JP7768929B2 - A device for identifying lines connecting specific points on an object - Google Patents

Description

The present invention relates to a device for identifying a straight line connecting specific points on an object, and relates to a device for determining and displaying a more appropriate position for, for example, the mechanical axis of the tibia (tibial functional axis), which is necessary during knee replacement surgery.

In knee prosthesis surgery, when performing tibial osteotomy, it is necessary to determine in advance the so-called mechanical axis, which is displayed by connecting a first reference position located on the proximal side of the tibia (specifically, the point on the intercondylar eminence on the superior articular surface of the proximal portion of the tibia, including the lateral and medial condyles) with a second reference position located on the distal side of the tibia (specifically, the center point in the width direction of the distal end of the tibia, including the medial malleolus of the tibia and the lateral malleolus of the fibula). The mechanical axis serves as a reference line for determining the plane along which the tibia will be cut, for example, to perform high tibial osteotomy on the proximal portion of the tibia. Therefore, determining the mechanical axis for the tibia is important.

As a technology related to tibial osteotomy, for example, Patent Document 1 discloses an optical tracking system 42 that includes a detection device 44 and a trackable element (e.g., a navigation marker 46) that is placed on a tracked object and can be detected by the detection device 44 (Figure 6 of Patent Document 1), and the detection device 44 is said to include a visible light-based detector that detects a pattern on the trackable element, or a stereoscopic camera pair that is sensitive to infrared light and can be placed in the operating room where the osteotomy surgery is performed.

JP 2016-505326 A

Patent document 1 discloses that the optical tracking system 42 includes a detection device 44 and a navigation marker 46, which enables the detection device 44 to track the orientation of an object (e.g., the tibia) to detect its movement as it moves within the coordinate system of the detection device 44. However, it does not disclose at all that these components 44, 46 can work together to determine the mechanical axis of the tibia.

In recent years, there has been a desire to determine the mechanical axis of the tibia more accurately than in the past, and there is also a strong desire to reduce the burden on the surgeon when performing osteotomy of the proximal part of the tibia by allowing the surgeon to check both the mechanical axis and the part of the bone to be cut within the same field of view.

The present invention has been made in consideration of the above circumstances, and aims to provide an inexpensive and simple device for identifying a straight line connecting specific points on an object, which can determine the mechanical axis of the tibia more accurately than conventional devices, and which reduces the burden on the surgeon by allowing them to check the mechanical axis and the part of the bone to be cut within the same field of view.

In order to solve the above problem, the present invention provides an apparatus for identifying a line connecting specific points on an object, which includes a plurality of fixed markers positioned relative to one another, a movable marker, a movable imaging means, a display means, and a processing means, wherein the three-dimensional coordinate system of each of the fixed markers is the reference for the three-dimensional coordinates of the specific point on the object, each of the fixed markers is used together with the movable marker to determine the specific point on the object, the imaging means is used to simultaneously photograph one of the fixed markers and the movable marker, the processing means is used to identify the specific points on the object and the line connecting those points, and the display means is used to display at least the line connecting the specific points on the object.

This invention assumes the use of a navigation system mechanism, i.e., a simple and inexpensive mechanism including a movable imaging means, a display means, multiple fixed markers, a movable marker, and a processing means. For example, when determining the mechanical axis of the tibia, the imaging means takes images of marker units consisting of fixed markers and movable markers for the number of locations where the movable markers are placed, taking into account the positional relationship between fixed markers placed simultaneously at multiple locations on the tibia and one movable marker placed sequentially in a position relatively close to each fixed marker, and the display means displays the marker units for the number of locations where the movable markers are placed. Next, in this invention, the processing means determines the positions of the fixed markers and movable markers in the three-dimensional coordinate system of the imaging means from the image data displayed in this way, and converts the position of a specific point corresponding to the movable marker into relative coordinates with respect to the fixed marker. Furthermore, in the present invention, because the positional relationship between the fixed markers is known, the position of a specific point determined using one fixed marker and a movable marker can be converted into relative coordinates with respect to the other fixed marker. As a result, the position information of the specific point converted into relative coordinates with respect to the fixed marker is stored in the calculation processing means, specific points on the tibia are determined, and these specific points are further displayed on the display means, thereby making it possible to determine the mechanical axis of the tibia.

