passive optically-tracked riding instruxnent_ The patient tracker 48 is rigidly attached to the patient in close proximity to the operative area. In THR, the patient tracker 48 is attached to the frontal iliac crests of the pelvis b0 and 62, as indicated in Figure Z, for example, using Kirschnar wires which are dxilled by the sargean into the iliac crests 60 and Zp 62 . For accurate image guidance, the pati~~xt tracker 48 cannot be significantly moveable relative to the patient 52 and is preferably substantially immoveable. It wih be appreciated that any movement of the patient txacker relative to the patient will affect the accuracy of any subsequent computation based on the location of the patient tracker 48.
The patient tracker 48 is used in conjunction with a position sensing system, as further described below. In the embodiment of Figure 1, the patiebt tracker 48 is provided with a plurality Qf passive reflective disks or visible Light emitting Diodes (LEDs) in a known geometry to yield orientation as web as positional information.
So Alternatively. active optical tx~ackers using infxared light emitting diodes FEB-O6-O1 17:40 FROM-GOWLIMG +613-563-9669 T-765 P.13/28 F-160 _~Q-(IRpps) may he attached unto the patient tracker 48. The trackers may be electronically connected to a control wait of the positron sensor system 54, which can control the emissions of the IRIrDs.
The position sensor 56 in the embodiraent of figure 1 is an optical camera set a distance away from the imaging device, in unobstructed view of all Crackers for which positional and arientarional information is desired. The position sensor 52 uses triangulation and real-time traclCixig algorithms to reconstruct three-dimensional coordinates of a tracker and is interfaced with the Computer system 40 to communicate tracking data. An example of a position 1 p sensor system 54 is the POLARIST"s system by Northern Digital lnc..
Alternate position sensor systems may used. Active optical sensor Systems may use IUDs as trackers. ~iybrid position sensors tracks the position and orientation of bath active and passive truckers.
'fhc position sens4r S6 of the emlxdixnent is interfaced via electronic cables 58 with the computer 42 for data coxnrnu~Iication.
The calibration plates 44 on the C-arm 22 are also provided with active or passive txackexs, the position of which are tracked by the position sensor system S4, such that C-arm 22 positional information is communicated to the computer 42. Alternatively, active or passive truckers may bt attached to predetermined 20 positions art the image intensifier 36, elsewhere on the image receptor assembly 32, or other mobile portion of the C-arm 22.
'fhe patient tracker 48 operates as a reference base attached tv the patient 52 while at the same time the C-atm 22 position and orientation in space is also tracked by the position tracking system 54. The patient tracker 48 and the C-ann 26 22 Crackers provide positia11a1 reference data for use with surgical navigation software.
?he function of the patient tracker 48 is to determine the transformation between image ordinate- and world coordinate systems (ie. the actual cu-ordinatcs of objects in the operating roam). These transfotznations are necessary 30 to render optically-tracked instruments (such as drill guides, probes, awls, ere.) on FEB-06-01 17:40 FROM-GOWLINO +613-563-9869 T-1B5 P.14/28 F-160 the fluoroscopic image in the correct anatomical position. The process of image registration is known and, for example, is described in IJ.S. patent number 5.772,594 titled "Fluoroscopic image guided orthopaedic surgery system with infra operative registration" issued to barrack, B.F. on lone ~0, 1998 and in Thomas ~. Y. Tang, "Calibration and Pairtt-Based Registration of Fluoroscopic Images", f~ueen's'Universiry, K-ingston, Ontario, Canada,:fan. 199.
While optical sensors are preferred as position sensors, other position sensors may be used, including mechanical sensors comprising articulated arms with potentiometers at each joint, sonic sensors comprising the detection of the 1 p speed and directiop of soundwaves from posiCioned acoustic emitters, or magnetic sensors, which detect phase and intensity of magnetic fields.
Preferably, the position sensor system 5h is also capable of localizing in space (tracking) the position of surgical instrumentation and tools doting intra-operative surgical procedure.
Tracbced surgical instrumentation and tools, which include probes, pointers, wands, drill guides, awls, suction units with inserts, reference clamps and pins, may be provided with integrated tracking technology embedded in the tool, or permanently err temporarily mounted with one or more trackexs.
Preferably, at least two trackers arc provided on tracked surgical ins~utnents so 2Q as to permit foal orientation, as well as position, to he determined.
Additional position trackers, active err passive, may he temporarily attached to various objects in the operating room for positioning and reference purposes, far etcample, on the patient table.
