Cutting method and system for far-end femur three-dimensional model

Abstract

The invention discloses a cutting method and system for a far-end femur three-dimensional model. According to the method, a far-end femur three-dimensional surface model obtained by a CT, MRI, digital or laser scanning system is analyzed, and an identified condylar surface contour key area is analyzed in combination with a parameterized ellipse fitting technology. The sagittal plane direction is adjusted through the analysis data, and the optimal cutting plane perpendicular to the supracondylar axis is found out. And finally, cutting the distal femur by using the corrected sagittal plane so as to obtain a three-dimensional model of the lateral condyle. Through the method disclosed by the invention, the lateral condyle of the thighbone can be accurately found out.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of quantitative analysis of geometric morphology. More particularly, the present application relates to a method and system for cutting a distal femoral three-dimensional model. BACKGROUND

[0002] In anterior cruciate ligament (ACL) reconstruction surgery, the lateral femoral condyle has important anatomical and surgical significance. First, the lateral condyle provides a key anatomical landmark for determining the correct position of the femoral tunnel, which is crucial for restoring the normal mechanical function of the knee joint. Second, the morphology and position of the lateral condyle provide an anatomical reference for surgeons to assess and ensure the correct placement of the graft, thereby improving the stability of the knee joint after surgery and preventing graft loosening or migration. In addition, precise use of the lateral condyle for tunnel drilling and graft fixation can reduce the risk of intraoperative damage to surrounding tissues and reduce the incidence of postoperative complications. Finally, based on the specific anatomical features of the patient's lateral condyle, surgeons can adjust the surgical technique to accommodate individual differences, thereby improving the success rate of surgery and the rehabilitation effect of patients. Therefore, the lateral condyle plays a key role in ACL reconstruction surgery, and its anatomical features have important implications for intraoperative decision-making and postoperative recovery.

[0003] Quantitative analysis of the geometric morphology of the distal femoral articular surface is of great significance in elucidating the intrinsic relationship between the anatomical structure and biomechanical function of the knee joint. Related studies have shown that significant progress has been made in the quantitative analysis of distal femoral morphology. Eckhoff et al. studied the distal femoral morphology through CT systems and clearly defined the differences between the transcondylar axis and the cylindrical axis, two types of knee flexion axes. Most et al. used a digitalizer to measure the femoral surface profile and confirmed the significant correlation between the kinematic characteristics of the knee joint and the selection of flexion axes. Based on this, Schmutz et al. successfully constructed a universal three-dimensional distal femoral model based on the profile information from CT data. Iwaki et al. used MRI technology to quantitatively measure the geometric morphology of the femoral and tibial articular surfaces. Further, Martelli and Pinskerova used a digital three-dimensional electric protractor to measure the geometric parameters of the femoral and tibial articular surfaces and compared them with MRI data. Subsequently, through systematic processing of digital data, they developed a software tool that integrates joint analysis and visualization functions. However, existing technologies have constructed three-dimensional distal femoral models and developed software tools with analysis and visualization functions, but have not cut and reconstructed the anatomical features of the key anatomical landmark - the lateral femoral condyle - in anterior cruciate ligament reconstruction surgery. SUMMARY

[0004] To at least address the technical issues described in the background section above, the present invention proposes a method and system for cutting a three-dimensional model of the distal femur. This method accurately obtains a three-dimensional model of the lateral femoral condyle, which can be used to determine the correct position of the femoral tunnel, which is crucial for restoring normal mechanical function of the knee joint. In view of this, the present invention provides solutions in the following aspects.

[0005] A first aspect of the present invention provides a method for cutting a three-dimensional model of a distal femur, comprising: reconstructing a three-dimensional surface model of the distal femur based on a CT image of the distal femur; establishing an initial sagittal plane of the distal femoral model based on the three-dimensional surface model of the distal femur, wherein the initial sagittal plane is a longitudinal section, perpendicular to the coronal plane and the transverse plane, and should be perpendicular to the cross-section of the distal femur, while separating the medial condyle and the lateral condyle along the center line of the intercondylar fossa; using a parameterized ellipse fitting technique, analyzing the joint area of ​​the condylar surface contour identified by the three-dimensional surface model of the distal femur, extracting and quantifying the geometric features describing the ellipse, including its size, shape, position and direction; constructing a set of parallel sections using the geometric features of the ellipse, cutting the medial condyle and the lateral condyle respectively, and calculating the eccentricity and focal dispersion of the elliptical sections; iteratively adjusting the sagittal plane direction to find the optimal cutting plane perpendicular to the supracondylar axis, wherein the optimal cutting plane is the cutting plane that minimizes eccentricity; and using the adjusted sagittal plane to cut the distal femoral model to obtain the lateral condyle.

