Localization device cross check

A method and apparatus for cross checking points palpated with a localization device. Points corresponding to the palpated points are obtained from a preoperative image of an object such as an X-Ray of a tibia bone. During a surgical procedure, the accuracy of palpated points are determined based on the palpated points and corresponding points obtained from the preoperative image. If a palpated point is inaccurate, an indicator is generated to indicate the inaccuracy of the palpated point.

FIELD OF THE INVENTION

The present invention relates to medical instruments and, more particularly, to a method for cross checking points palpated using a localization device.

BACKGROUND OF THE INVENTION

Localization devices have been developed to assist surgeons in performing surgical procedures. When utilized in a procedure, markers that can be tracked by a stereoscopic camera system are attached to bones. The stereoscopic camera system is connected to a data processing system that records the positions of the markers in space to establish a coordinate reference system relative to each bone bearing a marker. Additional markers may be attached to surgical tools and the localization device programmed with the location of the working portion of the surgical tool relative to the marker. The tool may then be used to palpate (touch) specific landmarks on the bones while the localization device records their location in order to ascertain the position of the landmarks in the coordinate reference systems of the bones. A monitor is used to display information developed from the coordinate reference system and the landmarks for use in guiding a surgeon during the procedure, such as navigating another surgical tool to a particular location in the coordinate reference system.

Typically, the surgical tool used to palpate points on a bone is a pointer. The marker can be mounted on the pointer in only one predetermined way. As noted, the localization device is programmed with data indicating the position of the working portion, e.g., the tip, of the pointer relative to the marker. By observing the marker, the localization device can determine the pointer tip position.

The accuracy of the pointer tip position as determined by the localization device is dependent on several factors. These factors include proper mounting of the marker on the pointer, accurate programming of the data indicating the position of the pointer tip relative to the marker, and trueness of the marker and the pointer. If any of these factors are amiss, the pointer tip position as determined by the localization device will be inaccurate, which, in turn, will result in the development of erroneous information that may be used to guide a surgeon during a surgical procedure. If the information used to guide the surgeon is erroneous, the surgical procedure may be affected adversely. Thus, there is a need to verify the accuracy of points palpated using the localization device to prevent the development of erroneous information based on the palpated points. The present invention fulfills this need among others.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus to verify the accuracy of points palpated using a localization device. The aforementioned need is satisfied by obtaining points from a preoperative image of an object to be palpated and, during a surgical procedure, determining the accuracy of palpated points based on the palpated points and corresponding points obtained from the preoperative image. If a palpated point is inaccurate, an indicator is generated to indicate the inaccuracy of the palpated point. Verifying the accuracy of palpated points increases the integrity of information developed from the palpated points for guiding a surgeon during a surgical procedure by allowing for corrective actions to be taken during the procedure if an inaccurate palpated point is identified.

One aspect of the present invention is a method for cross checking data points obtained with a localization device during a surgical procedure. The method includes palpating at least two points on an object during the surgical procedure using a surgical tool tracked by the localization device, determining the accuracy of one of the at least two palpated points based on the at least two palpated points and corresponding points obtained from a preoperative image of the object, and generating an indicator if one of the at least two palpated points is inaccurate.

Another aspect of the invention is an apparatus capable of cross checking palpated points acquired with a localization device. The apparatus includes sensors for sensing a surgical tool having a marker, the surgical tool for palpating an object to acquire at least two palpated points; a memory having stored therein data points obtained from a preoperative image of the object, the data points obtained from the preoperative image corresponding to the at least two palpated points; and a computer coupled to the sensors and the memory, the computer configured to determine the accuracy of one of the at least two palpated points based on the at least two palpated points and corresponding points obtained from a preoperative image of the object, the computer generating an indicator if one of the at least two palpated points is inaccurate.

