Abstract:
A non-invasive method for determining the articular point of a joint is disclosed. The method uses a surgical navigation system having a stereoscopic cameras which tracks the positions of infrared emitting or reflecting markers attached to appendages on either side of the joint. A movable marker is used to palpate known landmarks on the appendages to determine their positions. The appendages are moved to determine the trajectories of the landmarks relative to the joint. Positional information from the cameras is fed to a data processing system with resident software which uses the positional information and trajectories to mathematically determine the position of the joint articular point according to the laws of kinematics.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention concerns a method for the preoperative determination of the position of the articular point of a joint and specifically a knee joint.  
         BACKGROUND OF THE INVENTION  
         [0002]    Successful orthopaedic surgery, for example, the replacement of hip and knee joints with endoprostheses, the correction of knee deformities such as genu valgum (knock knee) and genu varum (bowleg) by osteotomy requires precise knowledge of the position of the articular point of the joint being operated on. The articular point represents an imaginary joint center about which the bones connected at the joint rotate. More precision in the knowledge of the articular point of a joint results in longer lasting replacement joints and more effective correction of deformities.  
           [0003]    To improve orthopaedic surgery, navigation systems have been developed, such as disclosed in U.S. Pat. No. 6,385,475 to Cinquin et al and hereby incorporated by reference. Such systems use markers attached to bones on opposite sides of the joint connecting the bones. The markers are observable by a stereoscopic camera system connected to a data processing system such as a computer that can record the positions of the markers in space and, using software, calculate the kinematic motion of the bones, as well as other mathematical parameters and relationships. The markers attached to the bones establish a coordinate reference system relative to each bone. Additional camera observable markers are freely moveable and may be used to palpate (touch) specific landmarks on the bones in order to ascertain the position of the landmarks in the coordinate reference systems of the bones. The positions of such landmarks are used by the data processing system software, along with the relative motion of the bones connected at the joint of interest, to calculate the geometric and kinematic relationships needed to guide the orthopaedic surgery. Included among these parameters are articular points or joint centers.  
           [0004]    Present methods for determining the position of articular points, such as the knee center, require that the knee joint be surgically opened to provide access to the anatomical center of the knee (a landmark point on the femur) so that this point may be palpated by a movable marker to establish its precise location in space relative to the femur coordinate reference system defined by the marker attached to the femur. Using the location of the anatomical knee center in conjunction with the positions of other landmarks (such as the medial and lateral epicondyles, also determined by palpation), as well as motion of the tibia relative to the femur, the data processing system software can calculate a relatively accurate position of the knee center in the femur and tibia coordinate systems. The position of the knee center is then used to provide further information directing the placement of endoprostheses or guiding the bone cutting in an osteotomy.  
           [0005]    Methods involving surgically opening the knee joint to determine the position of the knee center are acceptable when the contemplated operation also requires access to the knee joint, such as during a total knee arthroplasty (knee replacement). However, for less invasive procedures, such as the mere gathering of information for pre-operative diagnostic purposes, or an osteotomy to correct a knee deformity, that do not require the knee be surgically opened, it is not advantageous to open the knee merely to palpate the anatomical center to ascertain the knee center location. Clearly, there is a need for a non-invasive method for determining the position of the knee center, as well as the articular points of other joints.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0006]    The invention concerns a method for determining the position of an articular point of a central joint between two substantially rigid bodies, the central joint being located between first and second outer joints located at the ends of the first and second rigid bodies distal to the central joint. The steps of the method include identifying the position of a first point on the first rigid body located substantially at the central joint. Next, the position of a first articular point of the first rigid body at the first outer joint is determined. A first axis between the first articular point and the first point previously determined is defined. Then the respective positions of a second and a third point on the first rigid body on opposite sides of the central joint are identified. A plane, substantially perpendicular to the first axis and at substantially equal respective rectangular distances to the second and third points is defined. The intersection of the plane and the first axis is used as an initial estimate of the articular point of the central joint. Next the position of a second articular point of the second rigid body at the second outer joint is determined. A region in the plane having a predetermined size is identified, the region including the initial estimate of the articular point of the central joint as well as other points. The second articular point is then moved relatively to the first rigid body by rotating the second rigid body about the central joint. A multiplicity of different positions of the second articular point are identified during the rotation of the second rigid body. One point among a multiplicity of points within the region is identified for which the position is substantially invariant for each of the positions of the second articular point, the one point being the articular point of the central joint.  