Alternatively, additional position sensor systems (active optical sensors, sonic, mechanical, tuagnetic, radio freGluency, etc.) may be used to separately track various tools or reference objects in the operating roam. Such pasrtlon sensors would also be interfaced with a computer system 40 provided with surgical navigation sofrwam.
xl~e positioning of the tracked tools and positional truckers are pre-gp registered into the surgical navi~;ativn system prior to infra-operative asc by _n. . _..y~~.,~,. _. .... _..w. _._ FEB-O6-O1 17:40 FROM-COWLING +613-563-9669 T-785 P.15/26 F-160 conventional mesas.
Once the patient S2 within the free space 34 is fitted with the patiebt tracker 48, landmarks are identifed to define the frontal plane of the patient. The landmarks may be anatomically significant stntctures, points, virtual points, prominences, etc. With reference to the human pelvis G4, the frontal plane of a person standing in upright position is defined by three reference paints on the pelvis (Fig. Z): left anterior superior iliac spine 66, right anterior superior iliac spine 68 and the centre Qf pubis symphysis 70. As will be appreciated by persons skilled 1 ~ in the art, other reference points may be ascertained and used to define the frontal or other places for other anatomical structures, including other ball and socket joints, on human, mammalian or other vertebrates.
For an accurate definition of the frontal plane of the human pelvis, it is preferred that both anterior superior iliac spinm reference points 66 arid 68 are on the same height level in the anterior posterior and in the sagittal plane, and that the reference paint on the pubic symphysis b6 is centerad in the anterior posterior plane, as depicted in P'igurc 2.
These three reference points can be substantially identified by palpation by the surgeon using known techniques.
2p The surgeon uses a tracked probe 96 to digiti'e the position of the three reference points 66, G8 and 70. Preferably. the tracked pmbe 96 is a needle pointer capable of piercing the shin to contact the underlying bone- The needle pointer 96 has a tracking element 98 attached to the handle 100 of the probe.
The location and orientation of the tip trajectory of the tip 102 of the needle relative to the tracking clement 98 is known, and communicated to the navigation software.
Additionally, fluoroscopy images of the reference points may be taken with the C-arm 22 rotated swch that images are obtained of the operative area on at least two planes. Prmfcrably, only two images are taken of the operative area.
'However, depending on the size of the operative area, the size of the patient, and the diameter of the C-arm 22 imaging field, additional images rxtay be required in FEB-06-O1 1T:41 FROM-COWLING +613-563-9869 T-185 P.16/28 F-160 order to capture all reference points in fluoroscopic images. For example, where the field of view of the #luorascapic image is identical to that of a plain radiograph, and the size of the pelvis is greater than can be captured irt one image, each reference paint can be selected with orae image in the anterior gosterior orientation and by ant taken laterally, with six planar fluoroscopic images taken.
'fhe fluoroscopic images captured by the image intensifier 36 arc camrilunicaied to the computer system 44 where they are carx'ected for distortion and stored for use in surgical navigation, for example, to verify the location oftbe identified reference points.
Preferably, the surgical navigation so$ware in the computer 42 is provided with means to adjust the digitized positions of reference points in order to compensate for any inaccuracies in the surgeon's location of the reference points. fore particularly, the digitized position of the reference points as displayed on the coczsputer display 44 may be adjusted so as to coincide with the positions of the left anterior superior iliac spine b6, right anterior superior iliac spine 68 and the centre of pubis symphysis 7~ as displayed in the previously taken fluoroscopic images of the reference paints.
~'or the human pelvis in THIt surgery. the acetabular cup prosthetic is preferably orientated with angles of 15 degrees auteversion and 45 degrees abduction for human patients. A number of considerations are inv4lved in the selection of the targeted angles of approach including: a desire to obtain a maximum range of motion, to achieve a minimum residual pain, to avoid contact between the femur and other osseous structures in the pelvis, to avoid subsequent dislocation of the joint, and to otherwise avoid the need for a subsequent hip replacement due to improper placement of the prosthesis.
The anteversian and abduction angles are measured relative to the frontal, sagittal and axial plants. These planes are measured for the whole pelvis and the acetabular cup is placed relative to these planes. With reference to the left or right acatabulum 76 or 78 ari the human pelvis, the targeted acetabular cup 34 abduction angle 4f ~5 degrees is projected on the froxtial plane. An acetabular _~..~~.~~. . ..w.~.._~ n... v ... ..