In addition, with this device for identifying a straight line connecting specific points on an object, the fixed markers and movable markers are close to each other, and the imaging means is movable, so the imaging means can properly capture images of each marker unit consisting of a fixed marker and a movable marker. Therefore, by taking into account the positional relationship between the fixed markers from multiple image data, it is possible to more properly determine the mechanical axis of, for example, the tibia than in the past.

In addition, with this device for identifying a straight line connecting specific points on an object of the present invention, the imaging means is particularly movable. Therefore, when using this device to determine the mechanical axis of the tibia, for example, by using at least the imaging means and the display means as the same device, the surgeon can confirm the mechanical axis and the part of the tibia to be resected within the same field of view, thereby reducing the burden on the surgeon.

FIG. 1 is a plan view showing an apparatus for identifying a line connecting specific points on an object, in particular an apparatus for determining the mechanical axis of a tibia, according to an embodiment of the present invention, where (a) shows a state in which a removable marker is positioned near a first marker fixed to the tibia, and (b) shows a state in which a removable marker is positioned near a second marker fixed to the tibia. FIG. 2 is a perspective view of a system including the device shown in FIGS. 1(a) and 1(b) and a device for cutting the tibia.

<Device for identifying a line connecting specific points on an object>
The following describes in detail an apparatus for determining the mechanical axis of a tibia as an apparatus for identifying a line connecting specific points of an object according to an embodiment of the present invention (hereinafter, sometimes referred to as the "present apparatus"). However, the present apparatus is not limited to an apparatus for determining the mechanical axis of the tibia, and includes any apparatus for identifying a line connecting specific points of an object. Note that the example shown below is an example in which the imaging means, display means, and arithmetic processing means are implemented by the same device (for example, a smartphone with a camera (hereinafter, sometimes simply referred to as the "smartphone") on which an application for performing predetermined arithmetic processing is installed).

Figure 1 is a plan view showing the present device, particularly a device for determining the mechanical axis of the tibia, where (a) shows a state in which a removable marker is positioned near a first marker fixed to the tibia, and (b) shows a state in which a removable marker is positioned near a second marker fixed to the tibia.

As shown in Figures 1(a) and 1(b), the present device 10 includes a first marker 12a, a second marker 12b, a third marker 14, and a smartphone 16.

The first marker 12a is fixed directly to a proximal portion of the tibia T, or indirectly via a marker fixing member (not shown). The first marker 12a may be fixed to the tibia T in any manner as long as the portion to be read can be read by the smartphone 16 (when photographed together with the third marker 14, as described below). The first marker 12a may be a matrix-type two-dimensional code such as a QR code (registered trademark), but is not limited to this and various other markers can be used. The first marker 12a fixed to the tibia T is in a fixed positional relationship with the second marker 12b (fixed to the tibia) described below.

Similarly, the second marker 12b is fixed to a distal portion of the tibia T, either directly or indirectly via a marker fixing member (not shown). The second marker 12b may be fixed to the tibia T in any manner as long as the portion to be read can be read by the smartphone 16 (when photographed together with the third marker 14, as described below). The second marker 12b may be, for example, a matrix-type two-dimensional code such as a QR code (registered trademark), but is not limited to this and various other markers can be used. The second marker 12b fixed to the tibia T is in a fixed positional relationship with the first marker 12a (fixed to the tibia), described below.

