Method for aligning the tibia relative to the femur in a high tibial osteotomy and device for performing the procedure

Abstract

Procedure for aligning the tibia relative to the femur in a high tibial osteotomy (HTO), characterized by the following procedural steps: - Determine the position of the mechanical tibial axis in space; - Providing an ultrasound image of an ultrasound probe on a display unit, wherein a characteristic point located at and preferably on the mechanical femoral axis is detected with the ultrasound plane of the ultrasound probe and displayed in the ultrasound image, wherein a navigated ultrasound probe is used as the ultrasound probe, to which a surgical marking device tracked by a navigation device is fixed; - Determining an intersection point of the mechanical tibial axis with the ultrasound plane and displaying a corresponding intersection point marker in the ultrasound image; - Tracking the change in position of the mechanical tibial axis in space depending on the osteotomy; - Determining the changed intersection point of the mechanical tibial axis with the ultrasound plane and displaying the changed intersection point marker in the ultrasound image; - Determine that the intersection marker and the position of the characteristic point in the ultrasound image are the same or essentially the same.

Description

[0001] The invention relates to a method for aligning the tibia relative to the femur during a high tibial osteotomy (HTO).

[0002] Furthermore, the invention relates to a device for carrying out the method, which comprises a navigation device with a detection unit for detecting electromagnetic radiation, with a data processing unit for processing detection signals of the detection unit and with a display unit coupled to the data processing unit.

[0003] A high tibial osteotomy (HTO) is performed to correct leg axis deviations in which the mechanical femoral axis (imaginary connecting line between the center of the hip joint and the center of the knee joint in the frontal plane) forms an angle to the mechanical tibial axis (imaginary connecting line between the center of the knee joint and the center of the ankle joint in the frontal plane). The resulting mechanical leg axis is not straight. This manifests itself in a crooked position of the lower leg relative to the thigh, whereby in the frontal plane, bow legs (genu varum) are distinguished from knock knees (genu valgum). The load-bearing axis of the leg (also called the Mikulicz line), defined by an imaginary connecting line between the center of the hip joint and the center of the ankle joint, therefore runs at a distance from the knee joint. This manifests itself in one-sided wear of the knee joint medially (in bow legs) or laterally.lateral (in knock-knees).

[0004] To correct this misalignment, it is known to perform a procedure in the area of ​​the proximal tibia. In the so-called "open wedge" technique, an osteotomy incision is made in the tibia. The distal tibia segment is pivoted relative to the proximal tibia segment around a remaining bony bridge, thereby changing the alignment of the tibial axis. The tibial axis can be aligned with the mechanical femoral axis, thereby aligning the mechanical leg axis with the Mikulicz line. This results in medially and laterally balanced loading in the knee joint and correcting unilateral wear of the knee joint.

[0005] To perform high tibial osteotomy, it is also known that surgical markers can be placed on the femur and tibia. These markers can be tracked by a navigation system and can assist the surgeon in aligning the tibial axis relative to the femoral axis. Attaching the marker to the femur is inherently undesirable because the actual procedure is performed on the tibia, and fixating the marker to the femur leads to trauma, which can lead to reservations among surgeons and patients regarding navigation-assisted HTO.

[0006] Above, HTO was explained using the example of leg angulation in the frontal plane (bow leg or knock knee). However, this does not represent a limitation for the present invention. Leg malpositions in a sagittal plane, in which the tibia is angled forward or backward relative to the femur, are also conceivable. Of course, leg malpositions in a plane aligned at an angle to the frontal and sagittal planes are also conceivable. Furthermore, the invention can be used not only for an "open wedge" osteotomy, but also for other types of high tibial osteotomy.

[0007] US 2013 / 0006598 A1 describes a method for detecting the condition of a joint and preventing damage. Methods and devices for the non-invasive determination of prominent structures of the human or animal body are known from DE 10 2004 026 525 A1.

[0008] The object of the present invention is to provide a method of the type mentioned at the outset and a device for carrying out the method which is less invasive.