The steps of the method may be embodied in software in a computer readable medium or may form a system comprising means for performing the method steps.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1depicts a localization device100in which the method of the present invention may be employed. InFIG. 1, a patient102who is to undergo a surgical procedure on a leg104, e.g., a Total Knee Arthroplasty (TKA), is illustrated schematically lying on an operating table106. The localization device100includes a computer108loaded with software for surgical navigation, a memory110, a processor112, sensors (or cameras)114capable of detecting markers116, a foot pedal118, a keyboard120, and a monitor122with a display screen123for displaying surgical navigation information. The display screen123is available to a surgeon for guiding the surgeon during surgical procedures performed using the localization device100. The sensors114are positioned above and laterally from the patient102so that the patient's leg104is in the field of view of the sensors114. In general, the markers116are fixedly mounted on bones (e.g., a tibia105aand a femur105b) and surgical tools (e.g., a pointer124), so that the localization device100can track the exact location and orientation of the bones and surgical tools to which the markers116are mounted. A description of a suitable localization device100having these features is found in U.S. Pat. No. 6,385,475 to Cinquin et al., having a common inventor and commonly assigned to the same entity as the present application, incorporated fully herein by reference.

FIG. 2Adepicts a preoperative imaging device200for producing a preoperative image of an object, e.g., the tibia105awithin the leg104, andFIG. 2Bdepicts an illustrative preoperative image202of the tibia105aproduced by such a preoperative imaging device200. The preoperative image202is used to obtain points (i.e., preoperative image points) and features based on the preoperative image points, such as the distance between two points, that can be used to verify the accuracy of palpated points. As will be described in detail below with reference toFIG. 4andFIG. 5, the accuracy of a point palpated with the localization device100(FIG. 1) can be determined based on one or more other palpated points and corresponding preoperative image points. The preoperative imaging device200may include a conventional X-Ray, CT-scan, or MRI device, or essentially any device capable of acquiring a preoperative image of an object, e.g., a bone within an appendage. The illustrated preoperative image202is a two dimensional image (e.g., a conventional X-Ray image obtained with a conventional X-Ray device or a two dimensional print out of an image obtained with an MRI or CT-scan device). In an alternative embodiment, the preoperative image may be a three dimensional image (e.g., an electronic image obtained with an MRI or CT-scan device). Other imaging devices used for producing preoperative images will be readily apparent to those skilled in the art.

In the illustrated preoperative image202, the points obtained are the medial malleolus (M), the lateral malleolus (L), the middle of the intercondylar eminence (K), and the middle of the tibia (T) on the ankle extremity. Preferably, the features associated with the preoperative image202include measurements such as the distance between two points or the angle between two lines defined by three or more points. For example, the features associated with the preoperative image points may include by way of non-limiting example, the distance between M and L, the distance between K and T, and an angle α between a first line defined by L and M and a second line defined by K and T.

If the preoperative image202is a two dimensional “hard” copy, the features can be determined by hand in a known manner, e.g., by marking points directly on the preoperative image202and measuring distances between the marked points with a ruler and measuring angles between lines defined by the marked points with a protractor. If the preoperative image202is a two dimensional or three dimensional electronic image of the object, the features can be determined using known computer interface technologies, e.g., positioning a cursor on a first point with a conventional computer mouse and selecting that point by depressing a button on the computer mouse and, then, positioning the cursor on a second point and selecting that point. The distance between the two selected points may then be determined automatically in a known manner through the use of known software. Various alternative embodiments for identifying points and features associated with those points will be readily apparent to those skilled in the art.

The obtained points and/or features are stored for later comparison to points palpated using the localization device100(FIG. 1) to verify the accuracy of the palpated points. Preferably, the obtained points and/or features are stored in the memory110of the localization device100. In one embodiment, the features are input directly into the localization device100, e.g. via the keyboard120, for storage in the memory110. For example, the localization device100may display a prompt on the monitor122requesting that the distance between M and L as obtained from the preoperative image202be entered into the localization device. An operator, e.g., a surgeon or an assistant, then enters the information into the localization device100using the keyboard120.