The three-dimensional coordinate systems of the first marker 12a and the second marker 12b serve as the reference for the three-dimensional coordinates of a specific point used when determining the mechanical axis MA. To determine the specific point used when determining the mechanical axis MA, three three-dimensional coordinate systems are required: the three-dimensional coordinate system of the camera of the smartphone 16a, the three-dimensional coordinate systems of the first marker 12a and the second marker 12b (which are fixed markers), and the three-dimensional coordinate system of the third marker 14 (which is a movable marker). The position of the determined specific point is ultimately saved in the smartphone 16 as a relative coordinate with respect to the three-dimensional coordinate systems of the first marker 12a and the second marker 12b (which are fixed markers).

An example of a three-dimensional coordinate system for the first marker 12a and the second marker 12b is a coordinate system determined by the X-axis, Y-axis, and Z-axis shown in Figures 1(a) and 1(b), and with its origin at a point near the center of either the first marker 12a or the second marker 12b. These three axes each indicate the direction displayed on the screen of the smartphone 16; specifically, the X-axis indicates the direction connecting the two markers 12a and 12b, the Z-axis indicates the direction passing through each marker 12a and 12b, and the Y-axis indicates a direction perpendicular to both the X-axis and the Z-axis.

The third marker 14 can be attached to any point on the tibia T, including a point on the tibia T close to the first marker 12a and a point on the tibia T close to the second marker 12b, and can be detached from any of these points. Here, "the third marker 14 being attached to the tibia T" means that the third marker 14 is in contact with some part of the tibia T, regardless of whether or not other mechanical elements are present to maintain contact between the third marker 14 and the tibia T. In other words, the third marker 14 may be attached by piercing the tibia T directly with a needle-shaped tip, or the third marker 14 may simply be brought into contact with the tibia T using other mechanical elements.

As shown in FIG. 1(a), the third marker 14 is attached directly to a proximal portion of the tibia T (specifically, to point P1 on the intercondylar eminence on the superior articular surface of the proximal portion of the tibia, including the lateral and medial condyles), or indirectly via a marker fixing member (not shown). By attaching the third marker 14 in this manner, the first marker 12a and the third marker 14 form a first marker unit U1 at the proximal portion of the tibia T.

Furthermore, as shown in FIG. 1(b), the third marker 14 is attached directly to a point on the distal side of the tibia T (specifically, the point P2 on the medial malleolus of the tibia T that is farthest from the fibula F), or indirectly via a marker fixing member (not shown). By attaching the third marker 14 in this manner, the second marker 12b and the third marker 14 form a second marker unit U2 at the distal side of the tibia T.

Similarly, the third marker 14 is attached directly to a point on the distal side of the tibia T (specifically, point P3 on the lateral malleolus of the fibula F, the point farthest from the tibia T), for example, as shown in FIG. 1(b), or indirectly via a marker fixing member (not shown) (the third marker 14 is not shown at point P3). By attaching the third marker 14 in this manner, the second marker 12b and the third marker 14 form a third marker unit U3 (not shown) at the distal side of the tibia T.

The third marker 14 may be attached to the tibia T in any manner as long as the portion to be read can be read by the smartphone 16 (when photographed together with the first marker 12a or the second marker 12b, as described below). The third marker 14 may also be a matrix-type two-dimensional code such as a QR code (registered trademark), but is not limited to this and various other markers may be used.

The smartphone 16 separately photographs the above-mentioned first marker unit U1 (comprised of a first marker 12a fixed to the proximal side of the tibia T and a third marker 14 attached nearby on the tibia T), the second marker unit U2 (comprised of a second marker 12b fixed to the distal side of the tibia T and a third marker 14 attached nearby on the tibia T), and the third marker unit U3 (comprised of a second marker 12b fixed to the distal side of the tibia T and a third marker 14 attached nearby on the fibula F), and displays the information about them.

The smartphone 16 determines a first point at the proximal end of the tibia T (point P1 shown in FIG. 1(a)) and a second point at the distal end of the tibia T (point P4 in FIG. 1(b)) based on the images captured of the first, second, and third marker units U1, U2, and U3. The smartphone 16 then determines the mechanical axis MA based on the above two points (P1, P4), taking into account that the positional relationship between the first marker 12a and the second marker 12b when fixed to the tibia T is fixed. The various calculation processes performed when determining the mechanical axis MA can be performed, for example, by a specific application installed in the smartphone 16 and having calculation processing capabilities.