[0006] In one embodiment, the method of reconstructing a three-dimensional surface model of the distal femur based on a CT image of the distal femur includes: preprocessing the CT image to improve image quality and contrast; separating the femur from surrounding tissue using image segmentation technology; reconstructing the two-dimensional slice data obtained by segmentation into a three-dimensional model; and smoothing and repairing the generated three-dimensional model to eliminate noise and irregularities.

[0007] In one embodiment, the initial sagittal plane of the distal femur model is established based on the three-dimensional surface model of the distal femur, including: identifying key anatomical landmarks of the distal femur, including the medial condyle, the lateral condyle, and the intercondylar fossa located therebetween, which is a significant recessed area; selecting two reference points located on the center line of the anterior and posterior edges of the intercondylar fossa; and defining the sagittal plane through the two reference points.

[0008] In one embodiment, the parametric ellipse fitting technique is used to analyze the joint area of ​​the condyle surface contour identified by the distal femoral three-dimensional surface model, including: extracting the sagittal contours of the medial condyle and the lateral condyle respectively, wherein the sagittal contour is the shape and structure of the femoral condyle observed on the sagittal plane, and the contour line of the femoral condyle is displayed by a longitudinal section; parameterizing the contour line and expressing it as (x(t), y(t)) with parameter t, where x(t) and y(t) are the coordinates of the curve on the plane; for the parameterized curve, the curvature κ is defined as: where (x'(t),y'(t)) is the first derivative of the curve, and (x"(t),y"(t)) is the second derivative of the curve; the front end and the rear end of the articular surface are determined by identifying the local maximum curvature points with the maximum curvature value in the respective regions of the profile; in the geometric analysis of the condylar sagittal profile curve of the femur, in order to accurately describe the morphological characteristics, an elliptical fitting method is used for modeling; a set of discrete data points (x i ,y i ) are obtained from the condylar sagittal profile curve, which are distributed in an elliptical manner, and the general quadratic equation of the ellipse is Ax 2 +Bxy+Cy 2 +Dx+Ey+F=0, a design matrix X is constructed, where each row corresponds to a data point, and the form is The linear system X·p=0 is solved by the least square method, where p=[A,B,C,D,E,F] T is the undetermined coefficient vector; in order to ensure that the fitting result is indeed an elliptical shape, the discriminant B 2 -4AC<0 is used as a constraint condition.

[0009] In one embodiment, the iterative adjustment of the sagittal plane direction to find the best cutting plane perpendicular to the condylar axis comprises: selecting an initial direction and setting a convergence threshold and a maximum number of iterations; cutting the medial condyle and the lateral condyle, and calculating the eccentricity and the focal point dispersion of the elliptical section; in each iteration, the direction is fine-tuned and the indexes are recalculated; if the new indexes are better than the old indexes, the direction is updated; if the change in the indexes is less than the threshold or the maximum number of iterations is reached, the iteration is stopped and the result is output.

[0010] The second aspect of the application provides a cutting system for a distal femoral three-dimensional model, which utilizes any of the cutting methods for a distal femoral three-dimensional model described above.

[0011] The application analyzes the three-dimensional surface model of the distal femur obtained by CT, MRI, digital or laser scanning system, and analyzes the key regions of the identified condylar profile by combining the parameterized elliptical fitting technology. The sagittal plane direction is adjusted by analyzing the above data, and the best cutting plane perpendicular to the condylar axis is found. Finally, the distal femur is cut using the corrected sagittal plane, so as to obtain the three-dimensional model of the lateral condyle. The three-dimensional model of the lateral condyle of the femur can be obtained by the cutting method for the three-dimensional model of the distal femur disclosed in the application, which can be used to determine the correct position of the femoral tunnel, and is crucial for restoring the normal mechanical function of the knee joint. BRIEF DESCRIPTION OF DRAWINGS

