American Board of Internal Medicine (ABIM) Certification Practice Exam

Which mechanism of injury typically causes a complete anterior cruciate ligament tear?

1. A twisting of the knee when the foot is planted

2. A direct blow to the kneecap

3. Hyperextension of the knee

4. Excessive lateral movement of the knee

The correct answer is: A twisting of the knee when the foot is planted

A complete anterior cruciate ligament (ACL) tear is most commonly associated with a specific mechanism of injury where the knee is twisted while the foot remains planted on the ground, creating significant stress on the ligament. This occurs frequently in sports that require sudden changes in direction, jumping, or abrupt stops. The mechanism relies on a combination of forces acting on the knee joint, including rotational forces and the stability provided by the foot's position. When the foot is fixed and the knee undergoes twisting, the ACL bears the brunt of these forces, leading to the possibility of a tear. Factors such as muscle strength, control, and the angle of the knee at the time of injury also play crucial roles in whether the ligament will sustain damage. While other mechanisms, such as hyperextension or direct impacts, can contribute to ACL injuries, they are less common than the typical twisting motion associated with a planted foot. Each of the other options represents different types of stress or trauma to the knee but does not align as closely with the most prevalent cause of a complete ACL tear.