As described above, the examples shown above are examples in which the imaging means, display means, and arithmetic processing means are integrated into the same device (e.g., a smartphone), but other examples of this type include notebook PCs as well. However, the present invention is not limited to these examples. In other words, the present invention also includes examples A) and B) in which these three means are shared among multiple devices as follows.
A) An example in which a camera is used as the imaging means, a personal computer (hereinafter referred to as "PC") as the calculation means, and a display as the display means. B) An example in which a camera is used as the imaging means, and a notebook PC that combines calculation means and display means. Note that when these three means are provided by two or more devices, there are no other particular restrictions as long as the devices are able to communicate with each other via wired or wireless connections.

(Mechanism for determining the mechanical axis of the tibia T using the device of the present invention)
The present device 10 (specifically, a device for determining the mechanical axis MA of the tibia) including the first marker 12a, the second marker 12b, the third marker 14, and the smartphone 16 as described above operates as follows.

The first marker 12a is fixed at a predetermined position on the proximal side of the tibia T, and the second marker 12b is fixed at a predetermined position on the proximal side of the tibia T. Therefore, the first marker 12a and the second marker 12b, when fixed to the tibia T, are in a fixed positional relationship with each other. In other words, the first marker 12a is located at a position with a fixed direction and distance from the second marker 12b, and the second marker 12b is located at a position with a fixed direction and distance from the first marker 12a. This makes it possible to clarify the positional relationship between points whose positional relationship with the first marker 12a is clear and points whose positional relationship with the second marker 12b is clear.

Under these conditions, for example, first, the third marker 14 is attached to a point on the tibia T close to the first marker 12a (point P1 on the intercondylar eminence on the superior articular surface of the proximal portion of the tibia, including the lateral and medial condyles, in FIG. 1(a)), and the smartphone 16 photographs the first marker unit U1 composed of the first marker 12a and the third marker 14. Next, based on information about the photographed first marker unit U1, the smartphone 16 determines the position of point P1 in space using the position of the first marker 12a, which was previously fixed to the tibia T, as a reference, and point P1 is stored and displayed on the smartphone 16. This determines the first point (i.e., point P1) at the proximal end of the tibia T.

Next, the third marker 14 is removed from point P1 and attached to a point on the tibia T close to the second marker 12b (in Figure 1(b) , point P2 on the medial malleolus of the tibia T, the point farthest from the fibula F), and the smartphone 16 photographs the second marker unit U2, which is composed of the second marker 12b and the third marker 14 attached to point P2.

Furthermore, the third marker 14 is removed from point P2 and attached to a point on the fibula F close to the second marker 12b (in Figure 1(b) , point P3 on the lateral malleolus of the fibula F, the point farthest from the tibia T), and the smartphone 16 photographs the third marker unit U3 consisting of the second marker 12b and the third marker 14 attached to point P3.

Based on the information about the second marker unit U2 and the information about the third marker unit U3 captured in this manner, the smartphone 16 determines the positions of points P2 and P3 in space based on the position of the second marker 12b previously fixed to the tibia T, and also determines the midpoint between points P2 and P3 (point P4 in Figure 1(b)), which is then stored and displayed on the smartphone 16. This determines the second point (i.e., point P4) at the distal end of the tibia T.

Finally, based on the positions of a first point (point P1) on the proximal side of the tibia T determined with reference to the first marker 12a, and a second point (point P4) on the distal side of the tibia T determined with reference to the second marker 12b, the smartphone 16 determines the mechanical axis MA of the tibia T, taking into account that the positional relationship between these markers 12a and 12b is fixed, and the mechanical axis MA is stored and displayed on the smartphone 16.

