Rotational position determination

A method and a device is provided for determining a rotational position of a first feature of a first bone section relative to a second feature of a second bone section, the method comprises the steps of determining a longitudinal axis of a bone and determining a plane extending perpendicular to the longitudinal axis of the bone, determining a first projected vector representing a first direction defined by a feature of the first bone section, determining a second projected vector representing a second direction defined by a feature of the second bone section, and determining an angle between the first projected vector and the second projected vector.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/EP2012/004102 filed Sep. 27, 2012, published as WO 2014/048447 A1 on Apr. 3, 2014, which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to the field of computer assisted surgery. In particular, the invention relates to a method and a device for determining a rotational position of a first feature of a first bone section relative to a second feature of a second bone section of one bone. The method may be implemented as a computer program executable on a processing unit of the device.

BACKGROUND OF THE INVENTION

In a case in which a shaft of a long bone is fractured, a bone nail may be used to stabilize the parts of the bone during the healing of the fracture, wherein the bone nail may be inserted into a medullary channel of the bone in a longitudinal direction thereof. However, such a bone nail may allow a rotation of one part of the bone relative to another part of the bone, about the axis of the bone nail, at least until a locking screw is inserted through the bone nail in a lateral direction to fix the position of the rotatable part.

Accordingly, a physician should position the fractured parts as anatomically correct as possible. One approach may be to take into account features of the healthy counterpart of the fractures bone to provide information regarding how the parts of the fractured bone should be arranged. But this is difficult, since the bone of the counterpart is usually not visible.

SUMMARY OF THE INVENTION

It may be seen as an object of the invention to provide a method and a device for assisting a determination of an anatomical orientation of a first feature of a first bone section relative to a second feature of a second bone section. This is achieved by the subject-matter of each of the independent claims. Further embodiments are described in the respective dependent claims.

In general, a method is provided, namely a method for determining a rotational position of a first feature of a first bone section relative to a second feature of a second bone section, the method comprises the steps of determining a longitudinal axis of a bone and determining a plane extending perpendicular to the longitudinal axis of the bone, determining a first projected vector in the plane, for example by determining a first vector representing a first direction defined by a feature of the first bone section and projecting the first vector onto the plane, determining a second projected vector in the plane, the vector representing a second direction defined by a feature of the second bone section, and determining an angle between the first projected vector and the second projected vector.

Any bone, for example a bone at a hand or foot, in particular a long bone of the lower extremities, like the femur and the tibia, and of the upper extremities, like the humerus, may be subject to an embodiment of the method. That is, an orientation of a feature at for example the proximal section of a respective bone may be determined relative to a feature at the distal section of the respective bone. It is noted that such a determination of a feature may be performed based on gray scale image data which may be received for example from an x-ray imaging unit.

As used herein, the term “feature of a bone” refers to anything at a bone which may be suitable for determining a geometrical aspect, i.e. a point, a line, an arc, a center point, an axis, a cylinder surface, a ball surface, or the like, wherein such geometrical aspects are in particular suitable for a determination of a longitudinal axis and/or a vector. For example, a geometrical aspect of a femur may be the outer surface of the femur head, an axis defined by the neck between shaft and femur head, a longitudinal axis of the femur shaft, a most distal point on the bone surface, a line defined by the center points of the condyles, or a line defined by the most posterior points at the condyles. It will be understood that the other long bones provide other and/or comparable suitable geometrical aspects.

As used herein, the term “feature of a bone” may encompass any feature of an implant being already inserted into a bone or at least fixedly connected to a bone, said feature being suitable for determining a geometrical aspect as mentioned above.

As used herein, the term “projected vector” refers to a vector being projected on a plane, i.e. which results from a projection of a vector having any 3D orientation perpendicularly onto a plane. It is noted that a “projected vector” may also be achieved by a projection of only two points, for example end points, of a vector having any 3D orientation onto a plane, with the projected points defining the projected vector in the plane.