[0009] This object is achieved according to the invention by a method for aligning the tibia relative to the femur in a high tibial osteotomy, which is characterized by the following method steps: - Determine the position of the mechanical tibial axis in space; - Providing an ultrasound image of an ultrasound probe on a display unit, wherein a characteristic point located at and preferably on the mechanical femoral axis is detected with the ultrasound plane of the ultrasound probe and displayed in the ultrasound image, wherein a navigated ultrasound probe is used as the ultrasound probe, to which a surgical marking device tracked by a navigation device is fixed; - Determining an intersection point of the mechanical tibial axis with the ultrasound plane and displaying a corresponding intersection point marker in the ultrasound image; - Tracking the change in position of the mechanical tibial axis in space depending on the osteotomy; - Determining the changed intersection point of the mechanical tibial axis with the ultrasound plane and displaying the changed intersection point marker in the ultrasound image; - Determine that the intersection marker and the position of the characteristic point in the ultrasound image are the same or essentially the same.

[0010] The method according to the invention utilizes a navigated ultrasound probe to which a surgical marker is attached. A navigation device can track the marker and thus the ultrasound probe in space. The position of the ultrasound plane in space and of the characteristic point, which lies at and preferably on the mechanical femoral axis, can thereby be determined. This also allows the relative orientation of the mechanical tibial axis and the ultrasound plane, and in particular of the characteristic point in space, to be determined. This allows the method to be performed in a surgeon- and patient-friendly manner, as the surgeon can identify, via the intersection marker on the ultrasound image displayed on the display unit, where the mechanical tibial axis intersects the ultrasound plane.The surgeon can perform the osteotomy incision as explained above and move the distal tibial segment relative to the proximal tibial segment. This changes the position of the mechanical tibial axis in space, and a corresponding change in its intersection with the ultrasound plane can be determined and visualized via the intersection marker on the display unit. By moving the distal tibial segment, the surgeon can align the tibial axis so that it passes through the characteristic point and, accordingly, through the mechanical femoral axis. The surgeon can recognize this by the intersection marker in the ultrasound image coinciding or substantially coinciding with the image of the characteristic point.

[0011] In particular, the use of the navigated ultrasound probe eliminates the need for a surgical marker on the femur. The procedure can be performed less invasively than conventional navigation-assisted HTO, and patient trauma can be reduced.

[0012] Furthermore, the use of the navigated ultrasound probe also eliminates the need for fixation of the femur in space. This is made possible by the fact that, thanks to the navigated ultrasound probe and the tracking of the mechanical tibial axis in space, the navigation system can calculate its relative position at any time.

[0013] It is advantageous if the position of the mechanical tibial axis is determined based on position data of the ankle joint and a point at, and preferably on, the tibial axis, which is marked with a surgical marker tracked by a navigation device. The position of the mechanical tibial axis can be determined based on the first point, namely the ankle joint, and a second point, which second point is marked using the marker. If the position of the tibia changes in space, the mechanical tibial axis can be determined, in particular, based on the change in the position and / or orientation of the marker.

[0014] The sulcus of the patellar surface of the femur is preferably used as a characteristic point located on, and preferably on, the femoral axis. The patellar surface of the femur is the articular surface over which the patella can glide over the distal femur. The sulcus corresponds to a depression that, on the one hand, forms the center of the femoropatellar gliding surface. On the other hand, the sulcus lies, to a good approximation, on the mechanical femoral axis.

[0015] The ultrasound image is preferably acquired with the leg flexed, with the ultrasound probe positioned below the patella and a transverse cross-sectional plane of the leg selected as the ultrasound plane. The surgeon can identify the mechanical tibial axis using the intersection point marker, which is different from the position of the characteristic point in the ultrasound image. By moving the distal section of the tibia, the mechanical tibial axis can be changed as mentioned above and aligned so that it passes through the characteristic point. To acquire the ultrasound image, the ultrasound probe can be placed transversely on the leg below the patella with the leg flexed. The ultrasound probe can then be tilted proximally or distally if necessary until a prominent "hump-shaped" reflection becomes visible in the ultrasound image.This reflex originates from the sulcus of the patellar femoral surface and can be easily recognized by the surgeon.