[0012] The above and other objects, features and advantages of the present application will become more apparent from the following detailed description read in conjunction with the accompanying drawings, in which:

[0013] Figure 1 is a cutting method showing a distal femur three-dimensional model according to an embodiment of the present application;

[0014] Figure 2 is a distal femur three-dimensional surface model reconstructed from CT images according to an embodiment of the present application;

[0015] Figure 3 is a distal femur model establishing an initial sagittal plane according to an embodiment of the present application;

[0016] Figure 4 is a calculation of a curvature vector on a medial condyle and lateral condyle sagittal profile upper point to identify and determine a feature of a joint region according to an embodiment of the present application;

[0017] Figure 5 is an elliptical fitting of a condyle sagittal profile curve according to an embodiment of the present application;

[0018] Figure 6 is a revised condylar sagittal plane according to an embodiment of the present application;

[0019] Figure 7 is a minimization of eccentricity and dispersion to obtain a revised condylar sagittal plane according to an embodiment of the present application;

[0020] Figure 8 is a cutting of a distal femur model using a revised sagittal plane according to an embodiment of the present application;

[0021] Figure 9 is a distal femur preserving a lateral condyle portion according to an embodiment of the present application. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of, rather than all, embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0023] It should be understood that the terms "first", "second", "third", and "fourth" and the like in the description and in the claims of the present application are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. The terms "comprising", "including", "containing", and / or "having" as used herein, are meant to be open-ended and non-limiting.

[0024] It should also be understood that the terms used in the specification and the claims of the present application are for the purpose of describing particular embodiments and are not intended to be limiting of the present application. As used in this specification and the claims, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0025] As used in this specification and claims, the terms "if" and "when" can each be interpreted to mean "upon determination" or "in response to a determination” or “in response to detecting,” depending on the context. Similarly, the phrase “if it is determined” or “if [a described condition or event] is detected” can each be interpreted to mean “upon determining” or “in response to determining” or “upon detecting [the described condition or event]” or “in response to detecting [the described condition or event],” depending on the context.

[0026] The specific embodiments of the present application will now be described in detail with reference to the following drawings.

[0027] In a first aspect of the present application, a cutting method for a distal femoral three-dimensional model is provided. Figure 1 is a schematic diagram showing a cutting method for a distal femoral three-dimensional model according to an embodiment of the present application. The cutting method for a distal femoral three-dimensional model of the present application comprises:

[0028] Step S100: reconstructing a distal femoral three-dimensional surface model;

[0029] Step S200: establishing an initial sagittal plane for the distal femoral model.

[0030] Step S300: using a parametric ellipse fitting technique to analyze the identified joint region of the condylar contour, extracting and quantifying the geometric features describing the ellipse, including its size, shape, position, and orientation.

[0031] Step S400: adjusting the direction of the sagittal plane to find the best cutting plane perpendicular to the condylar axis.

[0032] Step S500: Using the corrected sagittal cut of the distal femur model, obtain the lateral condyle.

[0033] In a preferred embodiment of the present application, the specific implementation of the above-mentioned step S100: reconstructing the three-dimensional surface model of the distal femur can refer to Figure 2 .

[0034] Figure 2 is to show the reconstruction of the three-dimensional surface model of the distal femur from CT images according to the embodiment of the present application. The method proposed in the present application is suitable for analyzing the three-dimensional surface model of the distal femur obtained by using CT, MRI, digitizing or laser scanning system. Without loss of generality, the reconstruction of the three-dimensional surface model of the distal femur is taken as an example in the present embodiment. The CT images are pre-processed to improve the image quality and contrast. The femur is separated from the surrounding tissues using image segmentation techniques. Common methods include threshold-based segmentation, region growing, active contour model (such as Snake or Level Set method) and segmentation based on deep learning (such as U-Net, etc.). The two-dimensional slice data obtained by segmentation is reconstructed into a three-dimensional model. Marching Cubes algorithm or other volume rendering techniques are usually used to generate a three-dimensional surface model. The generated three-dimensional model is smoothed and repaired to eliminate noise and irregularities. Techniques such as mesh simplification, smoothing filtering, etc. can be used.

[0035] In a preferred embodiment of the present application, the specific implementation of the above-mentioned step S200: establishing an initial sagittal plane of the distal femur model can refer to Figure 3 .

