# Detection of Tibiofemoral Joint Injury in High-Impact Motion Based on Neural Network Reconstruction Algorithm

Authors: Hongbo Zheng
Journal: Journal of Healthcare Engineering
 (2021)

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## Abstract

In order to reduce the damage degree of joint bones, ligaments, and soft tissues caused by the high impact on the tibiofemoral joint during landing, a method for detecting the damage of tibiofemoral joint under high-impact action based on neural network reconstruction algorithm is proposed. Two dimensional X-ray images of knee joints from straightening to bending in 10 healthy volunteers were selected. CT scans were performed on the knee joint on the same side, and the 3D model from the acquired images was reconstructed. The kinematics data of the femur relative to the tibia with full degree of freedom were measured by registering the 3D model with 2D images. The results showed that in the extended position, the femur was rotated inward (5.5° ± 6.3°) relative to the tibia. The range of femoral external rotation is (18.7° ± 5.9°) from flexion to 90° in straight position. However, from 90° to 120°, a small amount of internal rotation occurred (1.4° ± 1.9°), so during the whole flexion process, the femur rotated (17.3° ± 6.9°), among which, from the straight position to 15°, the femur rotated (10.0° ± 5.6°). Damage in different areas is determined by the size of the interlayer displacement sample size method of sample space reduction. It is proved that the detection method of tibiofemoral joint injury in high-impact motion based on neural network reconstruction algorithm has high accuracy and consistency.


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## Body

## 1. Introduction

The tibiofemoral joint is a complex coupling joint with the longest lever arm [1]. Whether in daily life or sports activities, it is the part that mainly bears high-intensity impact from the ground [2]. For example, in the popular fitness activities such as brisk walking and jogging, for every contact between the foot and the ground, the supporting leg needs to bear the ground reaction force 1–3 times the weight. In the field of competitive sports, among the sports with running and jumping abilities [3], an athlete’s every second touchdown needs to bear the ground reaction force which is 3.5–7 times one's own weight. In the three-step layup and landing of basketball and the take-off pedal action of long jump, the lower limbs need to bear the ground reaction force which is 9 times the body weight [4]. In the field of sports science and medicine, a large number of studies have confirmed that the high load and high impact on the tibiofemoral joints during landing are important causes of injuries to bones, ligaments, and soft tissues. With the increase of landing height, the ground reaction force and joint load increased significantly [5]. The high load on the tibiofemoral joint will change the motion characteristics of the tibiofemoral joint, such as reducing the flexion angle of the tibiofemoral joint, increasing the valgus angle of the tibiofemoral joint, causing tear or fracture of the anterior cruciate ligament, and increasing the relative movement of the tibia and femur in different directions, resulting in meniscus tear. Then, knee osteoarthritis is induced, which brings huge medical burden to family and society [6].Therefore, a correct understanding of biomechanical characteristics such as kinematics and articular cartilage contact characteristics of the tibiofemoral joint during high-impact landing is of great significance for better understanding the biomechanical injury mechanism of the tibiofemoral joint and scientifically preventing and coping with various diseases of the tibiofemoral joint. The research on tibiofemoral joint movement has a long history and various technical means, including traditional sports biomechanics means, various medical scanning imaging means, and computer simulation means [7]. The commonly used biomechanical means of motion is motion capture technology, which mainly uses image capture to obtain kinematic indexes such as the rotation angle and angular velocity of the knee joint (approximately regarded as the tibiofemoral joint) during the real motion of the human body. The force and muscle moment acting on the joint are calculated in conjunction with the dynamometer and the reverse dynamic model. Delianides et al. studied the application of motion capture technology in the clinical treatment of tibial and knee implantation within one year. This represents a new NCE deployment on the new neural target in the only challenging area. It is easy to prolong the external application force, which makes the acute and chronic postoperative observation very important. A 27-year-old male with incomplete spinal cord injury (AIS C) at C3 to C4 levels received eight contact stimulation chips in the tibial and fibular nerves, near the knee joint. The four contacts of each cuff appear through temporary percutaneous lead externalization. After that, the percutaneous leads were removed and reconnected to the permanently implanted pulse generator with the best dorsiflexion (four kinds) and plantar flexion (four kinds) produced by the contacts. In order to ensure the safety of structure and personnel, reduce economic losses, and avoid catastrophic tragedy, one must study the damage identification technology. 13 months after implantation, through motor nerve conduction velocity (MNCV) study, clinical needle electrophotography, compound motor action potential (CMA), sensory nerve action potential (SNAP), and information collection induced by tonic myasthenia stimulation, neurological health and cuff performance and low limb circumference measurement were evaluated. Results showed that MNCV of the tibia and fibula remained stable, lower than 40 m/s, and the CMAP increased or stabilized after six months. The capture remained stable in all measurements [8].On the basis of this research, a new method for detecting tibiofemoral joint injury in high-impact motion based on neural network reconstruction algorithm is proposed. The 2D/3D registration in this new research method is based on the construction of personalized 3D bone model. The research work to be carried out is mainly focused on the first aspect. In practice, engineering structures are generally subjected to two kinds of damage, namely, sudden damage and accumulated damage. It does not contain joint soft tissue information, so it is impossible to analyze the contact characteristics of soft tissue, and there are hidden dangers of multiple exposure in the research. The image sequence obtained by MRI scan contains the information of bone, muscle, and soft tissue at the same time, which is a method that can replace CT scan. However, due to its imaging principle, the accuracy of the results obtained when the images obtained by this scanning method are used for 3D bone model reconstruction, and registration is always questioned. If it can be proved that the accuracy of the motion results measured by the 3D bone model established by MRI imaging is high, fitting errors can be avoided, data processing speed can be accelerated, and radiation dose can be reduced.