In an alternative embodiment, coordinates for the preoperative image points (i.e., data points) are entered directly into the localization device100. For example, the localization device may prompt the operator to enter the coordinates of the medial malleolus (M) and the lateral malleolus (L), which the operator enters via the keyboard120, and the localization device will determine the features, e.g., the distance between M and L as needed. In an alternative embodiment, it is contemplated that the data points and/or features are transferred to the localization device100directly from an electronic imaging device such as an MRI or CT-scan device. Various alternative embodiments for obtaining and storing data points and/or features associated with the points will be readily apparent to those skilled in the art and those embodiments are considered within the scope of the present invention.

FIG. 3depicts an object, e.g., the tibia105a, to be palpated with a surgical tool during a surgical procedure performed using the localization device100(FIG. 1). The object is palpated to obtain palpated points and features associated with those points for comparison to corresponding points and features associated with the preoperative image202(FIG. 2B). In the illustrated embodiment, the surgical tool is a pointer124. The pointer124has a marker116emounted thereon for tracking by the localization device100. The localization device100is preprogrammed with data that indicates the position of the working portion of the pointer124, i.e., the tip300, relative to the marker116emounted on the pointer124. During the surgical procedure, the localization device100observes the marker116eon the pointer124and determines the position of the pointer tip300relative to the tibia105a, which also has a marker116bmounted thereon. In a preferred embodiment, the object is palpated by touching the pointer tip300to a desired position on the object, e.g., the medial malleolus (M′), and recording the position of the pointer tip in a conventional manner that is well known to those of skill in the art.

The palpated points include by way of non-limiting example, a palpated medial malleolus (M′), a palpated lateral malleolus (L′), a palpated middle of intercondylar eminence (K′), and a palpated middle of tibia (T′) on the ankle extremity. Preferably, the palpated points correspond to known points on the preoperative image such as those described above with reference toFIG. 2B. In addition, the localization device100determines features associated with the palpated points for comparison to corresponding features associated with points obtained from the preoperative image202. Preferably, the features associated with the palpated points include measurements such as the distance between two points or the angle between two lines defined by three or more points. For example, the features associated with the preoperative image points may include by way of non-limiting example, the distance between M′ and L′, the distance between K′ and T′, and an angle α′ between a first line defined by L′ and M′ and a second line defined by K′ and T′. Preferably, the palpated features correspond to features obtained from the preoperative image202(FIG. 2B).

FIG. 4depicts a flow chart400for use in describing a method for cross checking points palpated using the localization device100(FIG. 1) in accordance with one embodiment of the present invention. At block410, the method begins. In a preferred embodiment, the method begins when invoked by the localization device100during one or more specific portions of a surgical procedure. For example, the localization device100may invoke the method during specific steps of a surgical procedure, e.g., whenever the surgical procedure requires that a point be palpated. In an alternative embodiment, the localization device100may invoke the method at the start of the surgical procedure and the method may be used to verify palpated points as needed throughout the entire surgical procedure.

At block420, an object is palpated using a surgical tool, e.g., pointer124(FIG. 1), that is tracked by the localization device100. In a preferred embodiment, the localization device100via the monitor122prompts the surgeon to palpate points on the object. For example, the localization device100may prompt the surgeon to palpate the following points on the tibia105a: the medial malleolus (M′), the lateral malleolus (L′), the middle of the intercondylar eminence (K′), and the middle of the tibia (T′) at the ankle extremity. Preferably, coordinates of the palpated points and/or features associated with the palpated points are stored in a memory110associated with the localization device100.

At blocks430and440, the accuracy of at least one palpated point is determined. In the embodiment depicted inFIG. 4, features associated with the points palpated at block420are compared to features associated with points obtained from a preoperative image202(FIG. 2B) of the object to determine if the compared features differ by more than a predefined amount (i.e., a threshold value). The features may include a distances between two points and/or angles defined by three or more points. The threshold value for a distance feature may be a predefined distance, e.g., 10 millimeters, or a predefined percentage, e.g., 2%. The threshold value for an angle may be a predefined angle, e.g., 2°, or a predefined percentage, e.g., 5%. It will be recognized by those skilled in the art that the distances may be scaled in a known manner prior to comparison to compensate for scale differences between palpated points obtained from the actual object and between corresponding points obtained from the preoperative image. In addition, known scaling techniques may be employed to compensate for apparent distance discrepancies attributed to representing a three dimensional object in two dimensions, if applicable. If the compared features exceed the predefined amount, processing proceeds to block450. Otherwise, processing ends at block460. Preferably, the features are compared by the localization device100.

