Your knee rotates primarily to allow for a greater range of motion, especially during full extension (straightening) and flexion (bending). This rotation is a natural and essential part of knee function.
Here's a more detailed explanation:
The "Screw Home" Mechanism
The most significant rotation occurs during the final degrees of knee extension. This is known as the "screw-home mechanism." It essentially "locks" the knee into a stable position.
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What it is: During the last 15-20 degrees of knee extension, the tibia (shin bone) externally rotates relative to the femur (thigh bone).
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Why it happens: This rotation is due to a combination of factors:
- The shape of the medial femoral condyle (the inner part of the femur where it articulates with the tibia). The medial condyle is larger and longer, guiding the rotation.
- The tension in the anterior cruciate ligament (ACL). As the knee extends, the ACL tightens, contributing to external rotation.
- The pull of the popliteus muscle. During initiation of knee flexion, this muscle helps "unlock" the knee by internally rotating the tibia.
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Purpose: The "screw-home" mechanism provides stability to the knee in the fully extended position, which is especially important for standing and maintaining posture.
Rotation During Flexion
While the "screw-home" mechanism describes the rotation during the final stages of extension, some degree of rotation also accompanies flexion.
- Why it happens: As the knee bends, the tibia internally rotates relative to the femur. This is necessary to allow the ligaments and menisci (cartilage pads in the knee) to move and accommodate the changing joint geometry.
- Muscle Involvement: Muscles surrounding the knee, like the hamstrings and quadriceps, also play a role in controlling the rotation during flexion and extension.
- Meniscus Role: The menisci also play an important role in guiding and facilitating the rotational movements of the knee.
In Summary
The rotation of your knee is a normal and necessary part of its function. It enhances stability during extension (thanks to the "screw-home" mechanism) and allows for a greater range of motion during flexion. The shape of the bones, the ligaments, the muscles, and the menisci all contribute to this rotational movement.