Final answer:
The knee can rotate internally and externally once it bends about 90 degrees or more, facilitated by the structure and ligaments within the joint.
Step-by-step explanation:
Once the knee bends about 90 degrees or more, it can rotate internally and externally. The knee joint functions much like a , enabling it to bend and straighten the lower leg. This flexibility is vital as it allows a person to perform everyday actions such as sitting, standing, and pivoting.
In addition to flexion and extension, the knee can also perform slight rotational movements when flexed due to the structures within the joint, such as the menisci and ligaments like the anterior and posterior cruciate ligaments.
The flexibility and capacity for internal and external rotation when the knee is flexed become possible because of the unique interactions between the femur's condyles and those of the tibia. When the knee comes into full extension, the femur slightly rotates medially to 'lock' the knee in place, providing stability and weight-bearing capability.
Conversely, a small lateral rotation of the femur is necessary to begin flexing the knee. Throughout these movements, the ligaments mentioned, along with the knee's articular capsule, work in coordination to maintain knee stability and prevent injuries.