According to an embodiment, at least one of the longitudinal axis of the bone, the feature of the first bone section and the feature of the second bone section is determined on the basis of a 3D image of at least a section of the bone.

It will be understood that a 3D image, i.e. a volume image may be generated from a stack of 2D images oriented in one direction or based on a plurality of 2D projection images generated from different imaging angles.

According to another embodiment, at least one of the longitudinal axis of the bone, the feature of the first bone section and the feature of the second bone section is determined on the basis of a 2D image of at least a section of the bone, wherein the 2D image further includes a visualization of a reference body. The reference body may comprise a structure forming a characteristic 2D projection image for determining a 3D orientation of the reference body. In other words, based on one 2D projection image, a 3D orientation of the reference body can be determined, leading to a determination of a 3D orientation of a feature of a bone.

According to a further embodiment, the reference body is adapted to be fixedly connected to the bone.

As used herein, each of the terms “fixedly connected”, “fixedly coupled” and “fixedly attached” encompasses a direct or an indirect connection of an element to another element. For example, a reference body may be directly attached at an implant or may be indirectly coupled to an implant, with for example an aiming device between the reference body and the implant. On the other hand, a reference body which is integrated into an implant, i.e. which can be considered as fixedly connected to the implant, may be considered as being indirectly coupled to a bone, i.e. via the implant.

According to an embodiment, the reference body may be at least a part of an implant. In other words, an implant which is adapted to be fixed at or in a bone may comprise elements which can be identified in an image of the bone or at least a section of the bone so that a vector may be determined based on the identified elements. For example, the elements may define points so that two elements may define a line or an axis, or the elements may define a contour so that a center axis may be determined.

According to an embodiment, the reference body defines an axis, i.e. the implant comprises an axis and the axis of the implant represents one of the longitudinal axis of the bone, the feature of the first bone section and the feature of the second bone section. In other words, based on the reference body, an axis or a vector may be determined. In case an implant is already implanted into or at a bone, a feature of the implant may be determined instead of a feature of the bone so that the feature of the implant may represent a feature of the bone.

According to yet another embodiment, the reference body is adapted to be fixedly connected to the first bone section while being visible in an image showing the second bone section.

As a first example, the reference body may be integrated into a leading end of a bone nail so that when a trailing end of the bone nail is already fixed to a first section of the bone, the reference body may be located within the second section of the bone and may thus be visible in an image of that second section.

As a second example, the reference body may be integrated into an aiming device for supporting an insertion of a locking screw through a bore in a leading end of a bone nail. Therefore, the aiming device may be adapted to be coupled to a trailing end of the bone nail and may extend outside the body of a patient as far as the bone nail extends inside the bone so that at least a portion of the aiming device can be visible in an image of the second section of the bone including the leading end of the bone nail.

According to an embodiment, the method does not comprise any step of connecting a reference body at a bone, in so far as it constitutes a treatment of a human or animal body by surgery.

According to another embodiment, a device for determining a rotational position of a first feature of a first bone section relative to a second feature of a second bone section, comprises a processing unit which is adapted to perform the steps of the method. Therefore, the processing unit may be adapted for determining a longitudinal axis of a bone based on received image data, and determining a plane extending perpendicular to the longitudinal axis of the bone, determining a first projected vector representing a first direction defined by a feature of the first bone section, based on received image data, determining a second projected vector representing a second direction defined by a feature of the second bone section, based on received image data, and determining an angle between the first projected vector and the second projected vector.

It is noted, that the processing unit may be realized by only one processor performing all the steps of the method, or by a group or plurality of processors, for example a system processor for processing the image data, a separate processor specialized on a determination of geometrical aspects, and a further processor for controlling a monitor for visualizing the result.

According to an embodiment, the device further comprises an imaging unit for providing 2D projection image data of at least a section of the bone. The imaging unit may be capable of generating images from different directions. Accordingly, the imaging unit of the device may be adapted to also provide 3D image data of at least a section of the bone.