[0016] Alternatively, the ultrasound image can be acquired with the leg extended, with the ultrasound probe positioned below the patella and a transverse cutting plane selected.

[0017] Preferably, the position of the characteristic point in the ultrasound image is marked, which makes it easier to identify and locate the characteristic point in the ultrasound image.

[0018] It is conceivable that a data processing unit that processes the ultrasound image signals from the ultrasound probe automatically detects the position of the characteristic point and displays the marker in the ultrasound image.

[0019] It is advantageous, especially with the leg extended, to determine the position of the hip and knee joints, and to determine the position of the mechanical femoral axis based on the position of the hip and knee joints, as well as the angle between the mechanical femoral axis and the mechanical tibial axis. This allows the surgeon to determine the angle formed by the mechanical femoral axis and the mechanical tibial axis. This not only allows the surgeon to plan the surgery, but it is also possible to determine the angle between the femoral and tibial axes intraoperatively or postoperatively to monitor the surgical outcome.

[0020] Preferably, the angle is shown on the display unit to facilitate the surgeon's workflow.

[0021] The positional data of the hip, knee, and / or ankle joints can be determined, for example, using ultrasound image signals from the ultrasound probe. This can be performed non-invasively and in a patient-friendly manner thanks to the navigated ultrasound probe.

[0022] Additionally or alternatively, the position data of the hip joint, knee joint, and / or ankle joint can be determined using a surgical probe to which a surgical marker is attached and tracked by a navigation device. The position data can also be determined non-invasively in a patient-friendly manner by palpation.

[0023] In addition or alternatively, it may be provided that the position data of the hip joint, the knee joint and / or the ankle joint are determined using X-ray images of the leg obtained with an X-ray machine.

[0024] As already mentioned, it is advantageous if the procedure is performed without surgical marking device on the femur.

[0025] Conveniently, the change in leg length resulting from the osteotomy is determined, preferably displayed on the display unit.

[0026] Assuming an intact, taut ligament system between the femur and the proximal portion of the tibia, the procedure also eliminates the need for a surgical marker that could be attached to the proximal portion of the tibia. During the osteotomy, in which the distal portion of the tibia is moved relative to its proximal portion, the intact, taut ligament system ensures that the position of the proximal portion relative to the femur remains unchanged. This makes it possible to determine the position of the mechanical tibial axis in space and its changes by simply tracking the surgical marker on the distal portion of the tibia.

[0027] As mentioned at the outset, the invention also relates to a device for carrying out the method. In a generic device, the object mentioned at the outset is achieved according to the invention in that the navigation device comprises a surgical marking device fixed to a patient's tibia for marking a point on the mechanical tibial axis, an ultrasound probe that emits ultrasound in an ultrasound plane and whose ultrasound image signals can be processed by the data processing unit and displayed as an ultrasound image on the display unit, and a surgical marking device attached to the ultrasound probe, wherein the marking devices are tracked in space by the navigation device, wherein a characteristic point detectable by the ultrasound field and located on and preferably on the mechanical femoral axis can be displayed in the ultrasound image.wherein the data processing unit is designed and programmed to determine the position of the mechanical tibial axis in space based on the position data of the marking device attached to the tibia and the position data of the ankle joint, to determine an intersection point of the mechanical tibial axis with the ultrasound plane, and to display a corresponding intersection point marker in the ultrasound image in an updateable manner, wherein it can be determined from the ultrasound image that the intersection point marker and the position of the characteristic point coincide or substantially coincide.

[0028] The device according to the invention is suitable for carrying out one of the aforementioned methods. The advantages achievable by applying the method can be achieved using the device. To avoid repetition, reference is therefore made to the above explanations.