[0036] Figure 3 is to show the establishment of the initial sagittal plane of the distal femur model according to the embodiment of the present application. As shown in Figure 3 , first, identify the key anatomical landmarks of the distal femur, including the medial condyle, the lateral condyle and the intercondylar notch between them, which is a significant concave area. At the anterior and posterior edges of the intercondylar notch, select two reference points located on the center line. Through these two reference points, define a sagittal plane. The sagittal plane is a longitudinal section, perpendicular to the coronal plane and the transverse plane, and should be perpendicular to the cross section of the distal femur, while separating the medial condyle and the lateral condyle along the center line of the intercondylar notch.

[0037] In a preferred embodiment of the present application, the specific implementation of the above-mentioned step S300: using parameterized ellipse fitting technique to analyze the joint region of the identified condylar contour, extracting and quantifying the geometric features of the ellipse, including its size, shape, position and direction. Its specific implementation can refer to Figure 4 .

[0038] Figure 4is a diagram showing the curvature vector of the point on the medial and lateral condylar sagittal profile for identifying and determining the features of the articular region according to an embodiment of the present application. When studying the three-dimensional surface model of the distal femur, the profile features of the condylar articular region in the sagittal plane are particularly concerned, especially the region close to the most prominent central part. As shown in Figure 4 , the sagittal profiles of the medial and lateral condyles are extracted respectively. The sagittal plane of the femoral condyle referred to herein is an anatomical plane passing through the front and back directions of the femoral condyle, which divides the femoral condyle into medial and lateral parts. The sagittal profile is the shape and structure of the femoral condyle observed in this plane, and the profile curve of the femoral condyle is shown by the longitudinal section. Then the aforementioned profile curve is parameterized, assuming that the curve can be represented by a parameter t as (x(t), y(t)), where x(t) and y(t) are the coordinates of the curve in the plane. For the parameterized curve, the curvature κ is defined as: where (x'(t), y'(t)) is the first derivative of the curve, and (x"(t), y"(t)) is the second derivative of the curve. The front and back ends of the articular surface are determined by identifying the local maximum curvature points with the maximum curvature value in the respective regions of the profile.

[0039] Figure 5 is a diagram showing the elliptical fitting of the condylar sagittal profile curve according to an embodiment of the present application. When performing geometric analysis on the condylar sagittal profile curve of the femur, in order to accurately describe its morphological features, the method of elliptical fitting is used to model it. Specifically, a set of discrete data points (x i ,y i ) are obtained from the condylar sagittal profile curve, which exhibit an elliptical distribution. The general quadratic equation of an ellipse is:

[0040] Ax 2 +Bxy+Cy 2 +Dx+Ey+F=0

[0041] A design matrix X is constructed, where each row corresponds to a data point, in the form of The linear system X·p=0 is solved by the least squares method, where p=[A,B,C,D,E,F] T is the vector of undetermined coefficients. To ensure that the fitting result is indeed an elliptical shape, the constraint condition that the discriminant B 2 -4AC<0 is imposed on the coefficients; in the solving process, singular value decomposition is used to enhance the stability and accuracy of the calculation. As shown in Figure 5 , the elliptical fitting of the sagittal profile curve is performed on the medial condyle as an example.

[0042] In a preferred embodiment of the present application, the aforementioned step S400: adjusting the sagittal plane direction to find the best cutting plane perpendicular to the condylar axis, can be implemented by referring to Figure 6 .

[0043] Figure 6 : is a diagram showing the modified condylar sagittal plane according to an embodiment of the present invention. Figure 6 As shown in the figure, a set of parallel sections cut through the medial and lateral condyles, respectively. When the cutting planes are not perpendicular to the supracondylar axis, the elliptical cross-sections appear eccentric. When these ellipses are superimposed or projected onto a common plane parallel to the cross-sections, their centroids or foci appear dispersed. As the cutting planes become increasingly perpendicular, the dispersion of the foci decreases, and the eccentricity of the cross-section decreases; when the planes are perfectly perpendicular, the foci completely coincide. The supracondylar axis is defined by a straight line between the medial and lateral supracondylar points. This axis is roughly parallel to the articular surface of the distal femur and is considered perpendicular to the mechanical axis of the femur.