           [0007]    The method according to the invention is specifically applicable to determining the position of the articular point of the knee joint (the knee center) between the femur and the tibia. Important landmarks on the knee joint for determining the articular point position include the patella (knee cap) and the medial and lateral epicondyles (the eminences of the femur above the knee joint). The method of determining the position of the knee articular point according to the invention comprises identifying the position of the patella and determining the position of the articular point of the femur at the hip joint. This information is then used to define a femoral axis that extends between the femur articular point and the patella. Next, the positions of the medial and lateral epicondyles are identified. A plane is defined that is substantially perpendicular to the femoral axis and at substantially equal respective rectangular distances to the positions of the medial and lateral epicondyles. The intersection of the plane and the femoral axis is the initial estimate of the articular point of the knee joint.  
           [0008]    Following these steps, a region in the plane having a predetermined size is defined. The region includes the initial estimate of the articular point position of the knee joint and is preferably a circle centered on this point. The position of an articular point of the tibia at the ankle joint is then defined and this point is moved relatively to the femur by rotating the tibia about the knee joint while recording multiple positions of the tibia articular point during the rotation. The one point within the aforementioned region containing the knee articular point is identified as the knee articular point whose position is substantially invariant for each of the positions of the tibia articular point.  
           [0009]    Preferably, the substantially invariant point is defined by a point within the region having the smallest standard deviation of distance between that point and the tibia articular point for all of the recorded positions of the tibia articular point.  
           [0010]    Preferably, the step of determining the position of the femur articular point comprises the steps of moving the patella by rotating the femur about the hip joint, identifying a plurality of positions of the patella during the motion of the patella about the hip joint and then determining mathematically a common point at the hip joint having substantially the same distance to all of the positions of the patella. The common point is the femur articular point.  
           [0011]    Preferably, the step of determining the position of the tibia articular point at the ankle comprises the steps of identifying the respective positions of medial and lateral malleoli (protuberances) on opposite sides of the ankle joint and the position of an anterior point of the ankle located in the sagittal plane of the tibia. Using this positional information, a first line between the medial and lateral malleoli is defined and a second line is projected from the anterior point to perpendicularly intersect the first line at an intersection point. The tibia articular point is determined as the intersection point of these two lines.  
           [0012]    It is an object of the invention to provide a method for determining the articular point or center of a joint.  
           [0013]    It is another object of the invention to provide a non-invasive method for determining the articular point of a joint.  
           [0014]    It is yet another object of the invention to provide a method of determining the articular point of a joint that does not require surgically opening the joint.  
           [0015]    It is still another object of the invention to provide a method of determining the articular point of a joint based upon the kinematics of the joint and landmarks whose location may be identified by palpation techniques.  
           [0016]    It is again another object of the invention to provide a method of determining the articular point of a joint to provide information for performing an osteotomy affecting the joint.  
           [0017]    These and other objects and advantages will become apparent upon consideration of the drawings and detailed description of the preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a perspective view of an apparatus used to perform the method of joint articular point determination according to the invention;  
         [0019]    [0019]FIG. 2 is a schematic diagram illustrating various landmarks, bones and mathematical constructs important to determining the knee articular point by the method according to the invention; and  
         [0020]    FIGS.  3 - 7  are elevational views of a leg showing the skeletal structure and illustrating various steps of the method according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 illustrates a type of orthopaedic surgical navigation device  10  preferably used to execute the steps of the method of determining the position of the knee articular point according to the invention, it being understood that the method is not limited to any particular device or any particular joint.  