[0029] It is advantageous if the data processing unit marks the characteristic point in the ultrasound image, which is located at and preferably on the femoral axis.

[0030] It may be provided that an operator specifies where the characteristic point is located in the ultrasound image via an input unit of the data processing unit.

[0031] Alternatively or additionally, it is conceivable that the data processing unit automatically determines the position of the characteristic point by analyzing the ultrasound image signals.

[0032] It is advantageous if the position data of the ankle joint and / or the hip joint and / or the knee joint can be determined by the data processing unit on the basis of ultrasound image signals from the ultrasound probe, alternatively or additionally on the basis of position data from a surgical sensing instrument of the device, on which a marking device tracked by the navigation device is fixed, additionally or alternatively on the basis of data from X-ray images of the patient's leg, which X-ray images are obtained using an X-ray device of the device.

[0033] Preferably, the data processing unit determines the position of the mechanical femoral axis based on the position data of the hip joint and the knee joint.

[0034] Preferably, the data processing unit determines the angle between the mechanical femoral axis and the mechanical tibial axis, and advantageously, the angle can be displayed by the data processing unit on the display unit.

[0035] It is advantageous if the device is free of any surgical marking device attached to the patient's femur.

[0036] Preferably, the data processing unit can determine the change in the length of the leg as a result of the osteotomy, wherein the change in the length of the leg can advantageously be displayed on the display unit.

[0037] It is advantageous if the device is free of a surgical marking device attached to a proximal portion of the tibia. In particular, the device can have only one surgical marking device attached to the tibia distal to the osteotomy incision to be performed.

[0038] The following description of preferred embodiments of the invention, taken in conjunction with the drawings, serves to explain the invention in more detail. They show: Fig. 1: a perspective view of a device according to the invention with which the method according to the invention can be carried out; Fig. 2: a schematic representation of a leg with malposition before performing a high tibial osteotomy and an ultrasound image on a display unit of the device from Fig. 1; Fig. 3: a schematic representation of the leg from Fig. 2 after performing the high tibial osteotomy and an ultrasound image on the display unit; Fig. 4: an enlarged view of detail A in Fig. 3; Fig. 5: schematically the flexion of a leg without soft tissue, whereby a navigated ultrasound probe of the device from Fig. 1 is shown and Fig. 6: A view of a knee without soft tissue and patella, looking at the patellar femoral surface from the dorsal side, with the femur flexed relative to the tibia.

[0039] Fig. 1 shows a perspective view of an advantageous embodiment of a device according to the invention, designated by reference numeral 10, with which the method according to the invention can be carried out. The device 10 comprises a surgical navigation device 12 with a detection unit 14, a data processing unit 16, and a display unit 18. The detection unit 14 comprises, in particular, a stereo camera 20 for detecting electromagnetic radiation (in particular infrared radiation), which can also be emitted by the detection unit 14. Detection signals from the detection unit 14 can be processed by the data processing unit 16. The display unit 18 is coupled to the data processing unit 16. The device 10 further comprises an input unit 22 for the data processing unit 16.

[0040] The device 10 comprises two surgical marking devices 24, 26, which are provided in a conventional manner with marker elements that reflect the radiation emitted by the detection unit 14. The reflected radiation is captured by the stereo camera 20. This allows the data processing unit 16 to determine the position and / or orientation of the marking devices 24, 26 in space based on the detection signals. This also makes it possible to detect any movement of the marking devices 24, 26 in space.

[0041] The marking device 24 is attached to an ultrasonic probe 28 of the device 10, so that when the ultrasonic probe 28 moves, the marking device 26 is also moved. The ultrasonic probe 28 emits the ultrasonic field in an ultrasonic plane 32.

[0042] The spatial position of the ultrasound plane 32 can be determined by the data processing unit 16 because the relative arrangement of the marking device 24 and the ultrasound plane 32 is known. Accordingly, the device 10 and the method that can be performed therewith utilize navigated ultrasound. The data processing unit 16 can also determine the position data of structures within the ultrasound field.