[0044] Figure 7 The figure shows the minimization of eccentricity and dispersion according to an embodiment of the present invention to obtain a corrected condylar sagittal plane. By iteratively adjusting the plane direction, the optimal cutting plane perpendicular to the supracondylar axis is found. First, the initial direction is selected and the convergence threshold and the maximum number of iterations are set. The medial and lateral condyles are cut, and the eccentricity and focal dispersion of the elliptical cross section are calculated. In each iteration, the direction is fine-tuned and these indicators are recalculated. If the new indicator is better than the old indicator, the direction is updated; if the indicator change is less than the threshold or the maximum number of iterations is reached, the iteration is stopped and the result is output.

[0045] The goal of the method of the present invention is to obtain the best cutting plane by adjusting the initial sagittal plane direction. The specific implementation process is: first, an initial sagittal plane based on the perpendicular direction of the supracondylar axis is selected, and a series of fixed parallel sections are set in this direction, and the convergence threshold and the maximum number of iterations are set. In each iteration, the elliptical sections of the medial condyle and the lateral condyle are obtained for all parallel sections in the current direction, the eccentricity of each section is calculated, and the average value of the eccentricity of all sections is taken as the evaluation index. Subsequently, a better direction is found by fine-tuning the initial sagittal plane direction (keeping the relative position between the parallel sections unchanged), and the direction is updated if the evaluation index under the new direction is better. When the eccentricity change is less than the set threshold or the maximum number of iterations is reached, the algorithm stops and outputs the final sagittal plane direction. This method can make the elliptical contours of the medial and lateral condyles on all parallel sections closest to the ideal circle, thereby better reflecting the true anatomical characteristics of the condyle.

[0046] By minimizing eccentricity and dispersion, e.g. Figure 7 As shown in the figure, the modified condylar sagittal plane is obtained. This iterative algorithm systematically adjusts the cutting plane to perform a series of cutting operations to approximate the most prominent parts of the two femoral condyles, with the goal of finding a cutting plane that minimizes eccentricity. The final cutting plane is considered the modified sagittal plane.

[0047] In a preferred embodiment of the present invention, the above step S500: using the modified sagittal plane to cut the distal femoral model to obtain the lateral condyle. Figure 8 .

[0048] Figure 8 FIG. 1 is a diagram showing a distal femur model cut using a modified sagittal plane according to an embodiment of the present invention. Figure 8 As shown, the distal femoral model was cut using the modified sagittal plane, the medial condyle area was removed, and only the lateral condyle was retained.

[0049] Figure 9 FIG. 1 is a diagram showing a distal femur retaining the lateral condyle portion according to an embodiment of the present invention. Figure 9 Figure 1 shows the portion of the lateral femoral condyle retained after distal femoral resection. The lateral femoral condyle plays a crucial anatomical and surgical role in anterior cruciate ligament (ACL) reconstruction surgery. First, the lateral femoral condyle provides a key anatomical landmark for determining the correct placement of the femoral tunnel, which is crucial for restoring normal knee mechanical function. Second, the morphology and position of the lateral femoral condyle provide surgeons with an anatomical reference to help assess and ensure correct graft placement, thereby improving postoperative knee stability and preventing graft loosening or migration. Furthermore, precise utilization of the lateral femoral condyle for tunnel drilling and graft fixation can reduce the risk of intraoperative damage to surrounding tissues and lower the incidence of postoperative complications. Finally, based on the specific anatomic features of the patient's lateral femoral condyle, surgeons can adjust surgical techniques to accommodate individual differences, thereby improving surgical success and patient recovery. Therefore, the lateral femoral condyle plays a critical role in ACL reconstruction surgery, and its anatomical characteristics significantly influence both intraoperative decision-making and postoperative recovery.

[0050] The second aspect of the present invention discloses a system for cutting a three-dimensional model of a distal femur, utilizing the above-disclosed method for cutting a three-dimensional model of a distal femur.

[0051] Although this specification has shown and described a plurality of embodiments of the present invention, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Those skilled in the art will appreciate that many modifications, variations, and alternatives can be made without departing from the spirit and scope of the present invention. It should be understood that in practicing the present invention, various alternatives to the embodiments of the present invention described herein may be employed. The appended claims are intended to define the scope of protection of the present invention and therefore cover modular compositions, equivalents, or alternatives within the scope of these claims.