## 2. Methods

### 2.1. Research Object

10 healthy volunteers (7 males and 3 females; 22–35 years old) participated in this study. There was no pain, injury, or surgical history of the knee joint.


### 2.2. Biplane X-Ray Image Acquisition

Vertical fixed and mobile X-ray machines take biplane X-ray films of stress position of one knee joint of the subject. The focal distance of X-ray machine is 3 m. The tested knee joint flexed from the straight position to 15, 30, 60, 90, and 120, the joint remained in the neutral position, and a marker ball (steel with a diameter of 30 mm) was placed in the middle thigh, measuring each angle with a joint protractor. Each angle (including the straight position) is maintained for about 5 seconds, and the posterolateral and posterolateral images of the knee joint are taken by two X-ray machines at the same time.


### 2.3. Image Registration

Spiral CT scans were performed on the knee joint on the same side, and the range is 12 cm above and below the knee joint line. The reconstruction layer is 1 mm thick, and the resolution is 0.35 × 0.35 pixels. CT images were processed by Mimics and Geomagic software applications to create a 3D model of the tibiofemoral joint. Import the 3D model and its corresponding biplane X-ray film into UG software, making them coincide with their two 2D images at the same time. Due to the inclusion of the structure of the knee joint, it is very difficult to measure the force of the knee joint and its ligaments in vivo. Its complex shape and uneven composition make it difficult to analyze its biomechanical properties. Especially in the case of load bearing, the knee joint plays the role of connecting the preceding and the following, and its stress characteristics are more complex. Also, when 3D-2D registration is completed for each buckling angle, the positional relationship between femur and tibia in the model reproduces the positional relationship between femur and tibia during flexion.