[0043] The surgeon attaches the marking device 26 to the tibia 34 in such a way that a point 36 is marked as close as possible to, or ideally on, the mechanical tibial axis 38. Based on the known geometry of the marking device 26, the data processing unit 16 can determine the spatial position data of the point 36.

[0044] In the present case, the device 10 and the method that can be performed with it are used for a high tibial osteotomy (HTO). An osteotomy incision is made on the tibia 34 between a proximal tibial section 40 and a distal tibial section 42. This will be discussed further below. In this case, the marking device 26 is fixed to the distal tibial section 42, preferably as proximally as possible to the osteotomy incision to be performed.

[0045] The device 10 and the method that can be performed with it are used here to correct a leg misalignment, for example, to correct a bow leg (genu varum) or a knock knee (genu valgum). However, the use of the device and the application of the method are not limited to leg misalignments within the frontal plane. Corrections of leg misalignments in a sagittal plane or in a plane oriented at an angle to the frontal and sagittal planes are also possible.

[0046] The Fig. 2 and Fig. 3 schematically illustrates the principle of HTO using the example of a preoperative bowleg. In a bowleg, the tibia 34 and the femur 44 form an angle such that the mechanical tibial axis 38 (imaginary line connecting the ankle joint 46 to the knee joint 48) is aligned at an obtuse angle to the mechanical femoral axis 50 (imaginary line connecting the hip joint 52 to the knee joint 48). This refers to an extended position of the leg 54. The mechanical leg axis 56, defined by the connection of the mechanical tibial axis 38 to the mechanical femoral axis 50, is accordingly not straight. In particular, the mechanical leg axis 56 deviates from the mechanical supporting axis 58 of the leg 54 (also called the Mikulicz line).

[0047] Therefore, loads on knee 60 are unilateral. In this example of a bowleg, greater wear occurs medially on knee 60 than laterally, leading to trauma for the patient.

[0048] To correct the leg misalignment, an osteotomy incision is made between the proximal tibial section 40 and the distal tibial section 42 during HTO ( Fig. 4). The distal tibia section 42 can be pivoted relative to the proximal tibia section 40 (a bone bridge 62 can remain for stabilization). Extension and compression of the tibia 34 is also possible. The tibia 34 is again fixed in the altered position of the tibia sections 40, 42 via a fixation device 64. In this case, the fixation device 64 comprises a plate 66, which is secured to the tibia sections 40, 42 with bone screws 68. It is also known to use an external fixator.

[0049] The HTO leads to a change in the position of the mechanical tibial axis 38, which can be aligned relative to the mechanical femoral axis 50 so that the mechanical leg axis 56 coincides with the mechanical bearing axis 58 (Mikulicz line). The malposition of the leg is thereby corrected, balancing the medial and lateral load in the knee 60 and preventing unilateral wear.

[0050] With the device 10, the navigated ultrasound probe 28 can be used to perform the HTO in a patient-friendly and minimally invasive manner. In particular, it is not necessary to fix a surgical marker on the femur 44, as is the case with typical navigation-assisted high tibial osteotomies. Fixation of the femur during the osteotomy is not required. With the device 10, in particular, the advantageous embodiment of the method according to the invention explained below can be performed.

[0051] First, particularly with the leg 54 in the extended position, the ankle joint 46, the knee joint 48, and the hip joint 52 can be scanned successively with the ultrasound probe 28 in order to determine their position data using the data processing unit 16. The angle between the mechanical tibial axis 38 and the mechanical femoral axis 50 can thus be calculated for planning the HTO. The angle can be displayed on the display unit 18.

[0052] The position of the mechanical tibial axis 38 in space can then be determined based on the position data of the ankle joint 46 and the point 36. When the tibial sections 40, 42 move relative to each other, the change in the position of the mechanical tibial axis 38 can also be determined solely based on the change in the orientation of the marking device 26.