Claims

1. A method for cutting a three-dimensional model of a distal femur, characterized in that: include: Reconstructing a three-dimensional surface model of the distal femur based on the distal femur CT image; Establishing an initial sagittal plane of the distal femoral model based on the distal femoral three-dimensional surface model, wherein the initial sagittal plane is a longitudinal section, perpendicular to the coronal plane and the transverse plane, and perpendicular to the cross section of the distal femur, while separating the medial condyle and the lateral condyle along the centerline of the intercondylar fossa; Using a parametric ellipse fitting technique, the articular region of the condylar contour identified by the three-dimensional surface model of the distal femur is analyzed to extract and quantify the geometric features describing the ellipse, including its size, shape, position, and orientation; constructing a set of parallel cross sections using the geometric features of the ellipse, cutting the medial condyle and the lateral condyle respectively, and calculating the eccentricity and focal dispersion of the ellipse cross sections; Adjusting the sagittal plane direction to find the optimal cutting plane perpendicular to the supracondylar axis, wherein the optimal cutting plane is the cutting plane that minimizes eccentricity; The distal femoral model was cut using the adjusted sagittal plane to obtain the lateral condyle.

2. The method for cutting a three-dimensional model of a distal femur according to claim 1, characterized in that: The method of reconstructing a three-dimensional surface model of the distal femur according to the distal femur CT image comprises: Preprocess CT images to improve image quality and contrast; Image segmentation techniques were used to separate the femur from surrounding tissue; Reconstruct the segmented two-dimensional slice data into a three-dimensional model; The resulting 3D model is smoothed and repaired to remove noise and irregularities.

3. The method for cutting a three-dimensional model of a distal femur according to claim 1, characterized in that: The step of establishing an initial sagittal plane of the distal femur model based on the distal femur three-dimensional surface model comprises: Identify key anatomical landmarks of the distal femur, including the medial and lateral condyles and the intercondylar notch, a prominent depression located between them. At the anterior and posterior edges of the intercondylar notch, select two reference points located on its centerline; The sagittal plane is defined by the two reference points.

4. A method for cutting a three-dimensional model of a distal femur according to any one of claims 1 to 3, characterized in that: The method of using a parameterized ellipse fitting technique to analyze the joint region of the condyle surface contour identified by the three-dimensional surface model of the distal femur includes: Extracting the sagittal contours of the medial condyle and the lateral condyle respectively, wherein the sagittal contours are the shape and structure of the femoral condyle observed on the sagittal plane, and the contour lines of the femoral condyle are displayed through a longitudinal section; The contour line is parameterized and expressed as (x(t), y(t)) with parameter t, where x(t) and y(t) are the coordinates of the curve on the plane; for the parameterized curve, the curvature κ is defined as: where (x′(t), y′(t)) is the first derivative of the curve, and (x″(t), y″(t)) is the second derivative of the curve; the front and back ends of the articular surface are determined by identifying the local maximum curvature points with the largest curvature values ​​in their respective regions of the contour; In the geometric analysis of the sagittal contour curve of the femur, in order to accurately describe its morphological characteristics, the ellipse fitting method was used to model it. A set of discrete data points (x i ,y i ), these points are distributed elliptically, and the general quadratic equation of the ellipse is: Ax 2 +Bxy+Cy 2 +Dx+Ey+F=0 Construct the design matrix X, where each row corresponds to a data point, in the form Solve the linear system X·p=0 by the least squares method, where p=[A,B,C,D,E,F] T is the unknown coefficient vector; To ensure that the fitting result is indeed an ellipse, the discriminant B is applied to the coefficients. 2 -4AC<0 is a constraint condition.

5. The method for cutting a three-dimensional model of a distal femur according to claim 4, characterized in that: The adjustment of the sagittal plane direction and the search for the optimal cutting plane perpendicular to the supracondylar axis include: Select an initial orientation and set a convergence threshold and maximum number of iterations; cut the medial and lateral condyles, and calculate the eccentricity and focal dispersion of the elliptical section; in each iteration, fine-tune the orientation and recalculate these indicators; if the new indicator is better than the old one, update the orientation; if the indicator change is less than the threshold or the maximum number of iterations is reached, stop the iteration and output the results.

6. A cutting system for a three-dimensional model of a distal femur, characterized in that: The method is performed using the cutting method of the distal femur three-dimensional model as described in any one of claims 1 to 5.

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