According to yet another embodiment, the processing unit of the device is further adapted for identifying a reference body in a projection image and for determining a 3D orientation of the reference body.

According to a further embodiment, the device further comprises a reference body including an arrangement of elements which allows a reconstruction of a 3D orientation of the reference body based on a 2D projection image.

The device may further comprise input means for manually identifying geometrical aspects of a bone in an image. Such input device may be for example a computer keyboard, a computer mouse or a touch screen.

According to a further embodiment, a computer software is provided including sets of instructions which when executed on an appropriate device, causes the device to perform the steps of the method as described above.

A corresponding computer program is preferably loaded into a work memory of a data processor. The data processor or processing unit is thus equipped to carry out the method. Further, the invention relates to a computer-readable medium such as a CD-ROM at which the computer program may be stored. However, the computer program may also be presented over a network like the World Wide Web and can be downloaded into the working memory of the data processor from such a network.

It has to be noted that embodiments are described with reference to different subject-matters. In particular, some embodiments are described with reference to method type claims (computer program) whereas other embodiments are described with reference to apparatus type claims (system). However, a person skilled in the art will gather from the above and the following description that unless other notified in addition to any combination of features belonging to one type of subject-matter also any combination between features relating to different subject-matters is considered to be disclosed with this application.

The aspects defined above and further aspects, features and advantages of the present invention can also be derived from the examples of the embodiments to be described hereinafter and are explained with reference to examples of embodiments also shown in the figures, but to which the invention is not limited.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The flow-chart inFIG. 1illustrates the principle of the steps performed in accordance with one embodiment of the disclosed method. It will be understood that the steps described, are major steps, wherein these major steps might be differentiated or divided into several sub-steps. Furthermore, there might be also sub-steps between these major steps.

In accordance with one method, in step S11, an image of the bone is received. In step S12, the longitudinal axis of the bone is determined. In step S13, a plane is determined extending perpendicular to the longitudinal axis.

In step S21, an image of the bone is received. In step S22, a first feature is identified representing a direction, so that at least two points can be determined defining a first vector in step S23.

Comparable with steps S21to S23, at least two points defining a second vector are determined in steps S31to S33.

As used herein, the term “receiving an image” basically refers to the fact that at least one image is necessary to perform the subsequent steps. That is, the term “receiving an image” may encompass also a loading of an image from a data memory into a processing unit. There is no need to generate a new image to receive an image. Accordingly, each of the images received in steps S11, S21or S31, may be any image suitable to identify a longitudinal axis of the bone, or a feature of the bone, i.e. a 3D image or a 2D image of at least the respective section of the bone, wherein the 2D image should additionally show a reference body.

On the other hand, it will be understood that only one image may be sufficient to determine the longitudinal axis as well as the first and second features of a bone, so that the same image may be received in steps S11, S21and S31.

In step S14, at least two points of the first vector determined in step S23and at least two points of the second vector determined in step S33are projected onto the plane determined in step S13, thus defining a first projected vector and a second projected vector.

It is note that the points are preferably projected onto the plane in a direction perpendicular to the plane.

Finally, an angle between the first projected vector and the second projected vector is determined in step S15.

It should be noted that these method steps may be performed to determine an angle between features at opposed ends of one healthy bone, but may also be performed to check whether an angle between features at opposed ends of one fractured bone are anatomically correctly arranged relative to each other. For example, the method may firstly be performed on a healthy counterpart of a fractured bone to determine an anatomically correct angle related to one patient, and may secondly be performed on the fractured bone to support the attempt to correctly arrange parts of the fractured bone.

FIG. 2shows an exemplary embodiment of a device. Substantially, necessary for performing the steps of the method, a processing unit100together with a monitor400is part of the device.

An exemplary imaging device200includes an X-ray source240, and an X-ray detector260, wherein these two devices are mounted on a C-arm220. It will be understood that the device may also comprise a non-invasive imaging modality like a computer tomography device, a magnetic resonance device, or an ultrasound device as imaging device instead of or additional to the shown C-arm based X-ray device.