FEB-06-O1 11:41 FROM-GOWLING +613-563-9869 T-765 P.1T/26 F-160 _ ~4 _ cup antevcrsion eagle Qf 15 degrees is projected on the sagittal plane which lies perpendicular to the frontal plane.
t~nce the franEal plane is determined, the sagittal plane and the axial plan can be determined. Whets the reference points 56, b8 and 70 are digitized, and fine-tuned with reference to the fluoroscopic images, if taken, the cross product of the vector from reference point 66 and 70 and the vector from reference point 68 and 70 defines the frontal plane with the normal (frontal normal) poiming ahead of the patient 52. The midpoint 72 between G6 and ~8 is then computed.
The sagittal plane is defined as the cross product of the vector from reference point 70 and midpoint 72 and the normal vector of the frontal plane (frontal normal). The normal of the sagittal plane (sagittal normal) points to the left side of the patient.
The axial plane is defined as by the cross product of the sagittal normal and the frontal natural and its normal points towards the head of the patient.
Preferably, the computations to determine the frontal plane, the sagittal planes and the axial planes are made, stared and applied in navigation safkware.
Referring to Figure 5, a coaveational acetahular cup positioner 80, for Gxamplc, ZirntnerTM 'l~ilagyTM Acetabular Cup, equipped with a tracking element 90 an the reamer 9~+, is used to compute the acetabular cup position in relation to Zp the patient's pelvic girdle. Preferably, active or passive optical tracking elements sre attached to the reamer 94 of the cup pasitioner 80. The location and orientation of the cup trajectory relative to the tracking elemern 90 is known, and registered.
Using the optics position sensor system 54, or an alternative position sensing systelxl, the position of the ~~k~ aeetabular cup pasitioncr 80 relative to the patient tracker 48, cad other tracked objects in the operating room, is communicated to the computer system 40 and displayed to the surgeon through the computer display 44, or other means, via navigation software, preferably in real-time.
R.efernng to Figures 3 and 6, the frontal plane 104 is described by the x,y FEB-O6-Ol 17:41 FROM-GOWLIHG +613-563-9869 T-785 P.18/28 F-160 coordinates and the sagittal plane 106 by the x,z coordinates. Alpha 82 represents the abductiarl angle and gamma 84 the anteversion angle. The U 86 represents the tracker position of the tracked cup positianer 80, as identified by its spatial position (x,y,z). The vector R 88 is the distance from the tracker an the tracked cup positiarler 80 to the instrument tip. The distance R 88 is determined prior to intro-operative use on calibration of the tracked cup pasitioner 80. The origin of the coordinate system is defined to be the centre of the pubic syrnphysis, reference point 70.
Both the origin of the coordinate system and U(x,y,z) are registered by the navigation software, allowing the calculation of bath angles alpha 82 and gamma 84 using the following relationships:
Ux ~ R*sin(gattutta)*cos(alpha) Uy ~ R *sin(gatnma)*sin(alpha) Uz ~ R *cas(gamma) R = sqr (UxstUy2+Uz~) The angle alpha 82 is the cup abduction angle, which is also the azimuth of U (spherical coordixrate). The relationship between alpha 82 and the position of the tracked cup SO is described as follows:
Alpha = axctan (UY, Ux) p0 with arctan(y,x) defined as:
if x~0 : tan'' (y/x) if x~4 : pi+tari'(ylx) if (xs0) and (ya0) : pi/2 if (x=fa) and (y<0) : -pv2 The angle gamma 84 is the cup anteversion angle, which is also the colatitude of U (spherical coordinate). The relationship between gamma 84 and the position of the tracked cup 85 is described as follows:
gamma ~ cas'' (U~R) Referring to figure 6, the &otttal 104, sagittal 10~ and axial 108 planes farm FEB-D6-D1 1T:42 FROM-COWLING +613-563-9869 T-T65 P.19/26 F-160 the pelvis coordinate system. Figure 7 depicts the same coordinate system using the plane x~ormals to represent the coordinate system for mathematical purposes.
The normal ofthe frontal plane (n~ 110, the normal of the sagittal plane (n=) 112 and the normal of the axis! plane (r~ 114 are identified.
'f o determixre the abduction angle 82 and the anteversion angle 84 for the left hip 78, first the direction vector vo of the cup positioner 8Q is determined:
va~R* [0Q 1]
Next, the projection of the cup positioner ar<to each plane normal is found:
projection onto x>f= vrt = (vo*n~nr ~ p Projection onto ox = v~, _ (vo*ri,)~
Projection onto n" ~ v~ = (va*n,~n, Next the projectiQa of the cup positioner onto each plane is found:
Projection onto frontal plane vi = v~, + v~, projection onto sagttal plane v, = v~f+ y.
~ b projection onto axial plane v, = v~ + v~f Next, the angle between the cup pasitioncr vector (v~ and each plane is found:
Angle cup positiouer to frontal plane af= (1$OIPI)cos(vo*vf)-' Angle cup pasitianer to sagittal plane a, _ (1 so/Plxos(vo*vy' Angle cup positioner to axial plane a. m (18UIP1)cos(v4*vJ'1 Za where the antevcrsion angle 84 is ar and the abducciou angle 82 is a&
Far the right hip 76, the negative of the abduction angle a, is used.
The computation of the angles alpha 82 and gamma 84 can be conducted within navigation software arid infermation regarding the traj ectory the tracked cup pasitioner 80, as well as the angles of approach with reference to alpha 82 and 25 gamma 84, displayed on the computer display 44.
As will be appreciated by persons skilled in the art, the above may be adapted to ascertaia the trajectory, including path, position and angle of approach, for any object relative to any anatomical structure, including skeletal structures, joints, soft tissue, prgans, etc., far which reference points to define a three dimensional 3d coordinate system ~r the anatomical structure can be identified.