(Actions of the device 10 of the present application)
The above-described device 10 of the present application is a type of so-called navigation system, since it uses a mechanism (smartphone 16) having an imaging function, a calculation function, and a display function. In such a system, the imaging function is realized by the camera of the smartphone 16, the calculation function is realized by an application for performing calculations installed on the smartphone 16, and the display function is realized by the screen of the smartphone 16. Note that, when the smartphone 16 is used, predetermined operations can be performed efficiently by a single surgeon (Action 1).

Alternatively, instead of using a smartphone 16, such a navigation system can be configured with the imaging means, processing means, and display means as essentially separate entities, provided that they are capable of communicating with each other via wired or wireless connections. As a result, by allowing these three means to be operated by different people, division of labor can be achieved, allowing the surgeon to specialize in operating one or two of these three means, thereby reducing the operational burden on each surgeon (Effect 2).

Next, conventionally, when determining the mechanical axis of the tibia, only one fixed marker is placed on the tibia, and removable markers are attached sequentially to predetermined positions at the proximal end and distal end of the tibia, with these two types of markers forming one marker unit. Images of the marker unit are taken for the same number of removable marker placement positions, and the mechanical axis of the tibia is determined based on the multiple images obtained in this way.

However, when there is only one fixed marker, the fixed marker must be relatively large in order to determine the mechanical axis with high accuracy, which in turn requires the imaging means to be positioned some distance away from the fixed marker when taking images. This not only makes the device itself difficult to handle, but also poses the risk of not being able to properly determine the mechanical axis. Furthermore, because the fixed marker is relatively large, there is a risk that it will get in the way of surgery, which in turn places a significant psychological burden on the surgeon.

In contrast, the present device 10 uses two relatively small, fixed markers on the tibia T (one fixed to the proximal side of the tibia T and the other fixed to the distal side of the tibia T) to determine the mechanical axis of the tibia T. The present device 10 also determines the positions of the first marker 12a and the second marker 12b (which are fixed markers) and the third marker 14 (which is a movable marker) in the three-dimensional coordinate system of the smartphone 16 (which is the imaging means), and converts the position of a specific point corresponding to the third marker 14 into relative coordinates for each of the first marker 12a and the second marker 12b. Furthermore, in the present device 10, because the positional relationship between the first marker 12a and the second marker 12b is known, the positions of specific points determined using the first marker 12a and the third marker 14, or specific points determined using the second marker 12b and the third marker 14, can be converted into relative coordinates with respect to the second marker 12b or the first marker 12a, which were not used to determine these specific points. As a result, in the present device 10, the position information of the specific points converted into relative coordinates with respect to the first marker 12a and the second marker 12b is stored in the smartphone 16 (as a processing means), specific points on the tibia T are determined, and these specific points are further displayed on the smartphone 16, thereby making it possible to determine the mechanical axis MA of the tibia T.

This allows the imaging means to be installed relatively close to the fixed marker for imaging, which not only makes the device itself easier to use, but also allows the mechanical axis to be determined more appropriately than before.Furthermore, because the fixed marker is relatively small, there is no risk that the fixed marker will interfere with the surgery, which in turn reduces the psychological burden on the surgeon (Effect 3).

Furthermore, with the present device 10, which determines the mechanical axis of the tibia T using the above-described method, the smartphone 16 is particularly movable, allowing the surgeon to check the mechanical axis of the tibia T and the portion of the tibia to be resected within the same field of view, thereby reducing the burden on the surgeon (effect 4).

As described above, the device 10 of the present application allows the surgeon to perform operations efficiently using a navigation system (effect 1) or reduce the burden on the surgeon (effect 2), while simultaneously resolving issues particularly in line with recent needs, namely, determining the mechanical axis of the tibia more appropriately than before (effect 3) and reducing the burden on the surgeon by allowing the surgeon to check the mechanical axis and the part of the bone to be cut within the same field of view (effect 4), thereby enabling osteotomy of the tibia T to be performed more easily and appropriately.

(Other Possible Embodiments)
In the device 10 of the present invention shown above, the distance between the first marker 12a and the second marker 12b shown in FIGS. 1(a) and 1(b) can be set to 5 cm or more and 35 cm or less.