Furthermore, the system inFIG. 2includes an input device300, by means of which for example a manual determination of a bone feature may be performed. Also shown is a connection (as dotted line) to a database600, located for example in a network.

Finally, there is shown a region of interest500. Within said region, for example a bone of a patient may be located which is subject to the method. Examples of (schematically) images from an imaging device200, can be seen inFIGS. 3 and 6.

FIGS. 3 to 6show schematically illustrations of exemplary images which may form a basis for performing the method. In this example, a rotational positioning of a first section12, i.e. a proximal section relative to a second section14, i.e. a distal section of a bone, i.e. a femur10is determined, with a fracture F in the shaft of the femur10.

FIG. 3is an image generated in an anterior to posterior direction, i.e. from above (assuming that the patient is lying on his/her back).FIG. 3shows a situation in which a bone nail20is already inserted into a medullary channel of the femur10in a longitudinal direction of the femur. Furthermore, the proximal section12of the femur10is rotationally fixed relative to the bone nail20by means of a locking screw30inserted through a bore in the bone nail and into the head of the femur. Fixedly attached to the bone nail20is an aiming device60with a sleeve70for an insertion of a further locking screw to be inserted in the distal section14of the femur.

Due to the fracture F, the distal section14of the femur can be rotated relative to the proximal section12of the femur10. This is indicated by the arrow R. The problem to be solved in this situation is to find the anatomically correct rotational position of the distal section14, to avoid a malposition of the knee joint and the lower leg relative to the hip joint.

With the aim to determine the current position of the distal section14relative to the proximal section12, a plane P perpendicular to the longitudinal axis of the bone (perpendicular to the drawing sheet), a first vector V1and a second vector V2may be firstly determined, wherein each of the two vectors represent a direction of a feature at the respective section of the femur.

It is noted that the vectors V1and V2are not shown as arrows, since it is irrelevant in which direction the vectors point, along the shown lines, respectively.

As a first approximation it can be assumed that the longitudinal axis X of the bone nail20is identical with the longitudinal axis of the shaft of the femur10. A plane P extending perpendicular to the longitudinal axis of the bone nail20will be identical to a plane extending perpendicular to the shaft of the femur, as long as the longitudinal axes are congruent or at least parallel to each other. In case there exists a deviation of the longitudinal axis of the bone nail from the longitudinal axis of the femur shaft, the angle between the longitudinal axes would be so small that a resulting error in the determination of an angle between two projected vectors on the plane can be neglected. Therefore, the longitudinal axis X of the bone nail20which is easier to determine, may be utilized to determine the plane P.

Having a locking screw30already inserted into the neck and head of the femur10, provides the possibility to utilize the axis of the locking screw as the first feature at the first section12of the femur10. Accordingly, a first vector V1may be determined based on the axis of the locking screw30.

Based on the projection image as shown inFIG. 3, the longitudinal axis X and thus the plane P as well as the first vector V1may be determined, wherein the known relation between for example the aiming device60, when fixedly connected to the bone nail, and the axis of the through bore in the bone nail through which the locking screw has been inserted, may be taken into account to determine the 3D orientation of the first vector V1.

Additionally, the second vector V2is shown inFIG. 3, although it would be difficult to determine the position and orientation of the second vector from an anterior-posterior image.

Indicated by dotted arrows, the method step of projecting at least two points of the first and second vectors onto plane P is visualized inFIG. 3.

InFIG. 4, a plan view onto plane P (identical with the drawing sheet) is shown together with features of the femur, i.e. the head of the femur at the first section12as well as the condyles at the second section14of the femur, as would be seen when viewed in a proximal to distal direction of the femur.FIG. 4further shows a locking screw30extending from the bone nail into the head of the femur and an aiming device60which is fixedly connected to the bone nail and extends outside of the bone, i.e. the leg. It is noted thatFIG. 4is a constructed image for explanation purposes. Therefore, the soft tissue surrounding the femur is omitted inFIG. 4.