[0053] Before the osteotomy cut, the ultrasound probe 28 is applied to the leg 54, for example, in a bent position of the leg 54 below the patella 70 ( Fig. 5). The ultrasound plane 32 is aligned transversely to the leg 54. In the ultrasound image 72 on the display unit 18, an easily recognizable, in practice "hump-shaped" reflex can be identified ( Fig. 2 and Fig. 3), whereby the ultrasound probe 28 is pivoted slightly distally or proximally if necessary. This reflex is recognizable to the surgeon by the lowest point 74 in the ultrasound image 72.

[0054] The deepest point 74 in the ultrasound image 72 corresponds to the image of a characteristic point 76 detected by the ultrasound field. The characteristic point 76 is considered to be located at and preferably on the mechanical femoral axis 50. In particular, the characteristic point 76 is the sulcus 77 of the patellar surface of the femur 78 ( Fig. 6). The patellar surface of the femur 78 is located distally on the femur 44, on the side facing the patella 70. The sulcus 77 represents the center of the femoropatellar gliding surface and lies, to a good approximation, on the mechanical femoral axis 50.

[0055] It is conceivable that the position of the sulcus 77 in the ultrasound image 72 is marked with a marker 80. This can be done by the surgeon via the input unit 22 or automatically by the data processing unit 16, which analyzes the ultrasound image signals with regard to the lowest point 74.

[0056] The mechanical tibial axis 38 intersects the ultrasound plane 32 at a distance from the femoral axis 50 (represented by the lowest point 74). The data processing unit 16 determines the position of the intersection point 82 with the ultrasound plane 32 and displays a corresponding intersection point marker 84 in the ultrasound image 72 ( Fig. 2).

[0057] The surgeon can then perform the osteotomy and move the tibial sections 40, 42 relative to one another. The change in position of the mechanical tibial axis 38 is tracked by the navigation device 12. Based on the ultrasound image 72, the surgeon can determine that the position of the intersection point 82 changes as a result by tracking the intersection point marker 84. The intersection point marker 84 is continuously updated by the data processing unit 16 in the ultrasound image 72. The surgeon aligns the tibial sections 40, 42 relative to one another such that the intersection point marker 84 coincides or substantially coincides with the lowest point 74 ( Fig. 3). Based on this information, the surgeon can see that the mechanical tibial axis 38 passes through the sulcus 77 and, accordingly, through the mechanical femoral axis 50.

[0058] The mechanical tibial axis 38 and the mechanical femoral axis 50 then coincide ( Fig. 3) and run along the mechanical support axis 58. The changed relative orientation of the tibial sections 40, 42 can then be fixed with the fixing device 64.

[0059] The data processing unit 16 can then calculate the change in leg length as a result of the HTO and display the leg length change, for example, on the display unit 18.