If the feature is a distance between two palpated points, such as the distance between the palpated medial malleolus (M′) and the palpated lateral malleolus (L′), the distance can be compared in the following manner. First, the localization device computes the distance between the palpated medial malleolus (M′) and the palpated lateral malleolus (L′) in a known manner. Then, the localization device compares the computed distance to a measured distance between the medial malleolus (M) and the lateral malleolus (L) in a preoperative image (scaling the computed distance if necessary). The distance between the preoperative image points may be stored in the memory110of the localization device for direct comparison to the computed distance or the distance between the preoperative image points may be computed from the coordinates of preoperative image points stored in the memory110.

If the feature is an angle between three or more palpated points, such as the angle α′ between a first line defined by a first vector extending between the palpated medial malleolus (M′) and the palpated lateral malleolus (L′) and a second line defined by a second vector extending between the palpated middle of the tibia (T′) on the ankle extremity and the palpated middle of the intercondylar eminence (K′), the angle feature can be compared in the following manner. First, the localization device computes the angle α′ between a first vector extending between the palpated medial malleolus (M′) and the palpated lateral malleolus (L′) and a second vector extending between the palpated middle of the tibia (T′) on the ankle extremity and the palpated middle of intercondylar eminence (K′). Then, the localization device compares the computed angle α′ to a measured angle α between a first vector extending between the medial malleolus (M) and the lateral malleolus (L) in a preoperative image and a second vector extending between the middle of the tibia (T) on the ankle extremity and the middle of intercondylar eminence (K′) in the preoperative image. The angle α between the preoperative image points may be stored in the memory110for direct comparison to the computed angle α′. Alternatively, the angle α defined by the preoperative image points may be computed from the coordinates of preoperative image points stored in the memory110.

In a preferred embodiment, a feature is compared as the points necessary to define that feature are acquired through palpation. In this manner, it is possible to identify an inaccurate palpated point when it is palpated (or shortly thereafter), thus maximizing the amount of time available to remedy the inaccuracy. For example, if the feature is a distance between two points, the feature for the palpated points will be compared to the corresponding feature obtained from points on a preoperative image immediately after acquiring the second of the two palpated points. At this time, if the palpated feature and the preoperative image feature differ by more than the threshold value for that feature, processing will proceed to block450.

At block450, an indicator is generated that indicates that the threshold value of a feature compared in blocks430and440has been exceeded. The indicator may be a visual indicator displayed on the monitor122of the localization device100and/or an audible indicator emitted from an audio device (not shown) associated with the localization device100. In a preferred embodiment, the presence of an indicator alerts a surgeon during the palpation of points to the possibility of a palpated point that is inaccurate, which may result in the development of erroneous information by the localization device100. Alerting the surgeon during the palpation of the points allows the surgeon to take corrective action during the procedure. For example, the surgeon may palpate one or more of the points again, replace the surgical tool, and/or replace the marker mounted on the surgical tool.

Thus, the method can be used to cross check points palpated on an object using a localization device by comparing features associated with the palpated points to corresponding features associated with points obtained from a preoperative image of the object in the following manner. The localization device100ofFIG. 1invokes the method ofFIG. 4during a step of a surgical procedure in which points are to be palpated using the localization device100. After at least two points are palpated, a feature associated with these palpated points, e.g., the distance between two of the palpated points, can be determined for comparison to a corresponding feature associated with a preoperative image, e.g., the distance between two points on the preoperative image that correspond to the two palpated points. If the difference between a feature associated with the palpated points and a corresponding feature associated with the preoperative image points differs by more than a predetermined amount, an indicator is generated to indicate that one of the palpated points on which the feature is based may be erroneous. Corrective action may then be taken in response to the identification of an erroneous palpated point to increase the integrity of information generated by the localization device from the palpated points for use in guiding a surgeon during a surgical procedure.