By setting the distance to 5 cm or more, the first marker 12a and the second marker 12b fixed to the tibia T will not overlap on the screen of the smartphone 16, and both markers can be clearly displayed on the smartphone 16. In contrast, given that the length of the tibia is generally approximately 40 cm or less, setting the distance to 35 cm or less will ensure that the first marker 12a and the second marker 12b are positioned reliably on the tibia.

Next, the distance between the first marker 12a (or second marker 12b) and the third marker 14 shown in Figures 1(a) and 1(b) can be 5 cm or more and 20 cm or less. When determining the mechanical axis in a conventional manner (using only one marker fixed to the tibia), if the markers previously used (e.g., 5 cm x 5 cm squares) are used, it was appropriate from the perspective of smartphone 16 operability to set the distance between these two types of markers 12a, 14 and the smartphone 16 to approximately 30 to 40 cm. Under these circumstances, if the two types of markers 12a, 14 were changed to relatively large ones in order to determine the mechanical axis with greater accuracy, the smartphone 16 would need to be moved relatively farther away from these two types of markers, which could potentially reduce the operability of the smartphone 16. Based on this rule of thumb, assuming that markers of the same size as those used up until now will be used, and taking into consideration the operability of the smartphone 16, it is essential that the distance between the first marker 12a (or second marker 12b) and the third marker 14 be between 5 cm and 20 cm.

Furthermore, in the present device 10 shown in FIG. 1(a) and elsewhere, the first marker 12a and the second marker 12b are square, but the shape of these markers 12a and 12b is not limited to square and can be any shape (e.g., circle, hexagon, octagon, star, etc.). Here, each of the shapes listed above is a shape that, when the markers 12a and 12b are rotated on their surfaces around their centers of gravity, will match their original shape multiple times during a 360° rotation. Such a shape allows for suitable imaging, particularly regardless of the angular relationship between the smartphone 16 and the markers 12a and 12b.

The reason why the above-described shape is preferable is as follows. Both the first marker 12a and the second marker 12b are fixed to the tibia T, and once fixed, their orientation cannot be changed. In order to properly photograph each of the markers 12a and 12b, which cannot be changed in orientation, using the smartphone 16, it is necessary to adjust the position of the smartphone 16 appropriately. However, if the longitudinal and lateral lengths of these markers 12a and 12b are significantly different, adjusting the position of the smartphone 16 may be difficult.

Furthermore, the subjects photographed at one time are the first (or second) marker 12a (12b) and the third marker 14 that make up each marker unit U1, U2, and U3. If the longitudinal and lateral lengths of these markers 12a and 12b differ significantly, the smartphone 16 may not be able to simultaneously and appropriately photograph the two types of markers 12a (12b) and 14 that make up each marker unit.

In contrast, if at least one of the first marker 12a and the second marker 12b is a square or a circle as listed above, the longitudinal and lateral lengths of these markers 12a and 12b are equal, which not only makes it easier to adjust the position of the smartphone 16, but also allows the smartphone 16 to simultaneously and appropriately photograph the two types of markers 12a (12b), 14 that make up each marker unit U1, U2, and U3.

<System including the present device and a device for cutting the tibia>
2 is a perspective view showing a system including the present device 10 and a device 20 for cutting the tibia T. The device 20 for cutting the tibia T includes a cutting guide 22 having a single slit defined therein, a blade 24 for cutting the tibia along the slit in the cutting guide 22, a blade holder 26 for holding the blade 24, and a handle piece 28 connected to the blade holder 26. The handle piece 28 includes a main body 28a and a button 28b attached to the main body 28a, and when the button 28b is pressed, the blade 24 vibrates in its short direction (i.e., left and right).

Once the mechanical axis MA of the tibia T has been determined using the present device 10 as described above, the cutting guide 22 is fixed to the tibia T so that the surgeon's planned bone resection angle is achieved when the tibia T is rotated in valgus, varus, and posterior tilt. Here, the surgeon's planned bone resection angle for the cutting guide 22 is determined by the obtained mechanical axis MA.