### 2.4. Kinematics Measurement

The kinematics data of the femur relative to the tibia were measured by the coordinate system established on the 3D model of the tibiofemoral joint. The femoral coordinate system has two axes: the long axis of distal femoral shaft and the geometric center axis [9]. The radius of medial and lateral posterior condyle is determined from the side of the femoral 3D model, and two spheres are created according to these two radii, which are fitted to the medial and lateral posterior condyles, respectively, so that the sphere center represents the center of the posterior condyle. The connection between the two ball centers is defined as GCA, and the midpoint of the connection is defined as the femoral center. The tibial coordinate system is created as the Cartesian coordinate system [10]. The inner and outer axes are perpendicular to the long axis of the proximal tibial shaft and bisect the tibial plateau. The midpoint of the inner and outer axes is defined as the tibial center. The far and near axes are parallel to the long axis and pass through the center of the tibia. The front and rear axes are perpendicular to the first two axes. The displacement of the femur is defined as the movement of femoral center relative to the tibial coordinate system, and the displacement of the femoral condyle is similarly defined as the movement of posterior condyle center. Flexion and extension are defined as the angle between the long axis of the femur and the far and near axes of the tibia in sagittal plane, internal and external rotation is defined as the angle between GCA and internal and external axes of the tibia in horizontal plane of the tibia, and internal and external rotation is the angle between these two axes in coronal plane [11, 12]. It is of great significance to actively prevent and reduce the occurrence of knee joint injury as well as the treatment and rehabilitation training of knee joint injury.


### 2.5. Accuracy of Biplane X-Ray Technology

Three normal knee specimens were obtained. The femoral shaft and tibial shaft, respectively, pass through two iron bars to fix the knee joint on the wooden board. Place the wooden board vertically and take biplane X-ray images of knee joint at six angles according to the above method, then perform CT scans on the knee joint at the same six angles, and at the same angle, perform X-ray shooting and CT scanning in turn, and then turn to the next angle. Three-dimensional models of tibiofemoral joints (CT models) with six angles were reconstructed from CT images of knee joint. Use the model in straight position to fit the models of the other five angles in Geomagic (standard deviation < 1 mm); at the same time, the straight position model is used to register with the biplane 2D images of six angles according to the above method (registration model). Then, the kinematics data of the registration model and the CT model are measured using the same coordinate system (the former takes the latter as a reference) [13, 14]. The accuracy of the biplane X-ray technique was evaluated by comparing the kinematic differences between them (as shown in Table 1). The mean difference between them in front and back displacement is (0.5 ± 0.9) mm; the average difference in internal and external rotation is (0.9 ± 1.1)°. There are similar differences in other degrees of freedom. This accuracy is acceptable for clinical application. A more accurate reconstruction of the 3D physical model of human knee joint was achieved by testing the self-intersecting triangles and bad edges.Table 1
Kinematics comparison of 6 degrees of freedom between CT model and registration model.
KneeBack and forth (AP)Inside and outside (ML)Far and near (PD)Flex (FE)Internal and external rotation (IE)Turn inside and outside (AA)1−0.13 ± 0.930.52 ± 0.260.62 ± 0.52-0.50 ± 1.24−1.17 ± 0.630.70 ± 0.242−0.40 ± 0.820.47 ± 0.490.28 ± 0.67−1.18 ± 0.69−1.57 ± 0.820.37 ± 0.793−1.02 ± 0.77−0.40 ± 0.55−0.12 ± 0.640.18 ± 0.690.15 ± 1.00−0.63 ± 0.34Average−0.52 ± 0.880.26 ± 0.650.19 ± 0.61−0.50 ± 1.03−0.86 ± 1.090.14 ± 0.76