FIG. 5depicts a flow chart500for use in describing an alternative method for cross checking points palpated using the localization device100(FIG. 1). At block510, the method begins as described above in reference to block410(FIG. 4). At block520, an object is palpated using a surgical tool, e.g., pointer124(FIG. 1), that is tracked by the localization device100to acquire at least three unaligned points. In a preferred embodiment, the localization device100via a monitor122prompts the surgeon to palpate points on the object. For example, the localization device100may prompt the surgeon to palpate the following points on the tibia105a: the medial malleolus (M′), the lateral malleolus (L′), and the middle of the tibia (T′) at the ankle extremity depicted inFIG. 3. Preferably, the coordinates of the palpated points are stored in the memory110associated with the localization device100.

At blocks530-560, the accuracy of at least one palpated point is determined (i.e., a check point). The check point is a palpated point other than the at least three unaligned points palpated at block520, e.g., the middle of the intercondylar eminence (K′) depicted inFIG. 3. In a preferred embodiment, as will be described in detail below, the accuracy of the check point is determined by developing a coordinate transformation based on the at least three unaligned points palpated at block520and corresponding points obtained from a preoperative image. The check point is then compared to a corresponding point obtained from the preoperative image, e.g., the middle of the intercondylar eminence (K) depicted inFIG. 2B. Prior to comparison, the corresponding point is transformed using the coordinate transformation. If the check point differs from the transformed corresponding point by more than a predefined amount, an indication is generated that indicates the palpated check point is erroneous.

At block530, the coordinate transformation is developed. The coordinate transformation is an algorithm based on the at least three unaligned points palpated at block520and corresponding points obtained from the preoperative image, e.g., the medial malleolus (M), the lateral malleolus (L), and the middle of the tibia (T) at the ankle extremity depicted inFIG. 2B. For illustrative purposes, the three unaligned palpated points are represented by the coordinate triplet (Ap,Bp,Cp) with each letter in the triplet corresponding to the coordinates of one of the three unaligned points, and the corresponding points obtained from the preoperative image (i.e., model) are represented by the coordinate triplet (Am,Bm,Cm) with similar capital letters indicating corresponding points.

In a preferred embodiment, the coordinate transformation is generated by, first, identifying points for first and second palpated points, e.g., Ap and Bp, that are within a neighborhood, i.e., within a predefined distance, of the palpated points, respectively. The number of points identified is dependent on the resolution of the system. For example, if the resolution of the system is 1,000 points per square inch and the neighborhood surrounding each palpated point is equivalent to one square inch, 100,000 points will be identified for each palpated point. For palpated points Ap and Bp, the points within the neighborhood of these points are represented by Ai and Bi, respectively, where i is 1 to n, n being the total number of points in a neighborhood. Then, all points in the neighborhood of the first palpated point, e.g., Ai, are identified where a point in Bi exists that is a distance d1from Ai, where the distance d1is the distance between corresponding points from the preoperative image, e.g., Am, Bm. These criteria are illustrated in the following equations 1-3:

d⁡(Ap,Ai)<δ(where,d⁢⁢is⁢⁢distance⁢⁢and⁢⁢δ⁢⁢is⁢⁢neighborhood⁢⁢size)(1)d⁡(Bp,Bi)<δ(2)d⁡(Ai,Bi)=d⁢⁢1⁢(Am,Bm)(3)
(Preferably, if no point Ai in the neighborhood of the first palpated point, e.g., Ap, satisfies equations 1-3, an indicator is generated to indicate that the coordinate transformation can not be developed based on the palpated points.)