FEB-06-01 17:42 FROM-COWLING +613-563-9869 T-785 P.20/28 F-160 Numerous modifications, variations, and adaptations may be made to the parncular embodiments of the invention described above without degarting from the scope of the invention, which are defined in tha claims.

Claims

What is claimed is:

1. A method of ascertaining the trajectory of an object in relation to an anatomical structure for use with computer assisted surgery, the method comprising the steps of:
identifying at least three landmarks associated with the anatomical structure in an operative area;
defining a first plane using at least two landmarks;
defining a second plane using at least two landmarks, wherein at least one landmark is different, said second plane being orthogonal to the first plane, such as to define a three dimensional coordinate system; and using the coordinate system to ascertain the trajectory off any object with respect to the anatomical structure.

2. A method of claim 1 wherein the steps are conducted intra-operatively.

3. A method of claim 1 wherein the anatomical structure is a pelvis.

4. A system for ascertaining the trajectory of an object in relation to an anatomical structure for use with computer assisted surgery, comprising:
means for identifying at least three landmarks associated with the anatomical structur in an operative area;
means for defining a first plane using at least two landmarks;
means for defining a second plane using at least two landmarks, wherein at least one landmark is different, said second plane being orthogonal to the first plane, such as to define a three dimensional coordinate system; and means for using the coordinate system to ascertain the trajectory of any object with respect to the anatomical structure.

5. A computer readable medium having computer executable software code stored thereon, the code for ascertaining the trajectory of an object is relation to an anatomical structure for use with computer assisted surgery, comprising:
code for identifying at least three landmarks associated with the anatomical structure in an operative area;
code for defining a first plane using at least two landmarks:
code far defining a second plane using at least two landmarks, wherein at least one landmark is different, said second plane being orthogonal to the first plane, such as to define a three dimensional coordinate system; and code for using the coordinate system to ascertain the trajectory of any object with respect to the anatomical structure.