         [0022]    Navigation device  10  comprises a stereoscopic sensor system  12  having sensors such as cameras  14  and  16  separated from one another so as to view markers  18  and  20  from different locations, thus allowing the positions of the markers in space to be deduced by techniques such as triangulation based upon comparison of the arrival times of signals from the markers at the different cameras. The cameras  14  and  16  are sensitive to infrared radiation so as to be usable in ambient visible light.  
         [0023]    The cameras  14  and  16  generate signals describing the detected positions of markers  18  and  20 , the signals being fed to a data processing system  22 , preferably comprising a microprocessor with resident software. The software is written to understand the camera signals and identify the positions of the markers which can then be stored and mathematically manipulated as needed to calculate or deduce further information. Information from the software is communicated to a user of the system by means of a computer monitor  24 , and the user communicates with the software by means of a keyboard  26  and foot pedals  27 .  
         [0024]    Markers  18  and  20  are attachable to the leg  28  of a patient on each side of the knee joint  30 , the articular point of which is to be determined. One marker,  18 , is attached to the upper leg portion  32  which includes the femur  34 , the other marker,  20 , is attached to the lower leg portion  36  which includes the tibia  38 . The markers are preferably attached by means of respective harnesses  40  that prevent each marker from shifting in position on the leg once attached. Harnesses  40  allow the markers to be attached without the use of invasive surgery, thus fulfilling one of the objects of the invention. The markers may also be attached directly to the femur and tibia using bone screws if the bone is surgically exposed. It is important that the markers be securely attached to the leg portions so that they always indicate the true locations of the leg portions and not a shift in the marker location relative to the leg. This ensures that any calculations based upon the locations of the markers will be meaningful with respect to the actual leg locations.  
         [0025]    A movable marker  42  is not attached to any part of the leg but is variably positionable at any point along the leg. Marker  42  is attached to a pointer  44 , having a tip  46 . The position of the tip  46  relative to the marker  42  is known to the software so that positioning the tip  46  at a landmark on the leg  28  (known as “palpating” the landmark) allows the software to precisely identify the position of the landmark relative to the position of the fixed markers  18  and  20 . Such relative positional information is useful for calculating parameters needed to determine the position of the knee articular point as described below.  
         [0026]    Markers  18 ,  20  and  42  emit infrared radiation visible to the cameras  14  and  16  allowing them to see and track the relative positions and motions of the bodies to which the markers  18  and  20  are attached or that marker  42  is palpating. The markers may have active emitters that generate their own infrared radiation, or passive emitters that reflect infrared radiation from an infrared radiation source associated with the navigation device  10 .  
         [0027]    In the method according to the invention, the locations of the markers  18  and  20  are identified to the software of the data processing system  22  by the sensor system  12 . As shown in FIG. 3, the marker locations establish frames of reference  48  and  50  on the femur  34  and tibia  38  respectively. The locations of landmarks on the leg  28  as well as the relative motion of the landmarks and the relative motion of the femur and tibia may be identified within the frames of reference.  
       Method Description  
       [0028]    A method of determining the position of the articular point of the knee joint (also called the “knee center”) is described below. The method is not limited to the knee joint but may be used on any joint connecting two rigid bodies.  
         [0029]    With reference to FIG. 1, the method according to the invention first requires that marker  18  be attached to the upper leg portion  32  which includes the femur  34 . Marker  20  is attached to lower leg portion  36  which includes the tibia  38 . Once securely attached by means of harnesses  40 , markers  18  and  20  are viewed by cameras  14  and  16  and defined to the software of the data processing system  22  as being attached to the femur and tibia respectively. For the active emitting markers illustrated, the software controls the infrared emissions from each marker via communication cables  18   a,    20   a  and  42   a  associated with each marker. Thus the software can distinguish between the markers by turning them on one at a time. It is preferred to dedicate one marker, for example  18 , to always be placed on the upper leg portion  32 , another marker,  20 , to always be placed on the lower leg portion  36 , and the third marker,  42 , to be the movable marker. This allocation of markers is programmed into the software, and when the markers are visible to the cameras  14  and  16  the software will know which marker is on which leg portion. Foot pedals  27  may be used to communicate interactively in response to the software to indicate that the markers are in place. With the markers  18  and  20  in position as shown in FIG. 1, foot pedal  27  is pressed in response to a prompting command from the software displayed on the monitor  24  to indicate that the markers are in position and ready. The software then activates the markers  18  and  20  which emit infrared signals that are read by the cameras  14  and  16 . The cameras identify the marker locations in space and transmit the information to the software which records their location and mathematically establishes coordinate reference frames  48  and  50  shown in FIG. 3 for the femur and the tibia.  