### 2.6. High-Impact Movements and Tibiofemoral Joint Injuries

Humans are upright walking creatures, and they complete various movements mainly with the foot in contact with the ground. According to Newton's second law, “the acceleration of an object is directly proportional to the combined external force (f) and inversely proportional to the mass (m) of the object, and the direction of acceleration (a) is the same as the direction of combined external force,” F = ma, the mass of human body is m, and it falls freely with the acceleration of gravity a (a = 9.8 m/s2). Ground reaction force will be generated when any part of the foot is in contact with the ground [15, 16]. Therefore, the impact force of human body in running and jumping mostly comes from the reaction force generated by the ground. The ground reaction force can reflect the strength and duration when the foot touches the ground. A large value indicates that the ground reaction force is large, that is, the impact force is large [17, 18]. If used for comparison among different individuals, the unit is body weight (BW), which is a multiple of the subject's body weight. The impact force is transmitted upward along the bone. The ground reaction force borne by human body comes from six directions: up and down, left and right, and front and back, and the forces in different directions have different influences on human body. When the human body jumps vertically, it often produces the ground reaction force in the up and down directions. When walking, the ground reaction force in front and back directions is often generated.A large number of studies take platform landing as the research object and assume that it is an action that can represent the human body's high-impact force. Researchers used the traditional three-dimensional mocap system to analyze the movement of the tibiofemoral joint (knee joint captured by traditional motion) under various landing modes. Its complex shape and uneven composition make it difficult to analyze its biomechanical properties, especially in the case of load bearing. It is found that when basketball players jump and shoot and land, the vertical reaction force on the ground reaches 3BW. When a professional gymnast rolls backward for two weeks and lands, the vertical ground reaction force reaches 8.8BW. The vertical ground reaction force of airborne troops landing from 40 cm platform is 7BW, which is obviously less than 10.31BW when landing from 80 cm platform. When basketball players and track and field jumping athletes land from a platform with a height of 60 cm, the vertical reaction force on the ground is 3200 N–3900 N. When normal people land from 30 cm platform to 60 cm platform (once every 5 cm), the vertical ground reaction force is getting higher and higher, ranging from 3.16BW to 4.49BW. Healthy sports enthusiasts landed from the 60 cm platform, and the vertical ground reaction force reached 4.38BW.The tibiofemoral joint is the longest joint in the human body, and it needs to bear the soft tissue and bone inside the joint. When it is subjected to direct or indirect impact force, it will exceed its own tolerance range, resulting in local microstructure fracture or even overall fracture. The actions include repeated flexion and extension, jumping and landing injuries, sudden stop and tearing, and falling under special violent circumstances. Common injury sites include bone injury, meniscus injury, and cartilage injury. These results indicate that PCL is the primary structure to resist tibial anterior displacement during knee flexion, and the stress is mainly concentrated at the proximal tibial insertion, which is consistent with the clinical occurrence of PCL fracture at the insertion of the femur.


## 2.1. Research Object

10 healthy volunteers (7 males and 3 females; 22–35 years old) participated in this study. There was no pain, injury, or surgical history of the knee joint.


## 2.2. Biplane X-Ray Image Acquisition

Vertical fixed and mobile X-ray machines take biplane X-ray films of stress position of one knee joint of the subject. The focal distance of X-ray machine is 3 m. The tested knee joint flexed from the straight position to 15, 30, 60, 90, and 120, the joint remained in the neutral position, and a marker ball (steel with a diameter of 30 mm) was placed in the middle thigh, measuring each angle with a joint protractor. Each angle (including the straight position) is maintained for about 5 seconds, and the posterolateral and posterolateral images of the knee joint are taken by two X-ray machines at the same time.


## 2.3. Image Registration

Spiral CT scans were performed on the knee joint on the same side, and the range is 12 cm above and below the knee joint line. The reconstruction layer is 1 mm thick, and the resolution is 0.35 × 0.35 pixels. CT images were processed by Mimics and Geomagic software applications to create a 3D model of the tibiofemoral joint. Import the 3D model and its corresponding biplane X-ray film into UG software, making them coincide with their two 2D images at the same time. Due to the inclusion of the structure of the knee joint, it is very difficult to measure the force of the knee joint and its ligaments in vivo. Its complex shape and uneven composition make it difficult to analyze its biomechanical properties. Especially in the case of load bearing, the knee joint plays the role of connecting the preceding and the following, and its stress characteristics are more complex. Also, when 3D-2D registration is completed for each buckling angle, the positional relationship between femur and tibia in the model reproduces the positional relationship between femur and tibia during flexion.