The position information of the mechanical axis MA is stored in the smartphone 16 (as a processing means) as the position of the first marker 12a and the second marker 12b (which are fixed markers) in a three-dimensional coordinate system. The installation angle when fixing the cutting guide 22 to the tibia T is the angle determined by converting the position information of the mechanical axis in the three-dimensional coordinate system of the first marker 12a and the second marker 12b into position information of the mechanical axis in the three-dimensional coordinate system of the camera of the smartphone 16 (as an imaging means), and then using this converted mechanical axis MA.

Next, the surgeon grasps the handle piece 28 and inserts the blade 24 into the slit. While pressing the button 28b on the handle piece 28, the surgeon presses the blade 24 against the tibia T to cut the desired portion of the tibia T. Once cutting of the tibia T is complete, the surgeon stops pressing the button 28b on the handle piece 28 and pulls the blade 24 out of the slit.

The cutting of the tibia T described above allows for the use of a more appropriate mechanical axis MA than conventional methods, which allows the cutting guide 22 to be fixed to the tibia T with greater precision, thereby achieving a higher level of cutting precision for the tibia T.

10 Apparatus of the present invention 12a First marker 12b Second marker 14 Third marker 16 Smartphone 20 Device for cutting tibia T 22 Cutting guide 24 Blade 26 Blade holding portion 28 Handle piece 28a Main body portion 28b Button F Fibula MA Mechanical axis P1 A predetermined point P2 on the superior articular surface of the proximal part of the tibia including the lateral condyle and the medial condyle A point P3 on the medial malleolus of the tibia T that is farthest from the fibula F A point P4 on the lateral malleolus of the fibula F that is farthest from the tibia T Midpoint between points P2 and P3 (second point)
T: tibia X: direction connecting the two markers 12a and 12b Y: direction perpendicular to both X and Z Z: direction passing through each of the markers 12a and 12b

Claims (6)

An apparatus for identifying a straight line connecting specific points of an object, the apparatus comprising: a plurality of fixed markers positioned relative to each other; a movable marker; a movable imaging means; a display means; and a calculation processing means,
a three-dimensional coordinate system of each of the fixed markers is a reference for three-dimensional coordinates of a specific point of the object, and each of the fixed markers is used together with the movable marker to determine the specific point of the object;
the imaging means is used to simultaneously image one of the fixed markers and the movable marker ;
the arithmetic processing means is used to identify a straight line connecting specific points of the object,
the display means is a device used to display at least a straight line connecting specific points of the object,
An apparatus for determining a mechanical axis of a tibia, the apparatus comprising: a first marker on a proximal side and a second marker on a distal side located between a proximal end and a distal end of the tibia and displaced in at least a longitudinal direction of the tibia; a third marker detachable from the tibia; a movable imaging means; a display means; and a processing means,
the first marker is used in conjunction with the third marker to determine a first point on the proximal end of the tibia;
the second marker is used in conjunction with the third marker to determine a second point on the distal end of the tibia;
the imaging means simultaneously images the first marker and the third marker, and simultaneously images the second marker and the third marker;
the calculation means determines a mechanical axis connecting the first point and the second point;
The display means displays at least the mechanical axis of the tibia . The apparatus of claim 1 , wherein the first point is determined and stored based on a positional relationship between the position of the first marker and the third marker. The apparatus of claim 1 , wherein the second point is determined and stored based on a positional relationship between the position of the second marker and the position of the third marker. The device of claim 1 , wherein the distance between the first marker and the second marker is greater than or equal to 5 cm and less than or equal to 35 cm. The device of claim 1 , wherein at least one of the distance between the first marker and the third marker and the distance between the second marker and the third marker is 5 cm or more and 20 cm or less. A system comprising a device according to any one of claims 1 to 5 and a device for cutting the tibia.