         [0030]    Next, as shown in FIG. 3, the position of the patella  52  (knee cap) is identified by palpating (touching) the patella with the tip  46  of pointer  44 . During palpation, the movable marker  42  attached to pointer  44  is viewed by the cameras  14  and  16  and the position of the patella  52  is identified to the software and thus known relative to both the femur and tibia reference frames  48  and  50 . Marker  42  may be identified to the software interactively by holding the pointer  44  stationary with its tip  46  on the patella  52  and marker  42  visible to the cameras  14  and  16  and then pressing the foot pedal  27  in response to a prompting command requesting input of the patella location, the command being visible on the monitor  24 . Using the communication cables  18   a,    20   a  and  42   a,  the software activates the markers  18 ,  20  and  42  in sequence, their infrared emissions are viewed by the cameras  14  and  16  and the software is able to distinguish between the three markers and identify and record their relative locations upon receipt of the information from the cameras.  
         [0031]    Knowing the position of the patella  52  in the femur reference frame  48 , the femur articular point  54  may be determined by moving the femur  34  about the hip joint  56  as illustrated in dashed line in FIG. 4 in response to a prompting command on the monitor  24 . Cameras  14  and  16  observe the motion of marker  18  and signal a number of discrete positions of the marker to the software. The software calculates the positions of the patella  52  from the positions of marker  18  observed by the cameras during the femur motion. Particularly since the patella  52  is at the end of a rigid body (the femur  34 ) whose opposite end is constrained in its motion by a ball joint (the hip joint  56 ), the software knows that the positions the patella may take all lie on a sphere centered at the articular point of the hip joint. Thus, from the marker positions observed and identified to the software during the motion of the femur and the known spatial relationships of the palpated patella  52  relative to the femoral marker  18 , the software can calculate the femur articular point  54  in the femur reference frame  48 , the articular point  54  being a common point at the hip joint that has substantially the same distance to all of the positions of the patella  52 .  
         [0032]    Knowing the position of the femur articular point  54 , the femoral axis  58  can be defined. As shown schematically in FIG. 2, the femoral axis  58  is an imaginary line extending between the femur articular point  54  (i.e., the center of the hip joint) and the position of the patella  52  and is defined mathematically by the software. The femoral axis  58  does not necessarily coincide with the femur  34  and is used in conjunction with the positions of the medial and lateral epicondyles  60  and  62  respectively, to determine an initial estimate of the knee articular point as described below. The epicondyles are eminences that protrude from either side of the femur  34  above the knee joint  30 .  
         [0033]    As shown in FIG. 5, the positions of the medial and lateral epicondyles  60  and  62  respectively are identified by palpating each with the tip  46  of pointer  44  and allowing the movable marker  42  to be observed by the cameras  14  and  16 . As each epicondyle is palpated, its respective position is identified to the software, again conveniently by responding to interactive prompts on the monitor  24 , the user positioning the pointer  44  appropriately in response to a command and pressing the pedal  27 . Next, as illustrated in FIG. 2, the software mathematically defines a plane  64  perpendicular to the femoral axis  58 . The plane  64  is located along the femoral axis where the plane is at substantially equal respective rectangular distances  66  and  68  to the positions of the medial and lateral epicondyles  60  and  62 . The point of intersection  70  between the plane  64  and the femoral axis  58  is the initial estimate of the knee articular point. Once the initial estimate point  70  is established, a region  72  in the plane  64  is defined that includes the estimate point. Preferably, the region  72  is a circle of 1 cm diameter centered on the initial estimate point  70 . Region  72  contains candidate points for the knee articular point, one of which will be selected mathematically, preferably using statistical methods as described below.  
