The knee joint consists of a double joint structure consisting of the tibiofemoral joint (medial tibiofemoral articular surface and lateral tibiofemoral articular surface) and the patellofemoral joint. The first 1/3 of the lateral tibiofemoral surface is a progressively ascending concave surface, while the second 2/3 is a progressively descending concave surface. The medial tibiofemoral articular surface is a bowl-shaped depression. However, the characteristic concave structure of the lateral tibiofemoral articular surface allows the lateral tibiofemoral articular surface to not be perfectly aligned, thus permitting the knee to flex and extend not coaxially but with multiple transient centers of motion.
Knee joint motion in cross-section
When the knee is fully extended, rotation of the knee is completely limited due to the interlocking femoral and tibial condyles, primarily because the medial femoral condyle is longer than the lateral condyle.
When the knee is flexed, the range of rotation increases, reaching a maximum of 0-45° of external rotation and 0-30° of internal rotation at 90° of knee flexion.
When flexion exceeds 90°, the range of internal and external rotation begins to decrease, primarily because the soft tissues limit the rotational motion.
Movement of the knee joint on the frontal plane
Abduction and adduction in the frontal plane are also influenced by the degree of joint flexion.
When the knee is fully extended, almost all movement on the frontal plane is prevented.
As the knee flexes to 30°, passive abduction and adduction increase, but the maximum change is only a few degrees.
As the knee flexes beyond 30°, frontal plane motion also begins to decrease due to functional limitations of the soft tissues.
Locking mechanism of the tibiofemoral joint
While the tibia is externally rotated during knee extension, this motion is reversed during knee flexion. The tibiofemoral joint is not a purely flexion joint; it has a spiral and a spiral plane of motion. The structure of the medial femoral condyle results in this spiral motion of the tibia relative to the femur during knee flexion and extension. In the normal knee joint the medial condyle is 1.7 cm longer than the lateral condyle, therefore, when the knee is flexed → straightened, a rotational movement is brought out between the femur and tibia in relation to each other.
This biomechanical mechanism is called the ‘screw-home mechanism’, or the spiral homing mechanism. If there is a problem with this mechanism (not enough or too much rotation), the muscles of the quadriceps muscle may change in the way they exert themselves, resulting in an imbalance in muscle tone, which is one of the main causes of patellofemoral pain syndrome.
Cruciate ligament
The anterior and posterior cruciate ligaments provide control and stability of the entire knee joint in flexion and extension.
Anterior Cruciate Ligament (ACL): The ACL pulls the femur forward during knee extension. Cutting this ligament allows the tibia to dislocate forward over the femur (anterior drawer sign). Severing the ACL in a cadaver shows a 7 mm forward displacement of the tibia over the femur. in normal subjects this movement is very small. The mean value of the anterior drawer test in healthy college students at 90° of knee flexion is 1.2 to 2.7 mm. The subordinate function of the ACL is usually considered to be the restriction of internal and external rotation.
Posterior cruciate ligament (PCL): the PCL limits the posterior displacement of the tibia over the femur (posterior drawer sign). Conversely, the PCL helps prevent forward displacement of the femoral condyle over the tibial condyle (dislocation) when landing on the running foot in closed chain sports. the PCL normally allows only a small amount of passive motion. The mean value of displacement in the back-drawer test at 90° of knee flexion in healthy college students ranged from 0.6 to 1.0 mm in men and 1.2 to 1.9 mm in women.
Patellofemoral joint
The patella slides approximately 7 cm between the femoral condyles from full extension to full flexion of the knee.
In flexion greater than 90°, the patella is externally rotated and only the medial articular surface of the femur forms an articulation with the patella
In full flexion, the patella sinks into the intercondylar groove.
The contact area increases with increasing knee flexion and with increasing quadriceps tension.
Relationship between the contact surface of the patellofemoral joint and the angle of knee flexion and extension
Normally, the patella is not completely within the femoral talus, and the interpatellofemoral contact surface changes continuously with knee flexion and extension.
When the knee is flexed at 10° to 20°, the medial and lateral articular surfaces of the inferior pole of the patella are in contact with the femoral talus at the same time. As the number of degrees of flexion increases, the contact surface between the patella and the trochanter gradually moves proximally and laterally.
At 45° of knee flexion, the contact area of the patellofemoral joint reaches its maximum.
After 90° of flexion, with further increase in flexion, the medial and lateral contact surfaces of the patella corresponding to the femur gradually separate and become independent of each other.
Q-angle
The Q angle is the angle between the quadriceps muscle force line and the patellar ligament force line, that is, the line from the anterior superior iliac spine to the midpoint of the patella is the quadriceps muscle force line, and the line from the midpoint of the patella to the highest point of the tibial tuberosity is the patellar ligament force line, and the angle formed by the two lines is the Q angle.
Factors affecting the Q angle: The Q angle is affected by the tilt of the femoral neck and tibial rotation, and is measured in the supine position with the hip and knee fully extended.
If the knee joint is mildly flexed, the Q angle will decrease due to internal rotation of the tibia relative to the femur. With internal rotation of the femur, the Q angle increases.
The greater the Q angle, the greater the outward force on the patella, the more unstable the patella and the more abnormal the distribution of pressure on the patellofemoral joint.