What Is the Biomechanics of Joints?

The load of the knee joint varies greatly with the movement and gait of the human body. The static force of the knee joint in standing position (two feet on the ground) is 0.43 times the weight, and it can reach 3.02 times the weight when walking. It can reach 4.25 times. The normal knee joint forces are transmitted by the creep of the meniscus and articular cartilage to increase the contact surface between the tibia and femur, thereby reducing the force load per unit area. In the coronal plane, when standing on one foot, the human body's gravity is transmitted along the vertical center of gravity and passes through the medial side of the knee joint. This gravity forces the femur toward the medial tibial condyle. At this time, the fascia lata and gluteus maximus muscles are maintained in balance by the lateral force of the iliotibial bundle. The sum of these forces represents the total supporting force of the knee joint on this side, and the resultant force passes through the center of the knee joint.

The load of the knee joint varies greatly with the movement and gait of the human body. The static force of the knee joint in standing position (two feet on the ground) is 0.43 times the weight, and it can reach 3.02 times the weight when walking. It can reach 4.25 times. The normal knee joint forces are transmitted by the creep of the meniscus and articular cartilage to increase the contact surface between the tibia and femur, thereby reducing the force load per unit area. In the coronal plane, when standing on one foot, the human body's gravity is transmitted along the vertical center of gravity and passes through the medial side of the knee joint. This gravity forces the femur toward the medial tibial condyle. At this time, the fascia lata and gluteus maximus muscles are maintained in balance by the lateral force of the iliotibial bundle. The sum of these forces represents the total supporting force of the knee joint on this side, and the resultant force passes through the center of the knee joint.

The motion mode of the knee joint is not a simple flexion and extension movement, but a complex multi-degree-of-freedom movement mode that combines flexion and extension, rolling, sliding, side shifting, and axial rotation. In the sagittal plane, the extension and flexion of the knee joint does not surround the same rotation center, but generates multiple instantaneous rotation centers according to the process of the movement. When the particle surface velocity direction of the contact surface is cut on the joint surface, the resistance to movement is the smallest, and the instantaneous rotation center can be obtained based on the vertical line of the contact surface. If the instantaneous center is not on this line, the knee joint will slide. On a normal knee joint, the velocity direction of any instantaneous center will be cut at the joint contact surface. During the process of knee flexion and extension, the instantaneous rotation center of each movement will be continuously marked, and a " J "shaped trajectory.

The force transmission and stress distribution of normal tibiofemoral joints are closely related to the function of normal meniscus and articular cartilage. In the movement and stress phase of the knee joint, the change in pressure between the joints tends to be moderated due to the relative displacement of the meniscus with joint activity and the strain of the normal meniscus and articular cartilage tissue with viscoelastic properties. In addition, the rotation of the knee joint in the horizontal plane is centered on the medial condyle. This rotation method makes the medial space of the knee joint prone to degeneration. This is also the knee joint osteoarthritis lesions that tend to focus on the medial space, even the typical medial side. Single-chamber osteoarthritis and knee varus deformity.

The patellofemoral joint is an important structure involved in knee flexion and extension, and it has a special chapter in the knee joint activity. In addition to transmitting the tensile force of the quadriceps femoris and the tension of the patellar ligament, the metatarsal bone itself bears the stress during the flexion of the knee joint and the stress distribution on the articular surface is the focus of the biomechanical research of the patellofemoral joint. The lateral tilt and lateral displacement of the sacrum are the main forms of abnormal patella-femoral alignment. The reasons may include abnormal development of the femoral condyle, abnormal development of the sacrum and high sacrum, abnormal increase in knee valgus and Q angle, and loosening of the medial support band. , Lateral support belt contracture and other factors.

The metatarsal position is actually a subluxation of the metatarsal. The metatarsal is easily pushed to the outside when in the straight position. When the knee is bent at 20 °, it can be found that the central metatarsal and the lowest point of the trough are not in a corresponding relationship, but outward. Displacement, the degree of displacement is significant for the evaluation of zygomatic subluxation. Therefore, the evaluation of the patellofemoral alignment during knee flexion of 20 ° -30 ° is the key to the diagnosis of patellofemoral abnormal alignment during joint examination. The direct result of patellofemoral abnormality is the abnormal distribution of joint surface stress or patellofemoral contact pressure. On the one hand, localized stress concentration on the articular surface can cause articular cartilage damage. On the other hand, the reduction of contact pressure and loss of contact on the articular surface can also lead to the degradation of cartilage. The loss of cartilage thickness due to the degeneration of the cartilage surface can also lead to the redistribution of stress on the normal cartilage surface, leading to the expansion of the entire cartilage lesion. Based on this, it can be considered that the uneven distribution of patellofemoral contact pressure caused by abnormal patellofemoral alignment is a potential cause of cartilage lesions. Similarly, abnormal patellofemoral alignment is the main cause of patella complications after TKA.

The uneven distribution of stress on the metatarsal articular surface is the direct cause of cartilage degeneration on the patellofemoral articular surface. Under normal physiological conditions, the patellar-femoral contact pressure gradually increases during the movement of the knee joint from extension to 90 °, and gradually decreases after 90 °. Because the contact surface of the normal patellofemoral joint increases with the increase of patellofemoral contact pressure, the stress acting on the patellofemoral joint surface is dispersed, and its pressure changes little. During knee replacement, the matching of the patella and the femur in different phases is an important indicator in the design of the prosthesis. The patella can be falsely implanted through reasonable prosthesis design, internalization of the patella and external rotation installation of the femoral The body is easier to match with the tackle.

The knee joint can maintain static and dynamic stability due to the combined action of the aforementioned bony structure, meniscus, joint capsule and attached ligament structure. When the knee joint is fully extended, the joints will lock up and achieve maximum joint stability. This is because when the knee is fully extended, the femur rotates inward on the tibia; while in the hyperflexed position, the femur Rotating outwards will increase joint stability through articulation of the articular surface and guidance of the cruciate ligament.