         [0034]    Next, as shown in FIGS. 2 and 6, the position of the articular point  74  of the tibia at the ankle joint  76  is determined with respect to the fixed marker  20 , preferably using the positions of the medial and lateral malleoli  78  and  80 . The malleoli are the protuberances on either side of the ankle joint. The positions of the medial and lateral malleoli  78  and  80  respectively are identified by palpating them with the tip  46  of the pointer  44  attached to the movable marker  42  when prompted by the software. During respective palpation, the marker  42  is observed by the cameras  14  and  16  which signals the respective positional information of the malleoli to the software. Actuation of foot pedal  27  may be used to effect data capture as with previous palpations. Next, the position of an anterior point  82  of the ankle joint  76  is identified by palpating the anterior region of the ankle in the sagittal plane of the tibia  38 . Knowing the positions of the malleoli  78  and  80 , the software mathematically defines an imaginary line  84  (see FIG. 2) between the medial and lateral malleoli  78  and  80 . Knowing the position of anterior point  82 , the software then mathematically projects another line  86  from the anterior point  82  that intersects line  84  substantially perpendicularly. The point  74  where lines  84  and  86  intersect is defined as the tibia articular point.  
         [0035]    As shown in FIG. 7, the tibia  38  is next rotated about the knee joint  30  through a specified range of motion. Preferably, the tibia  38  is initially placed at a substantially right angle to the femur  34  and then rotated in flexure (away from the femur), as shown in dashed line, through an angle between about 10° and about 90°, but preferably between about 10° and about 40°. Moving the tibia moves the tibia articular point  74  through a trajectory  88  relative to the knee joint  30 , the motion being constrained by the nature of the knee joint, which may be approximated as a hinge for angular motion between about 10° and about 40°, and the fact that the articular point is at the end of a rigid body (the tibia  38 ) movable about the hinge joint (the knee  30 ). Cameras  14  and  16  observe the motion of marker  20  on the tibia  38  relative to the marker  18  on the femur  34  and signal the positions of a multiplicity of points of the marker  20  to the software. The software knows the position of the tibia articular point  74  in the tibia reference frame  50  (i.e., relatively to the marker  20 ) for all motions of the tibia. Using this information, the software can calculate the corresponding motion of the articular point  74  relative to the points within the region  72  (fixed in the femur reference frame  48  as shown in FIG. 2) through the trajectory  88  defined by the constraints of a rigid body (the tibia  38 ) rotating about a hinge joint (the knee joint  30 ) relative to another rigid body (the femur  34 ). The software then uses the positional information of the tibia articular point  74  during its motion through trajectory  88  to choose a point from among the points in the region  72  (see FIG. 2) which represents the best estimate of the position of the knee articular point  90  based upon a particular set of criteria.  
         [0036]    Preferably, the point within region  72  having a position that is substantially invariant for a multiplicity of positions of the tibia articular point along its trajectory  88  about the knee joint  30  is selected as the knee center or knee articular point  90 . For example, the point in region  72  that has the smallest standard deviation of distance to the tibia articular point  74  for a multiplicity of positions of the tibia articular point  74  along the trajectory  88  may be selected as the knee articular point or knee center  90 . The accuracy with which the position of the knee articular point  90  is determined will be proportional to both the number of positions of the tibia articular point  74  that are measured along the trajectory  88  and the number of points in the region  72  each of these points is compared with. Generally, the more points used the greater the accuracy of the answer. Natural physical limits on the ability of the cameras to accurately measure small differences in position, as well as the accumulation of numerical errors within the mathematical algorithms used by the software will of course limit the accuracy of the answer.  
         [0037]    The method of determining the articular point of a joint according to the invention provides a fast procedure for acquiring accurate preoperative information respecting the position of a joint center which does not require surgery and, thus, enables the surgeon to avoid inflicting unnecessary trauma on a patient.