As can be seen inFIG. 4, the most posterior points at the condyles are used to determine a tangent line. The tangent line at the condyles serves to determine a second vector V2pat the second section14of the femur. At the first section12of the femur, the axis of the locking screw30is a basis for determining the first vector V1p.

Further visualized inFIG. 4is the aspect that the second section14of the femur may be rotated relative to the first section12of the femur, about the longitudinal axis of the bone nail. Such a rotation is indicated by arrow R, causing for example a change of the orientation of the second vector V2pto an orientation of a vector V2′p.

Assuming that the orientation of the second vector V2prepresents an anatomically correct position of the condyles, and that vector V2′pis an orientation as currently determined, it would be possible to determine the angle of rotation about which the second section14of the femur has to be rotated to achieve an anatomically correct arrangement.

FIG. 5is similar toFIG. 4, showing a view onto plane P.FIG. 5differs fromFIG. 4in that an angle A is drawn in, between the first vector V1pand the second vector V2p. By means of the method such an angle A can be determined, indicating a rotational relation of a first feature at a first section and a second feature at a second section of a bone.

FIG. 6shows a schematically image which would be appropriate to determine a tangent lines at most posterior points of the condyles. The image is generated from a lateral direction but also inclined in a proximal to distal direction so that both condyles at the second section14of the femur can be identified in one image. Additionally shown in the image ofFIG. 6, is an aiming device60including an adjusting element62for adjusting a height for inserting a locking screw through a bore in the leading end of a bone nail20, and a sleeve70for facilitating the insertion of a locking screw.

Further shown inFIG. 6is a plurality of elements64forming an example of a reference body. Each of these elements64is a small radiopaque sphere so that each element is shown as a point in an X-ray image. Due to a specific 3D distribution of the elements64at or within the adjusting device62, a projection of the elements onto an imaging plane will lead to a unique 2D distribution so that an actual 3D orientation of the reference body can be determined based on the projected 2D distribution. Knowing the 3D orientation of the reference body allows the determination of an actual direction of for example a tangent line at the condyles.

By means of a single image, like that exemplarily shown inFIG. 6, it is possible to determine a second projected vector V2.

One possibility to determine the second projected vector V2is to determine a tangent at each of the condyles, with tangent T1defining a most posterior point at the first condyle and tangent T2defining a most posterior point at the second condyle. Both tangents T1and T2may be parallel to each other as well as parallel to the longitudinal axis X of the femur. It will be understood that the height of the tangents T1, T2differ from each other in an inclined lateral view, like that ofFIG. 6. Accordingly, a distance Dm between the tangent T1and the tangent T2may be measured in a direction perpendicular to both tangents.

Keeping in mind that a plane is determined which is perpendicular to the longitudinal shaft axis, it will be understood that each tangent T1, T2intersects this plane at one point. The resulting points are schematically illustrated inFIGS. 3 and 5as points TP1and TP2.

Further illustrated inFIG. 5are geometrical aspects which allow a determination of an angle A between the first projected vector V1pand the second projected vector V2p. For example from a viewing direction VD, a distance Dm can be measured between the tangents T1and T2, that is between the intersection points TP1and TP2of the tangents with the plane.

Based on the knowledge, for example from a data base of femurs, that dimensions at a femur have usually specific relations (with deviations), for example the width of the shaft or the diameter of the femur head relative to the distance between the condyles, the distance Dc between the condyles may be estimated from for example a measured width of the femur shaft. The angle B can therefore be calculated as arc sinus of Dm/Dc.

The angle C between the viewing direction VD and the first projected vector V1pcan be determined on the basis of the orientation information which is given by the reference body. Finally, angle A can be calculated as subtraction of angle C from angle B.

While embodiments has been illustrated and described in detail in the drawings and afore-going description, such illustrations and descriptions are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments.

The mere fact that certain measures are recited and mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. The computer program may be stored/distributed on a suitable medium such as an optical storage medium or a solid-state medium supplied together with or as a part of another hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SIGNS