Next, all points within the neighborhood of a third palpated point, e.g., Cp, are identified. For palpated point Cp, the points within the neighborhood are represented by Ci. Then, using all first and second points, e.g., (Ai,Bi), that satisfy the criteria of equations 1-3 above, all points in the neighborhood of the third palpated point, e.g., Ci, are identified where a point in Ci exists that is a distance d2from Ai, where d2is the distance between two corresponding points from the preoperative image, e.g., Am, Cm; and where an angle α between a first vector, e.g., (AiBi), and a second vector, e.g., (AiCi), is the same as an angle from the preoperative image between a third vector, e.g., (AmBm), and a fourth vector, e.g., (AmCm). These criteria are illustrated in the following equations 4-6:
d(Cp,Ci)<δ  (4)
d(Ai,Ci)=d2(Am,Cm)  (5)
α(AiBi,AiCi)=α(AmBm,AmCm)  (6)
(Preferably, if no set of points (Ai,Bi) exists that satisfies equations 1-6, an indicator is generated to indicate that the coordinate transformation can not be developed based on the palpated points.)

Then, for all sets of points, e.g., (Ai,Bi,Ci), that satisfy equations 1-6, a transformation triplet (Ao,Bo,Co) is identified wherein the sum of the distances between each point of the transformation triplet and a corresponding palpated point is minimum. This criteria is illustrated in the following equation 7:
MinimumΣid(Ai,Ap)+d(Bi,Bp)+d(Ci,Cp)  (7)
The transformation triplet (Ao,Bo,Co) satisfies equations 1-7 and is used to generate the coordinate transformation. The generation of the coordinate transformation from the triplet (Ao,Bo,Co) will be readily apparent to those of skill in the art.

At block540, a check point, Dp, is palpated. As noted above, the palpated check point is a point other than the three unaligned points palpated at block520, e.g., the middle of the intercondylar eminence (K′). The check point is palpated as described above with reference to block520. Preferably, the coordinates of the palpated point are stored in the memory110associated with the localization device100.

At blocks550and560, the check point palpated at block540is compared to a corresponding point, Dm, from the preoperative image, e.g., the middle of the intercondylar eminence (K). Prior to comparison, the corresponding point from the preoperative image is transformed using the coordinate transformation developed at block530. Using the coordinate transformation developed at block520, the coordinates of the corresponding point Dm are transformed in a known manner to obtain a transformed point Do. The check point Dp is then compared to the transformed point Do to see if the check point Dp is within the neighborhood of the transformed point Do. Specifically, the points are compared to determine if the check point differs from the transformed point Do by more than a predefined amount, e.g., 10 millimeters or 2%. The criteria for determining if the transformed point differs from the transformed corresponding point is illustrated in the following equation 8:
d(Dp,Do)<δ  (8)
If the check point differs from the transformed image point, processing proceeds to block570—where an indicator is generated as described above with reference to block450. Otherwise, processing ends at block580.

Thus, as with the method described above with reference toFIG. 4, the alternative method described with reference toFIG. 5can be used to cross check points palpated on an object using a localization device. The embodiment depicted inFIG. 5accomplishes cross checking of palpated points in the following manner. The localization device100ofFIG. 1invokes the method ofFIG. 5during a step of a surgical procedure in which points are to be palpated using the localization device100. After at least three points are palpated, a coordinate transformation can be developed based on the palpated points and corresponding points from a preoperative image. The coordinate transformation is then used to transform the coordinates of any point from a preoperative image for comparison to a corresponding palpated point to determine the accuracy of the palpated point. If the difference between the transformed point and the corresponding palpated point exceeds a predetermined amount, an indicator is generated to indicate that the palpated point may be erroneous. Corrective action may then be taken in response to the identification of an erroneous palpated point to increase the integrity of information generated by the localization device from the palpated points for use in guiding a surgeon during a surgical procedure.

The processing steps of the present invention can be implemented by computer programs in conjunction with hardware components. Software programming code which embodies the present invention may be stored on any of a variety of known media such as a diskette, hard drive, or CD-ROM, and may be distributed on such media. The techniques and methods for embodying software programming code on physical media and/or distributing software code are known in the art.