The knee can rotate primarily due to the inequality of the articular surface of the femur condyles. This anatomical feature facilitates a necessary rotation for healthy knee movement, especially during the final stages of knee extension.
The Role of Femoral Condyle Shape
The femoral condyles are the rounded surfaces at the end of the femur (thigh bone) that articulate with the tibia (shin bone). Their uneven shape is key to allowing rotation.
- The medial condyle (inner side) is typically larger and more curved than the lateral condyle (outer side).
- This difference in shape and size contributes to the screw-home mechanism.
The Screw-Home Mechanism
The screw-home mechanism refers to the automatic rotation that occurs during the last 30 degrees of knee extension.
- Open Chain (Tibia Rotating): When the leg is not weight-bearing (e.g., sitting and extending the leg), the tibia externally rotates about 10 degrees.
- Closed Chain (Femur Rotating): When the leg is weight-bearing (e.g., standing up), the femur internally rotates about 10 degrees.
This rotation provides additional stability to the knee joint when fully extended. It effectively "locks" the knee, reducing the need for muscular effort to maintain a standing posture.
Importance of Knee Rotation
Knee rotation is crucial for several reasons:
- Stability: The screw-home mechanism enhances knee stability, especially during activities like standing.
- Efficient Movement: Rotation allows for smoother transitions between flexion (bending) and extension (straightening) of the knee.
- Injury Prevention: Proper rotation helps distribute forces evenly across the knee joint, reducing the risk of injury.