## 2.4. Kinematics Measurement

The kinematics data of the femur relative to the tibia were measured by the coordinate system established on the 3D model of the tibiofemoral joint. The femoral coordinate system has two axes: the long axis of distal femoral shaft and the geometric center axis [9]. The radius of medial and lateral posterior condyle is determined from the side of the femoral 3D model, and two spheres are created according to these two radii, which are fitted to the medial and lateral posterior condyles, respectively, so that the sphere center represents the center of the posterior condyle. The connection between the two ball centers is defined as GCA, and the midpoint of the connection is defined as the femoral center. The tibial coordinate system is created as the Cartesian coordinate system [10]. The inner and outer axes are perpendicular to the long axis of the proximal tibial shaft and bisect the tibial plateau. The midpoint of the inner and outer axes is defined as the tibial center. The far and near axes are parallel to the long axis and pass through the center of the tibia. The front and rear axes are perpendicular to the first two axes. The displacement of the femur is defined as the movement of femoral center relative to the tibial coordinate system, and the displacement of the femoral condyle is similarly defined as the movement of posterior condyle center. Flexion and extension are defined as the angle between the long axis of the femur and the far and near axes of the tibia in sagittal plane, internal and external rotation is defined as the angle between GCA and internal and external axes of the tibia in horizontal plane of the tibia, and internal and external rotation is the angle between these two axes in coronal plane [11, 12]. It is of great significance to actively prevent and reduce the occurrence of knee joint injury as well as the treatment and rehabilitation training of knee joint injury.


## 2.5. Accuracy of Biplane X-Ray Technology

Three normal knee specimens were obtained. The femoral shaft and tibial shaft, respectively, pass through two iron bars to fix the knee joint on the wooden board. Place the wooden board vertically and take biplane X-ray images of knee joint at six angles according to the above method, then perform CT scans on the knee joint at the same six angles, and at the same angle, perform X-ray shooting and CT scanning in turn, and then turn to the next angle. Three-dimensional models of tibiofemoral joints (CT models) with six angles were reconstructed from CT images of knee joint. Use the model in straight position to fit the models of the other five angles in Geomagic (standard deviation < 1 mm); at the same time, the straight position model is used to register with the biplane 2D images of six angles according to the above method (registration model). Then, the kinematics data of the registration model and the CT model are measured using the same coordinate system (the former takes the latter as a reference) [13, 14]. The accuracy of the biplane X-ray technique was evaluated by comparing the kinematic differences between them (as shown in Table 1). The mean difference between them in front and back displacement is (0.5 ± 0.9) mm; the average difference in internal and external rotation is (0.9 ± 1.1)°. There are similar differences in other degrees of freedom. This accuracy is acceptable for clinical application. A more accurate reconstruction of the 3D physical model of human knee joint was achieved by testing the self-intersecting triangles and bad edges.Table 1
Kinematics comparison of 6 degrees of freedom between CT model and registration model.
KneeBack and forth (AP)Inside and outside (ML)Far and near (PD)Flex (FE)Internal and external rotation (IE)Turn inside and outside (AA)1−0.13 ± 0.930.52 ± 0.260.62 ± 0.52-0.50 ± 1.24−1.17 ± 0.630.70 ± 0.242−0.40 ± 0.820.47 ± 0.490.28 ± 0.67−1.18 ± 0.69−1.57 ± 0.820.37 ± 0.793−1.02 ± 0.77−0.40 ± 0.55−0.12 ± 0.640.18 ± 0.690.15 ± 1.00−0.63 ± 0.34Average−0.52 ± 0.880.26 ± 0.650.19 ± 0.61−0.50 ± 1.03−0.86 ± 1.090.14 ± 0.76