Claims

[1] Procedure for aligning the tibia relative to the femur in high tibial osteotomy (HTO), characterized by the following procedural steps: - Determining the position of the mechanical tibial axis in space; - Providing an ultrasound image from an ultrasound probe to a display unit, wherein the ultrasound plane of the ultrasound probe captures and displays a characteristic point located on and preferably on the mechanical axis of the femur, wherein a navigated ultrasound probe is used as the ultrasound probe, on which a surgical marking device tracked by a navigation device is attached; - Determining the intersection point of the mechanical tibial axis with the ultrasound plane and displaying a corresponding intersection point marker in the ultrasound image; - Tracking the change in position of the mechanical tibial axis in space depending on the osteotomy; - Determining the altered intersection point of the mechanical tibial axis with the ultrasound plane and displaying the altered intersection point marker in the ultrasound image; - Determine that the intersection marker and the location of the characteristic point in the ultrasound image coincide or substantially coincide. [2] Method according to claim 1, characterized by that the position of the mechanical tibial axis is determined using positional data of the ankle joint and a point on and preferably on the tibial axis, which is marked with a surgical marking device tracked by a navigation device. [3] Method according to claim 1 or 2, characterized by , that the sulcus of the patellar surface of the femur is used as a characteristic point. [4] Method according to any of the preceding claims, characterized by, that the ultrasound image is obtained with the leg flexed, with the ultrasound probe positioned below the patella and a transverse section plane of the leg being chosen as the ultrasound plane. [5] Method according to any of the preceding claims, characterized by , that the location of the characteristic point in the ultrasound image is marked. [6] Method according to any of the preceding claims, characterized by that the position data of the hip joint and the knee joint are determined, that the position of the mechanical femoral axis is determined based on the position data of the hip joint and the knee joint, and the angle between the mechanical femoral axis and the mechanical tibial axis is determined, preferably that the angle is displayed on the display unit. [7] Method according to any of the preceding claims, characterized by, that the positional data of the hip joint, knee joint and / or ankle joint are determined using ultrasound image signals from the ultrasound probe, or using a surgical probe to which a surgical marking device tracked by a navigation system is attached, or using X-ray images of the leg obtained with an X-ray machine. [8] Method according to any of the preceding claims, characterized by that the procedure is performed without a surgical marking device on the femur. [9] Method according to any of the preceding claims, characterized by , that the change in leg length as a result of the osteotomy is determined, preferably displayed on the display unit. [10] Device for carrying out the method according to one of the preceding claims, comprising a navigation device (12) with a detection unit (14) for detecting electromagnetic radiation, with a data processing unit (16) for processing detection signals from the detection unit (14) and with a display unit (18) coupled to the data processing unit (16), a surgical marking device (26) fixed to a tibia (34) of a patient for marking a point (36) of the mechanical tibial axis (38), an ultrasound probe (28) which emits ultrasound in an ultrasound plane (32) and whose ultrasound image signals can be processed by the data processing unit (16) and displayed as an ultrasound image (72) on the display unit (18), and a surgical marking device (24) attached to the ultrasound probe (28), wherein the marking devices (24, 26) are tracked in space by the navigation device (12),wherein a characteristic point (76) detectable by the ultrasound field and located on, and preferably on, the mechanical femoral axis (50) can be displayed in the ultrasound image (72), and wherein the data processing unit (16) is designed and programmed such that it determines the position of the mechanical tibial axis (38) in space based on the position data of the marking device (26) fixed to the tibia (34) and the position data of the ankle joint (46), that it determines an intersection point (82) of the mechanical tibial axis (38) with the ultrasound plane (32) and displays a corresponding intersection point marker (84) in the ultrasound image (72) in an updateable manner, wherein it can be determined from the ultrasound image (72) that the intersection point marker (84) and the position of the characteristic point (76) coincide or substantially coincide. [11] Device according to claim 10, characterized by, that the data processing unit (16) provides the characteristic point (76) located on and preferably on the femoral axis (50) in the ultrasound image with a marker (80). [12] Device according to claim 10 or 11, characterized by , that the positional data of the ankle joint (46) and / or the hip joint (52) and / or the knee joint (48) can be determined by the data processing unit (16) using ultrasound image signals from the ultrasound probe (28) or using positional data of a surgical probe of the device on which a marking device tracked by the navigation device is attached or using data from X-ray images of the patient's leg, which X-ray images are obtained with an X-ray device of the device. [13] Device according to any one of claims 10 to 12, characterized by, that the data processing unit (16) determines the position of the mechanical femoral axis (50) based on the position data of the hip joint (52) and the knee joint (48), and that the data processing unit (16) determines the angle between the mechanical femoral axis (50) and the mechanical tibial axis (38), preferably that the angle can be displayed by the data processing unit (16) on the display unit (18). [14] Device according to any one of claims 10 to 13, characterized by that the device (10) is free of a surgical marking device attached to the femur (44) of the patient. [15] Device according to any one of claims 10 to 14, characterized by , that the change in the length of the leg (54) as a result of the osteotomy can be determined by the data processing unit (16), preferably displayed on the display unit (18).

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