## 2.6. High-Impact Movements and Tibiofemoral Joint Injuries

Humans are upright walking creatures, and they complete various movements mainly with the foot in contact with the ground. According to Newton's second law, “the acceleration of an object is directly proportional to the combined external force (f) and inversely proportional to the mass (m) of the object, and the direction of acceleration (a) is the same as the direction of combined external force,” F = ma, the mass of human body is m, and it falls freely with the acceleration of gravity a (a = 9.8 m/s2). Ground reaction force will be generated when any part of the foot is in contact with the ground [15, 16]. Therefore, the impact force of human body in running and jumping mostly comes from the reaction force generated by the ground. The ground reaction force can reflect the strength and duration when the foot touches the ground. A large value indicates that the ground reaction force is large, that is, the impact force is large [17, 18]. If used for comparison among different individuals, the unit is body weight (BW), which is a multiple of the subject's body weight. The impact force is transmitted upward along the bone. The ground reaction force borne by human body comes from six directions: up and down, left and right, and front and back, and the forces in different directions have different influences on human body. When the human body jumps vertically, it often produces the ground reaction force in the up and down directions. When walking, the ground reaction force in front and back directions is often generated.A large number of studies take platform landing as the research object and assume that it is an action that can represent the human body's high-impact force. Researchers used the traditional three-dimensional mocap system to analyze the movement of the tibiofemoral joint (knee joint captured by traditional motion) under various landing modes. Its complex shape and uneven composition make it difficult to analyze its biomechanical properties, especially in the case of load bearing. It is found that when basketball players jump and shoot and land, the vertical reaction force on the ground reaches 3BW. When a professional gymnast rolls backward for two weeks and lands, the vertical ground reaction force reaches 8.8BW. The vertical ground reaction force of airborne troops landing from 40 cm platform is 7BW, which is obviously less than 10.31BW when landing from 80 cm platform. When basketball players and track and field jumping athletes land from a platform with a height of 60 cm, the vertical reaction force on the ground is 3200 N–3900 N. When normal people land from 30 cm platform to 60 cm platform (once every 5 cm), the vertical ground reaction force is getting higher and higher, ranging from 3.16BW to 4.49BW. Healthy sports enthusiasts landed from the 60 cm platform, and the vertical ground reaction force reached 4.38BW.The tibiofemoral joint is the longest joint in the human body, and it needs to bear the soft tissue and bone inside the joint. When it is subjected to direct or indirect impact force, it will exceed its own tolerance range, resulting in local microstructure fracture or even overall fracture. The actions include repeated flexion and extension, jumping and landing injuries, sudden stop and tearing, and falling under special violent circumstances. Common injury sites include bone injury, meniscus injury, and cartilage injury. These results indicate that PCL is the primary structure to resist tibial anterior displacement during knee flexion, and the stress is mainly concentrated at the proximal tibial insertion, which is consistent with the clinical occurrence of PCL fracture at the insertion of the femur.


## 3. Results

### 3.1. Displacement of the Femur relative to the Tibia

As shown in Figure1, from the straight position to 120°, the femur is located behind the tibia. In the straight position, the femur is (4.3 ± 2.1) mm behind. During the whole flexion process, the femur gradually moved backward, with a range of (5.1 ± 2.3) mm. Among them, it moved (1.9 ± 2.4) mm from the straight position to 15°. The femur flexes from the straight position to 120°and is located inside the tibia. In the straight position, the femur is located (5.0 ± 1.1) mm inside. With the increase of buckling, the femur gradually moved outward with a range of (3.1 ± 1.8) mm, of which (1.5 ± 1.1) mm occurred during the flexion from the straight position to 15°. During the whole flexion, the femur is always located near the tibia. In the straight position, the femur is located (25.3 ± 2.4) mm proximal. From the straight position to 15°, the range of distal movement of the femur is (2.3 ± 0.8) mm, accounting for more than half of the total distal movement range of (4.1 ± 1.2) mm.Figure 1
Change of femoral displacement with flexion angle.


### 3.2. Anteroposterior Displacement of the Femoral Condyle relative to the Tibia

As shown in Figure2, the lateral condyle gradually moves backward with flexion. In the extended position, the lateral condyle is located (1.8 ± 3.9) mm behind the tibia. From the straight position to 120°, the lateral condyle moved backward by (12.6 ± 3.4) mm, of which (6.2 ± 3.7) mm occurred before 15°.Figure 2
Changes of anteroposterior displacement of femoral ankle joint with flexion angle.Contrary to the backward movement of the lateral condyle, the medial condyle moves forward from the straight position to 90°. In the extended position, the medial condyle is located (4.7 ± 2.6) mm behind the tibia. This displacement value has been adjusted because in the process of straightening the femur from 10°, the medial condyle moves backward by about 2 mm, which is not really backward translation but caused by its upward rotation. The medial condyle moved forward (3.4 ± 3.1) mm from the straight position to 90° (before 15°, (1.5 ± 2.9) mm) and moved backward from 90° to 120° by (3.0 ± 1.5) mm, so the medial condyle moved forward very little by (0.4 ± 3.9) mm during the whole flexion. The stress of ACL, PCL, and LCL was small at tibial insertion, indicating that MCL is the main structure to resist leg valgus.


### 3.3. Rotation of the Femur relative to the Tibia

It can be found in Figure3 that from the straight position to the 120° flexion, the femur gradually rotates outward. In the extended position, the femur is rotated (5.5 ± 6.3)° with respect to the tibia. From the straight position to 90°, the range of femoral external rotation is (18.7 ± 5.9)°, while from 90° to 120°, a small amount of internal rotation occurs (1.4° ± 1.9°). Therefore, in the whole flexion process, the femoral external rotation angle is (17.3 ± 6.9)°, the femoral varus changes little, and the varus angle is basically (3.9 ± 2.2)°.Figure 3
Femur has no change with the increase of flexion, external rotation, and varus.


## 3.1. Displacement of the Femur relative to the Tibia

As shown in Figure1, from the straight position to 120°, the femur is located behind the tibia. In the straight position, the femur is (4.3 ± 2.1) mm behind. During the whole flexion process, the femur gradually moved backward, with a range of (5.1 ± 2.3) mm. Among them, it moved (1.9 ± 2.4) mm from the straight position to 15°. The femur flexes from the straight position to 120°and is located inside the tibia. In the straight position, the femur is located (5.0 ± 1.1) mm inside. With the increase of buckling, the femur gradually moved outward with a range of (3.1 ± 1.8) mm, of which (1.5 ± 1.1) mm occurred during the flexion from the straight position to 15°. During the whole flexion, the femur is always located near the tibia. In the straight position, the femur is located (25.3 ± 2.4) mm proximal. From the straight position to 15°, the range of distal movement of the femur is (2.3 ± 0.8) mm, accounting for more than half of the total distal movement range of (4.1 ± 1.2) mm.Figure 1
Change of femoral displacement with flexion angle.


## 3.2. Anteroposterior Displacement of the Femoral Condyle relative to the Tibia

As shown in Figure2, the lateral condyle gradually moves backward with flexion. In the extended position, the lateral condyle is located (1.8 ± 3.9) mm behind the tibia. From the straight position to 120°, the lateral condyle moved backward by (12.6 ± 3.4) mm, of which (6.2 ± 3.7) mm occurred before 15°.Figure 2
Changes of anteroposterior displacement of femoral ankle joint with flexion angle.Contrary to the backward movement of the lateral condyle, the medial condyle moves forward from the straight position to 90°. In the extended position, the medial condyle is located (4.7 ± 2.6) mm behind the tibia. This displacement value has been adjusted because in the process of straightening the femur from 10°, the medial condyle moves backward by about 2 mm, which is not really backward translation but caused by its upward rotation. The medial condyle moved forward (3.4 ± 3.1) mm from the straight position to 90° (before 15°, (1.5 ± 2.9) mm) and moved backward from 90° to 120° by (3.0 ± 1.5) mm, so the medial condyle moved forward very little by (0.4 ± 3.9) mm during the whole flexion. The stress of ACL, PCL, and LCL was small at tibial insertion, indicating that MCL is the main structure to resist leg valgus.


## 3.3. Rotation of the Femur relative to the Tibia

It can be found in Figure3 that from the straight position to the 120° flexion, the femur gradually rotates outward. In the extended position, the femur is rotated (5.5 ± 6.3)° with respect to the tibia. From the straight position to 90°, the range of femoral external rotation is (18.7 ± 5.9)°, while from 90° to 120°, a small amount of internal rotation occurs (1.4° ± 1.9°). Therefore, in the whole flexion process, the femoral external rotation angle is (17.3 ± 6.9)°, the femoral varus changes little, and the varus angle is basically (3.9 ± 2.2)°.Figure 3
Femur has no change with the increase of flexion, external rotation, and varus.


## 4. Discussion

In this study, the biplane X-ray technique was used to measure the positional relationship of the femur and the tibia in normal knee joint load, and the movement track of the femur relative to the tibia and the change of anterior and posterior displacement of the femoral condyle were obtained. Many previous studies have applied the biplane X-ray technique to evaluate the kinematics of knee joint. In some studies, tantalum beads need to be placed in the knee joint cavity of the subject. This invasive operation limits the research to a small number of people. In this study, the placement of marker balls on the surface of knee joint was used to assist in the biplane X-ray technique, which could avoid the placement of tantalum beads [18, 19]. Only by combining with other research data and clinical test results, can the conclusion truly reflect the stress state of specimens. Other scholars use double c-arm X-ray fluoroscopy machine to obtain double-beam X-rays, and this method requires a complicated calibration procedure to correct the distortion of the image. In this study, by increasing the focal distance, the edge magnification of X-ray image of tibiofemoral joint is reduced, and the geometric distortion of the image is reduced, so that the image without significant deformation is obtained. In this study, it was found that the femur was in the movement of the terminal extension arc from the extension position to 15°, and the amplitude of kinematic changes is significant, which is about 1/2 or more of the overall motion amplitude. This may be determined by the bone structure of the femoral condyle and tibial plateau. This study measured the kinematics of the tibiofemoral joint under static state. Although several flexion angles are measured, they are not a dynamic way of joint continuous motion. Referring to previous studies, this study also used joint protractor to measure the flexion angle. Although it can produce angle deviation, it has little influence on the kinematics trend of knee joint. Under the weight load, the femur moves backward, outward, and far relative to the tibia during flexion and rotates outward and remains in the varus position. The lateral condyle of the femur gradually moves backward with flexion, and the medial condyle has less anteroposterior movement. Kinematics changes significantly during the movement of the femoral terminal extension arc. Using the biplane X-ray technique assisted by marker ball, the kinematics of the full degree of freedom of knee joint in vivo can be measured. The model can reflect the anatomical structure of human knee joint with fine division and high precision.


## 5. Conclusion

In the process of normal knee flexion load, the femur moves backward, outward, and far and outward relative to the tibia, and the varus changes little. The movement of medial and lateral condyles of the femur is different. In the extended position, the femur was rotated inward (5.5° ± 6.3°) relative to the tibia. The range of femoral external rotation is (18.7° ± 5.9°) from flexion to 90° in straight position. However, from 90° to 120°, a small amount of internal rotation occurred (1.4° ± 1.9°), so during the whole flexion process, the femur rotated (17.3° ± 6.9°), among which, from the straight position to 15°, the femur rotated (10.0° ± 5.6°). Damage in different areas is determined by the size of interlayer displacement sample size method of sample space reduction. Therefore, the detection method of tibiofemoral joint injury in high-impact motion based on neural network reconstruction algorithm is a method with high accuracy and precision and good consistency. It can be used for clinical measurement of 6DOF motion and soft tissue contact characteristics of tibiofemoral joint.



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*Source: 5800893-2021-12-01.xml*